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+The Project Gutenberg eBook, Wireless Transmission of Photographs, by
+Marcus J. Martin
+
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+
+
+
+Title: Wireless Transmission of Photographs
+       Second Edition, Revised and Enlarged 1919
+
+
+Author: Marcus J. Martin
+
+
+
+Release Date: October 9, 2010  [eBook #34052]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+
+***START OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF
+PHOTOGRAPHS***
+
+
+E-text prepared by Robert Cicconetti, Keith Edkins, and the Online
+Distributed Proofreading Team (http://www.pgdp.net) from page images
+generously made available by Internet Archive/Canadian Libraries
+(http://www.archive.org/details/toronto)
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 34052-h.htm or 34052-h.zip:
+      (http://www.gutenberg.org/files/34052/34052-h/34052-h.htm)
+      or
+      (http://www.gutenberg.org/files/34052/34052-h.zip)
+
+
+      Images of the original pages are available through
+      Internet Archive/Canadian Libraries. See
+      http://www.archive.org/details/wirelesstransmis00martuoft
+
+
+Transcriber's note:
+
+      A carat character indicates that the following character
+      (or characters in curly braces) is a superscript. Examples:
+      A^2 (A raised to the second power), 10^{-7} (10 raised to
+      the -7 power).
+
+      Text enclosed between underscores was italicized in the
+      original (_italics_).
+
+      A single underscore indicates that the following character
+      or characters is a subscript. Example: CS_2 (the "2" is a
+      subscript).
+
+      Numbers enclosed by curly braces within the text are page
+      numbers (example: {100}), which have been incorporated to
+      enable the reader to use the index.
+
+
+
+
+
+WIRELESS TRANSMISSION OF PHOTOGRAPHS
+
+[Illustration: FIG. 10.]
+
+       *       *       *       *       *
+
+
+WIRELESS TRANSMISSION OF PHOTOGRAPHS
+
+by
+
+MARCUS J. MARTIN
+
+SECOND EDITION
+REVISED AND ENLARGED 1919
+
+
+
+
+
+
+
+The Wireless Press, Ltd.
+12-13 Henrietta Street, Strand
+London, W.C. 2
+
+
+
+{v}
+
+PREFACE TO SECOND EDITION
+
+Although during the last few years very little, in common with other
+wireless work, has been possible in connection with the practical side of
+the wireless transmission of photographs, yet, now that the prospect of
+experimental work is once again occupying the minds of all wireless
+workers, advantage has been taken of a reprint of this little volume to
+amplify a few points that were insufficiently dealt with in the first
+edition, and also to add some fresh matter.
+
+To Chapter V. has been added a short description of the Nernst lamp, and
+also some useful information regarding photographic films, and a few notes
+relating to enlarging included in the Appendix B.
+
+A fresh appendix dealing with the principles of optical lenses has also
+been added. This is a subject that plays an important part in any system of
+wireless photography, and to those experimenters whose knowledge of optics
+is limited this section should prove useful.
+
+To serious workers engaged on the problem of the wireless transmission of
+photographs, attention {vi} is called to a series of articles which are
+being published from time to time in the _Wireless World_, on the design
+and construction of wireless photographic apparatus.
+
+  M. J. M.
+
+  MAIDSTONE, 1919.
+
+{vii}
+
+PREFACE
+
+In these progressive times it is only reasonable to expect that some
+attempt would be made to utilise the ether-waves for other purposes than
+that of telegraphic communication, and already many clever minds are at
+work trying to solve the problems of the wireless control of torpedoes and
+airships, wireless telephony, and, last but not least, the wireless
+transmission of photographs.
+
+It may seem rather premature to talk about the wireless transmission of
+photographs at a time when the ordinary systems are not fully developed;
+but the prospects of wireless photography are of a very encouraging nature,
+especially for long over-water distances, as there are great difficulties
+to be overcome in long-distance transmission over ordinary land lines and
+cables which will be entirely eliminated by wireless methods.
+
+From a perusal of Chapter I. the reader will be able to understand
+something of the difficulties that are to be encountered in working over
+long distances, and he will also be able to appreciate something of the
+advantages that would be derived {viii} from a reliable wireless system.
+Apart from the value of such a system for transmitting news pictures, it
+would also be of great advantage to transmit to ships at sea photographs of
+criminals for identification purposes. In such a small volume as this it
+would be impossible to deal with the working of wireless apparatus and the
+many systems that have been devised for the transmission of photographs
+over metallic circuits. The Author has taken it for granted that other
+works have been studied in connection with these subjects, and will
+therefore only describe such apparatus as is likely to be of use in
+wireless transmission. At present the transmission of photographs by
+wireless methods is in a purely experimental stage, and this book will have
+served its purpose if it helps to put future experimenters on the right
+track and prevent them from making expensive and fruitless experiments, by
+showing them the right direction in which investigations are being carried
+out. As there is no claim to originality in respect of a good many pieces
+of apparatus, etc., described, I have not thought it necessary to state the
+various sources from which the information has been obtained.
+
+  M. J. M.
+
+  ASHFORD, 1916.
+
+       *       *       *       *       *
+
+
+{ix}
+
+CONTENTS
+
+                                                                       PAGE
+  PREFACE TO SECOND EDITION                                               v
+
+  PREFACE                                                               vii
+
+CHAPTER I
+
+  INTRODUCTORY                                                            1
+
+    Foreword--Early experiments--Advantages of
+    Radio-Photography--Difficulties in Cable working--Bernochi's
+    System--Knudsen's System.
+
+CHAPTER II
+
+  TRANSMITTING APPARATUS                                                 13
+
+    Wireless Apparatus--Preparing the Photographs--Transmitting
+    Machines--Transmitting Apparatus--Effects of
+    Arcing--Spark-Gaps--Contact Breakers--Complete Station--Professor
+    Korn's Apparatus--Poulsen Company's Photographic Recorder--Comparison
+    of various systems--Practical applications.
+
+CHAPTER III
+
+  RECEIVING APPARATUS                                                    37
+
+    Methods of Receiving--Author's Photographic Receiver--Decohering
+    Apparatus--Description of Einthoven Galvanometer--Use of Galvanometer
+    in Receiving--Belin's Application of Blondel's
+    Oscillograph--Description of Charbonelle's Receiver--Use of Telephone
+    Relay--Description of Telephone Relay--Telephotographic
+    Receiver--Polarisation Receiver--Kathode-Ray Receiver--Electrolytic
+    Receiver--Atmospherics in Long-Distance working.
+
+{x}
+
+CHAPTER IV
+
+  SYNCHRONISING AND DRIVING                                              63
+
+    Driving Motors--Isochronising the Electrolytic System--Professor Korn's
+    method--Description of Hughes Governor--Author's Speed
+    Regulator--Problem of Synchronising--Methods of Synchronising--Advances
+    made in Radio-Photography.
+
+CHAPTER V
+
+  THE "TELEPHOGRAPH"                                                     74
+
+    Author's System of
+    Radio-Photography--Requirements--Advantages--Transmitting
+    machine--Description of Differential Relay--Wireless Receiving
+    Apparatus--Photo-Telegraphic Receiving Apparatus--Circuit
+    Breaker--Friction Brake--Magnetic Clutch--Description of
+    Isochroniser--Method of working--Types of Nernst Lamp--Action of Nernst
+    Lamp--Comparison of Actinic Value--Inertia of Photographic
+    Films--Choosing Films--Speed of Films--Standard of Speed--Comparative
+    Film Speeds--Effects of Minimum Exposure--Effects of Maximum
+    Exposure--Considerations in working and choosing Films.
+
+APPENDIX A
+
+  SELENIUM CELLS                                                        109
+
+    Nature of Selenium--Preparation of Selenium--Forms of Selenium
+    Cells--Action of Selenium Cells--Characteristics of Selenium
+    Cells--Effects of Inertia in Photo-Telegraphy--Methods of counteracting
+    Inertia--Sensitiveness of Selenium to Light--Effect of Heat on
+    Selenium.
+
+APPENDIX B
+
+  PREPARING THE METAL PRINTS                                            115
+
+    Outline of Process--Line Screens--Choice of Camera--Fixing Line Screen
+    in Camera--Lenses and Stops--Taking the Photograph--Copying
+    Stands--Choice of Photographic Plates--Sources of Illumination--Metal
+    Prints--Coating the {xi} Metal Sheets--Sensitising Solution--Printing
+    Operations--Developing--Intensifying--Precautions to be observed in
+    working--Preparing Sketches on Metal--Apparatus for Reducing or
+    Enlarging--Improvements to Copying Board--Lenses for Copying--Formula
+    for Copying.
+
+APPENDIX C
+
+  LENSES                                                                126
+
+    Action of Light--Law of Refraction--Lenses--Prisms--Action of
+    Lenses--Focal Length of Lenses--Formation of Images--Apparent Magnitude
+    of Objects--Real and Virtual Images--Formation of Virtual Images--Power
+    of Magnification--Defects of Lenses--Aberration.
+
+       *       *       *       *       *
+
+
+{xiii}
+
+ILLUSTRATIONS
+
+  FIG.                                                                 PAGE
+
+  1. Diagram showing effects of capacity on an intermittent current       5
+
+  2. Bernochi's wireless apparatus                                        7
+
+  3. Knudsen's wireless apparatus                                        10
+
+  4. Wireless transmitting station                                       13
+
+  5. Diagram of experiment illustrating principle of line photograph     16
+
+  6. Drawing of transmitting machine                                     17
+
+  7. Drawing of transmitting machine                                     18
+
+  8. Drawing of stylus                                                   18
+
+  9. Electrical connections of machine                                   19
+
+  10. Photograph of Author's experimental machine            _Frontispiece_
+
+  10a. End view of Author's experimental machine  }
+                                                  }        _facing page_ 21
+  10b. View of image broken up by a "cross" screen}
+
+  11. Connections of complete transmitting apparatus                     23
+
+  12. Drawing of ordinary type of spark-gap                              27
+
+  13. Synchronous rotating spark-gap                                     28
+
+  14. Non-synchronous rotating spark-gap                                 28
+
+  15. Connections for complete wireless photographic station             30
+
+  16. Connections of Professor Korn's apparatus                          31
+
+  17. Connections of Poulsen's photographic recorder                     33
+
+  18. Author's photographic receiver                                     38
+
+  19. Enlarged drawing of cone                                           39
+
+  20. End view of Author's photographic receiver                         39
+
+  21. Connections of decohering apparatus                                41
+
+  22. Connections for complete photographic receiver                     42
+
+  {xiv}
+  23. Arrangement of Einthoven galvanometer                              45
+
+  24. Einthoven galvanometer arranged for receiving                      46
+
+  25. Connection of telephone relay                                      49
+
+  26. Drawing of Author's improved photographic receiver                 51
+
+  27. Diagram giving ratio of vibrating arm                              51
+
+  28. Arrangement of polarisation receiver                               53
+
+  29. Arrangement of kathode-ray receiver                                54
+
+  30. Connections of electrolytic receiver                               56
+
+  31. Drawing of improved stylus for receiving                           58
+
+  32. Drawing of Hughes telegraph governor                               66
+
+  33. Arrangement of simple speed regulator                              68
+
+  34. Diagram of connections of simple speed regulator                   68
+
+  35. Author's arrangement for complete radio-photographic station       77
+
+  36. Drawing of transmitting machine and circuit breaker                78
+
+  37. Drawing of special transmitting stylus showing adjusting
+      arrangements                                                       79
+
+  37a. End view of transmitting stylus                                   79
+
+  38. Connections of new type of relay designed by the Author            80
+
+  39. Arrangement of mercury containers and dipping rods for relay       82
+
+  40. Drawing of Author's receiver                                       84
+
+  41. Enlarged drawing of diaphragm and steel point                      84
+
+  41a. Drawing showing arrangement of bush and counter-weight            84
+
+  42. Optical arrangements of receiver                                   85
+
+  43. Optical arrangements of receiver                                   86
+
+  44. Drawing of circuit breaker                                         88
+
+  45. Drawing of friction brake                                          89
+
+  46. Sectional drawing of magnetic clutch                               90
+
+  47. Plan of magnetic clutch                                            90
+
+  48. Details of Isochroniser                                            92
+
+  49. Connections of Isochroniser                                        94
+
+  50. Dial of Isochroniser                                               94
+
+  51. Diagram of driving mechanism                                       96
+
+  {xv}
+  52. Diagram showing starting positions of machines                     97
+
+  52a. Arrangement of small type Nernst lamp                             99
+
+  52b. Ballasting resistances for Nernst lamps                          100
+
+  52c. Arrangement of large type Nernst lamp                            101
+
+  53. Connections of selenium cell elements                             110
+
+  53a. Form of selenium cell used by Bell and Tainter                   110
+
+  54. Diagram showing construction of modern cell                       111
+
+  55. Resistance curve of selenium cell                                 111
+
+  55a. Actual curve of selenium cell                                    112
+
+  56. Diagram of Professor Korn's method for counteracting inertia      113
+
+  57. Arrangement of plate sheath and line screen                       117
+
+  58. Details of clips to hold line screen                              118
+
+  59. Arrangement of apparatus for copying                              119
+
+  60. Drawing showing method of arranging camera and copying stand for
+     adjustment                                                         119
+
+  61. Photograph of line screen and metal print  }
+                                                 } _facing page_        124
+  62. Photograph of sketch drawn upon metal foil }
+
+  63. Method of marking out copying board                               124
+
+  64. Diagram illustrating law of refraction                            127
+
+  65. Forms of lenses                                                   128
+
+  66. Action of light passed through a prism                            129
+
+  67. Diagram illustrating action of a lens                             130
+
+  68. Formation of principal focus of a lens                            130
+
+  69. Formation of conjugate foci of a lens                             131
+
+  70. Apparatus illustrating principle of camera                        132
+
+  71. Formation of an image by a lens                                   133
+
+  72. Diagram illustrating apparent magnitude                           134
+
+  73. Formation of virtual image by a convex lens                       137
+
+  74. Formation of virtual image by a concave lens                      138
+
+  75. Diagram showing spherical aberration                              139
+
+  76. Combination of plano-convex lenses                                139
+
+  77. Combination of meniscus and convex lenses                         139
+
+       *       *       *       *       *
+
+
+{1}
+
+RADIO-PHOTOGRAPHY
+
+CHAPTER I
+
+INTRODUCTORY
+
+Those who desire to experiment on radio-photography, _i.e._ transmitting
+photographs, drawings, etc., from one place to another without the aid of
+artificial conductors, must cultivate at least an elementary knowledge of
+optics, chemistry, mechanics, and electricity; photo-telegraphy calling for
+a knowledge of all these sciences. There are, no doubt, many wireless
+workers who are interested in this subject, but who are deterred from
+experimenting owing to a lack of knowledge regarding the direction
+developments are taking, besides which, information on this subject is very
+difficult to obtain, the science of photo-telegraphy being, at the present
+time, in a purely experimental stage.
+
+The wireless transmission of photographs has, no doubt, a great commercial
+value, but for any system to be commercially practicable, it must be
+simple, rapid, and reliable, besides being able to work {2} in conjunction
+with the apparatus already installed for the purpose of ordinary wireless
+telegraphy.
+
+As far back as 1847 experiments were carried out with a view to solving the
+problem of transmitting pictures and writing by electrical methods over
+artificial conductors, but no great incentive was held forth for
+development owing to lack of possible application; but owing to the great
+public demand for illustrated newspapers that has recently sprung into
+being, a large field has been opened up. During the last ten years,
+however, development has been very rapid, and some excellent results are
+now being obtained over a considerable length of line.
+
+The wireless transmission of photographs is, on the other hand, of quite
+recent growth, the first practicable attempt being made by Mr. Hans Knudsen
+in 1908. It may seem rather premature to talk about the wireless
+transmission at a time when the systems for transmitting over ordinary
+conductors are not perfectly developed, but everything points to the fact
+that for long-distance transmission a reliable wireless system will prove
+to be both cheaper and quicker than transmission over ordinary land lines
+and cables.
+
+The effects of capacity and inductance--properties inherent to all
+telegraph systems using metallic conductors--have a distinct bearing upon
+the two questions, how far and how quickly can {3} photographs be
+transmitted? Owing to the small currents received and to prevent
+interference from earth currents it is necessary to use a complete metallic
+circuit. If an overhead line could be employed no difficulty would be
+experienced in working a distance of over 1000 miles, but a line of this
+length is impossible--at least in this country--and if transmission is
+attempted with any other country, a certain amount of submarine cable is
+essential. It has been found that the electrostatic capacity of one mile of
+submarine cable is equal to the capacity of 20 miles of overhead line, and
+as the effect of capacity is to retard the current and reduce the speed of
+working, it is evident that where there is any great length of cable in the
+circuit the distance of possible transmission is enormously reduced.
+
+If we take for an example the London-Paris telephone line with a length of
+311 miles and a capacity of 10.62 microfarads, we find that about half this
+capacity, or 5.9 microfarads,[1] is contributed by the 23 miles of cable
+connecting England with France.
+
+In practice the reduction of speed due to capacity has, to a great extent,
+been overcome by means of apparatus known as a line-balancer, which hastens
+the slow discharge of the line and {4} allows each current sent out from
+the transmitter--the current in several systems being intermittent--to be
+recorded separately on the receiver. Photographs suitable for press work
+can now be sent over a line which includes only a short length of cable for
+a distance of quite 400 miles in about ten minutes, the time, of course,
+depending upon the size of the photograph. In extending the working to
+other countries where there is need for a great length of cable, as between
+England and Ireland, or America, the retardation due to capacity is very
+great. On a cable joining this country with America the current is retarded
+four-tenths of a second. In submarine telegraphy use is made of only one
+cable with an earth return, but special means have had to be adopted to
+overcome interference from earth currents, as the enormous cost prohibits
+the laying of a second cable to provide a complete metallic circuit. The
+current available at the cable ends for receiving is very small, being only
+1/200000th part of an ampere, and this necessitates the use of apparatus of
+a very sensitive character. One system of photo-telegraphy in use at the
+present time, employs what is known as an electrolytic receiver (see
+Chapter III.) which can record signals over a length of line in which the
+capacity effects are very slight, with the marvellous speed of 12,000 a
+minute, but this speed rapidly decreases with an increase of distance
+between the {5} [Illustration] two stations. The effect of capacity upon an
+intermittent current is clearly shown in Fig. 1. If we were to send twenty
+brief currents in rapid succession over a line of moderate capacity in a
+given time, we should find that instead of being recorded separately and
+distinctly as at _a_, each mark would be pointed at both ends and joined
+together as shown at _b_, while only perhaps fifteen could be recorded. If
+the capacity be still farther increased as at _c_, only perhaps half the
+original number of currents could be recorded in the same time, owing to
+the fact that with an increase of resistance, capacity, and inductance of
+the line a longer time is required for it to charge up and discharge,
+thereby materially lessening the rate at which it will allow separate
+signals to pass; the number of signals that can therefore be recorded in a
+given time is greatly diminished. If we were to attempt to send the same
+number of signals over a line of great capacity, as could be sent, and
+recorded separately and distinctly over a line of small capacity--the time
+limit being of course the same in both instances--we should find that the
+{6} signals would be recorded practically as a continuous line. The two
+latter cases _b_, and _c_, Fig. 1, clearly shows the retardation that takes
+place at the commencement of a current and the prolongation that takes
+place at the finish. If the photo-telegraphic system previously mentioned
+could be rendered sensitive enough to work on the Atlantic cables, we
+should find that only about 1200 signals a minute could be recorded, and
+this would mean that a photograph which could be transmitted over ordinary
+land lines in about ten minutes would take at least fifty minutes over the
+cable. This would be both costly and impracticable, and time alone will
+show whether, for long-distance work, transmission by wireless will be both
+cheaper and more rapid than any other method. At present wireless
+telegraphy has not superseded the ordinary methods of communicating over
+land, but there can be no doubt that wireless telegraphy, if free from
+Government restrictions, would in certain circumstances very quickly
+supersede land-line telegraphy, while it has proved a formidable commercial
+competitor to the cable as a means of connecting this country with America.
+Likewise we cannot say that no system of radio-photography will ever come
+into general use, but where there is any great distance to be bridged,
+especially over water, wireless transmission is really the only practical
+solution. From the {7} foregoing remarks, it is evident that a reliable
+system of radio-photography would secure a great victory in the matter of
+time and cost alone, besides which, the photo-telegraphic apparatus would
+be merely an accessory to the already existing wireless installation.
+
+[Illustration: FIG. 2.]
+
+There have been numerous suggestions put forward for the wireless
+transmission of photographs, but they are all more or less impracticable.
+One of the earliest systems was devised by de' Bernochi of Turin, but his
+system can only be regarded interesting from an historical point of view,
+and as in all probability it could only have been made to work over a
+distance of a few hundred yards it is of no practical value. Fig. 2 will
+help to explain the apparatus. A glass cylinder A' is fastened at one end
+to a threaded steel shaft, which runs in two bearings, one bearing having
+an internal thread corresponding with that on the {8} shaft. Round the
+cylinder is wrapped a transparent film upon which a photograph has been
+taken and developed. Light from a powerful electric lamp L, is focussed by
+means of the lens, N, to a point upon the photographic film. As the
+cylinder is revolved by means of a suitable motor, it travels upwards
+simultaneously by reason of the threaded shaft and bearing, so that the
+spot of light traces a complete spiral over the surface of the film. The
+light, on passing through the film (the transmission of which varies in
+intensity according to the density of that portion of the photograph
+through which it is passing), is refracted by the prism P on to the
+selenium cell S which is in series with a battery B and the primary X of a
+form of induction coil. As light of different intensities falls upon the
+selenium cell,[2] the resistance of which alters in proportion, current is
+induced in the secondary Y of the coil and influences the light of an arc
+lamp of whose circuit it is shunted. This arc lamp T is placed at the focus
+of a parabolic reflector R, from which the light is reflected in a parallel
+beam to the receiving station.
+
+The receiver consists of a similar reflector R' with a selenium cell E
+placed at its focus, whose resistance is altered by the varying light
+falling upon it from the reflector R. The selenium cell {9} E is in series
+with a battery F and the mirror galvanometer H. Light falls from a lamp D
+and is reflected by the mirror of the galvanometer on to a graduated
+aperture J and focussed by means of the aplanatic lens U upon the receiving
+drum A^2, which carries a sensitised photographic film. The two cylinders
+must be revolved synchronously. The above apparatus is very clever, but
+cannot be made to work over a distance of more than 200 yards.
+
+A system based on more practical lines was that invented and demonstrated
+by Mr. Hans Knudsen, but the apparatus which he employed for receiving has
+been discarded in wireless work, as it is not suitable for working with the
+highly-tuned systems in use at the present time.
+
+Knudsen's transmitter, a diagrammatic representation of which is given in
+Fig. 3, consists of a flat table to which a horizontal to-and-fro motion is
+given by means of a clockwork motor. Upon this table is fastened a
+photographic plate which has been prepared in the following manner. The
+plate upon which the photograph is to be taken has the gelatine film from
+three to four times thicker than that commonly used in photography. In the
+camera, between the lens and this plate, a single line screen is
+interposed, which has the effect of breaking the picture up into parallel
+lines. Upon the plate being developed and before it is {10} [Illustration]
+completely dry, it is sprinkled over with fine iron dust. With this type of
+plate the transparent parts dry much quicker than the shaded or dark parts,
+and on the iron dust being sprinkled over the plate it adheres to the
+darker portions of the film to a greater extent than it does to the lighter
+portions; a picture partly composed of iron dust is thus obtained. A steel
+point attached to a flat spring rests upon this plate and is made to travel
+at right angles to the motion of the table. As the picture is partly
+composed of iron dust, and as the steel needle is fastened to a delicate
+spring it is evident that as the plate passes to and fro under the needle,
+both the spring and needle are set in a state of vibration. This vibrating
+spring makes {11} and breaks the battery circuit of a spark coil, which in
+turn sets up sparking in the spark-gap of the wireless apparatus.
+
+The receiver consists of a similar table to that used for transmitting, and
+carries a glass plate that has been smoked upon one side. A similar spring
+and needle is placed over this plate, but is actuated by means of a small
+electro-magnet in circuit with a battery and a sensitive coherer. As the
+coherer makes and breaks the battery circuit by means of the intermittent
+waves sent out from the transmitting aerial, the needle is made to vibrate
+upon the smoked glass plate in unison with the needle at the transmitting
+end. Scratches are made upon the smoked plate, and these reproduce the
+picture on the original plate. A print can be taken from this scratched
+plate in a similar manner to an ordinary photographic negative.
+
+The two tables are synchronised in the following manner. Every time the
+transmitting table is about to start its forward stroke a powerful spark is
+produced at the spark-gap. The waves set up by this spark operate an
+ordinary metal filings coherer at the receiving end which completes the
+circuit of an electro-magnet. The armature of this magnet on being
+attracted immediately releases the motor used for driving, allowing it to
+operate the table. The time taken to transmit a photograph, quarter-plate
+size, is about fifteen minutes. {12} Although very ingenious this system
+would not be practicable, as besides speed the quality of the received
+pictures is a great factor, especially where they are required for
+reproduction purposes. The results from the above apparatus are said to be
+very crude, as with the method used to prepare the photographs no very
+small detail could be transmitted.
+
+       *       *       *       *       *
+
+
+{13}
+
+CHAPTER II
+
+TRANSMITTING APPARATUS
+
+Let us now consider the requirements necessary for transmitting photographs
+by means of the wireless apparatus in use at the present time.
+
+[Illustration: FIG. 4.]
+
+The connections for an experimental syntonic wireless transmitting station
+are shown in the diagram Fig. 4. A is the aerial; T, the inductance; E,
+earth; L, hot-wire ammeter. The closed oscillatory circuit consists of an
+inductance F, spark-gap G, and a block condenser C. H is a spark-coil for
+supplying the energy, the secondary J being connected to the spark-gap. A
+{14} mercury break N and a battery B are placed in the primary circuit of
+the coil. The Morse key K is for completing the battery circuit for
+signalling purposes. When the key K is depressed, the battery circuit is
+completed, and a spark passes between the balls of the spark-gap G
+producing oscillations in the closed circuit, which are transposed to the
+aerial circuit by induction. For signalling purposes it is only necessary
+for the operator by means of the key K to send out a long or short train of
+waves in some pre-arranged order, to enable the operator at the receiving
+station to understand the message that is being transmitted.
+
+If a photograph could be prepared in such a manner that it would serve the
+purpose of the key K, and could so arrange matters that a minute portion of
+the photograph could be transmitted separately but in succession, and that
+each portion of the photograph having the same density could be given the
+same signal, then it would only be necessary to have apparatus at the
+receiving station capable of arranging the signals in proper sequence (each
+signal recorded being the same size and having the same density as the
+transmitted portion of the photograph) in order to receive a facsimile of
+the picture transmitted.
+
+The following method of preparing the photograph[3] is one that has been
+adopted in several {15} systems of photo-telegraphy, and is the only one at
+all suitable for wireless transmission. The photograph or picture which is
+to be transmitted is fastened out perfectly flat upon a copying-board. A
+strong light is placed on either side of this copying board, and is
+concentrated upon the picture by means of reflectors. The camera which is
+used for copying has a single line screen interposed between the lens and
+sensitised plate, and the effect of this screen is to break the picture up
+into parallel lines. Thus a white portion of the photograph would consist
+of very narrow lines wide apart, while the dark portion would be made up of
+wide lines close together; a black part would appear solid and show no
+lines at all. From this line negative it will be necessary to take off a
+print upon a specially prepared sheet of metal. This consists of a sheet of
+thick lead- or tinfoil, coated upon one side with a thin film of glue to
+which bichromate of potash has been added; the bichromate possessing the
+property of rendering the glue waterproof when acted upon by light. The
+print can be taken off by artificial light (arc lamps being generally
+used), but the exact time to allow for printing can only be found by
+experiment, as it varies considerably according to the thickness of the
+film. The printing finished, the metal print is washed under running water,
+when all those parts not acted upon by light, _i.e._ the parts between the
+lines, are {16} washed away, leaving the bare metal. We have now an image
+composed of numerous bands of insulating material (each band varying in
+width according to the density of the photograph at any point from which it
+is prepared) attached to a metal base, so that each band of insulating
+material is separated by a band of conducting material. It is, of course,
+obvious that the lines on the print cannot be wider apart, centre to
+centre, than the lines of the screen used in preparing it. A good screen to
+use is one having 50 lines to the inch, but one is perhaps more suitable
+for experimental work a little coarser, say 35 lines to the inch. To use a
+screen having 50 or more lines to the inch, the transmitting apparatus, as
+will be evident later on, will require to be very nearly perfect.
+
+[Illustration: FIG. 5.]
+
+Before proceeding further it will perhaps be as well to make an experiment.
+If we take one of the metal prints or, more simple, draw a sketch in
+insulating ink upon a sheet of metal A, Fig. 5, and connect a battery B and
+the galvanometer D as shown, we shall find on drawing the free end of the
+wire across the metal plate that all the time the wire is in contact with
+the lines of insulating material the needle of the galvanometer will remain
+{17} at zero, but where it is in contact with the metal plate the needle is
+deflected.
+
+From this experiment it will be seen that we have in our metal line print,
+which consists of alternate lines of insulating and conducting material, a
+method by which an electric circuit can be very easily made and broken. It
+is, of course, necessary to have some arrangement whereby the whole of the
+surface of the metal print is utilised for this purpose to the best
+advantage. One type of transmitting machine used for this purpose is
+represented by the diagram, Fig. 6. The cylinder A is fastened to the steel
+shaft B, which runs in the two bearings D and D', the bearing D' having an
+internal thread corresponding to that on the shaft. The stylus in this
+class of machine is a fixture, the cylinder being given a lateral as well
+as a revolving movement. As it is impossible to use a rigid drive, a
+flexible coupling F is employed between the shaft B and the motor.
+
+[Illustration: FIG. 6.]
+
+Another type of machine is shown in Fig. 7. The drum in this case is
+stationary, the table T moving laterally by reason of the screwed shaft
+{18} [Illustration] and half nut F. The table, shown separate in Fig. 8,
+carries a stiff brass spring A, to which is attached a holder B made to
+take a hardened steel point. The holder is provided with a set screw P for
+securing the steel point Z. The spring and needle are insulated from the
+rest of the machine, as shown in the drawing. In working, the metal print
+is wrapped tightly round the cylinder of the machine, the glue image being,
+of course, uppermost. To fasten the print a little seccotine should be
+applied to one edge, and the joint carefully smoothed down with the
+fingers. [Illustration] If there is any tendency on the part of the print
+to slip round on the drum, a couple of small spring clips placed over the
+ends of the drum will act as a preventive. It is necessary to place the
+print upon the drum in such a manner that the stylus draws away from the
+edge of the lap and not towards it, and the metal prints should be of such
+a size that when placed round the drum of the {19} machine a lap of about
+3/16ths of an inch is allowed.
+
+[Illustration: FIG. 9.]
+
+The steel point Z (ordinary gramophone needles may be used and will be
+found to answer the purpose admirably) is made to press lightly upon the
+metal print, and while the pressure should be sufficient to make good
+electrical contact, it should not be sufficient to cause the needle to
+scratch the surface of the foil. The pressure is regulated by means of the
+milled nut H. The electrical connections are given in Fig. 9. One wire from
+the battery M is taken to the terminal T, and the other wires from M and F
+lead to the relay R. The current flows from the battery M through the
+spring Y, through the drum and metal print, the stylus Z, spring A, down to
+the relay R, and from R back to the battery M. As the drum carrying the
+single line half-tone print is revolved, the stylus, by reason of the
+lateral movement given to the table or cylinder as the case may be, will
+trace a spiral path over the entire surface of the print. As the stylus
+traces over a conducting strip the circuit is completed, and the tongue of
+the relay R is attracted, making contact with the stop S. {20} On passing
+over a strip of insulation the circuit is broken and the tongue of the
+relay R returns to its normal position.
+
+As already stated, the conducting and insulating bands on the print vary in
+width according to the density of the photograph from which it is prepared,
+so that the length of time that the tongue of the relay R is held against
+the stop S, is in proportion to the width of the conducting strip which is
+passing under the stylus at any instant. The function of the transmitter is
+therefore to send to the relay R an intermittent current of varying
+duration.
+
+The two photographs Figs. 10 and 10_a_ are of a machine designed and used
+by the writer in his experiments. In this machine the drum is 3.5 inches
+long and 1.5 inches in diameter. The lead screw has 30 threads to the inch,
+and the reduction between it and the drum is 3:1, so that the table has a
+movement of 1/90th inch per revolution of the drum.
+
+From the brief description of the various types of machines that have been
+given it will be apparent that in the design of the machine proper there is
+nothing very complicated, although the addition of the driving and
+synchronising apparatus complicates matters rather considerably. The
+questions of driving and synchronising the machines at the two stations is
+fully dealt with in Chapter IV.
+
+[Illustration: FIG. 10a.]
+
+[Illustration: FIG. 10b. Enlarged view of an image broken up by a cross
+screen.]
+
+{21} Although the design of the machines is rather simple great attention
+must be paid both to accuracy of construction and accuracy of working, and
+this applies, not only to the machines (whether for transmitting or
+receiving) but for all the various pieces of apparatus that are used. Too
+much care cannot be bestowed upon this point, as in the wireless
+transmission of photographs there is a large number of instruments all
+requiring careful adjustment, and which have to work together in perfect
+unison at a high speed.
+
+The machine shown in Figs. 10 and 10_a_ was designed and used by the writer
+solely for experimental work. It will be noticed in the description given
+in the appendix of the method of preparing the metal prints that a 5" × 4"
+camera is recommended, while the machine, Fig. 10, is designed to take a
+print procured from a quarter-plate negative. This size of drum was adopted
+for several reasons, and although it will be found quite large enough for
+general experimental work the writer has come to the conclusion that for
+practical commercial work a drum to take a print 5" × 4" will give better
+results.
+
+In making a negative of a picture that is required for reproduction
+purposes, the line screen in the camera is replaced by a "cross screen,"
+_i.e._ two single line screens placed with their lines at an angle of 90°
+to one another, and this breaks the {22} image up into small squares
+instead of lines. By looking at any ordinary newspaper or book illustration
+through a powerful magnifying glass the effects of a cross screen will
+readily be seen. With a cross screen a certain amount of detail is
+necessarily lost, but with a single line screen the amount lost is much
+greater. If there is any very small detail in the picture most of this
+would be lost in a coarse screen, hence the necessity of employing as fine
+a line screen as practicable in order to get as much detail in as possible.
+It is mainly on this account that a 5" × 4" print is recommended, as, if
+fairly bold subjects are used for copying, the small detail (this is, of
+course, a very vague and indefinable term) will not be too fine, and the
+time required for transmitting reasonable. For obvious reasons it is a
+great advantage to put the print under pressure to cause the glue image to
+sink into the soft metal base and leave a perfectly flat and smooth
+surface. It is essential that the bands on the print lie along the axis of
+the cylinder, so that the stylus traces its path across them, and not with
+them.
+
+We have now an arrangement that is capable of taking the place of the key
+K, Fig. 4, and the diagram, Fig. 11, gives the connections for the complete
+transmitter. A is the aerial, E earth, T inductance, L ammeter. The closed
+oscillatory circuit consists of a spark-gap G, inductance F, {23}
+[Illustration] and a condenser C. The secondary J of the coil H is
+connected to the spark-gap, and the primary P is in circuit with the
+mercury break N, the battery B, and the local contacts of the relay R. The
+action is as follows. When contact is made between the stylus Z and the
+drum V by means of the conducting bands on the line print, the circuit of
+the relay R and the battery M is completed. The closing of the local
+circuit of the relay R actuates the second relay R', allowing the primary
+circuit of the coil H to be closed. As soon as the primary circuit of the
+coil is completed sparks pass between the electrodes of the spark-gap G,
+causing waves to radiate from the aerial. The duration of the wave-trains
+radiated depends upon the duration of contact made by the relays {24} R and
+R', and this in turn depends upon the width of the conducting strip that is
+passing under the stylus. The battery M should be about 4 volts, and the
+battery D about 2 volts. The two-way switch X is connected up so that the
+relay R' can be thrown out and the key K switched in for ordinary
+signalling purposes. If any sparking takes place at the point of the
+stylus, a small condenser C' (about 1 microfarad capacity) should be
+connected as shown. In the present instance the condenser should be used
+more as a preventive than as a cure, as in all probability the voltage from
+M will not be sufficient to cause destructive (if any) sparking; but, as
+most wireless workers know, anything in the nature of a spark occurring in
+the neighbourhood of a detector (this, of course, only applies when the
+receiving apparatus is placed in close proximity to the transmitter) is
+liable to destroy the adjustment.
+
+In transmitting over ordinary conductors where the initial voltage is
+fairly high and the self-induction of the circuit very great, the use of
+the condenser will be found to be absolutely essential. It has also been
+noted that the angle which the stylus presents to the drum has a marked
+effect upon the sparking, an angle of about 60° being found to give very
+good results.
+
+If the size of the single line print used is 5 inches by 4 inches, and a
+screen having 50 lines {25} to the inch is used for preparing it, then the
+stylus will have to make 250 contacts during one revolution of the drum.
+Assuming the drum to make one revolution in three seconds, then the time
+taken to transmit the complete photograph can be found from the equation T
+= w × t × s, where w is the width of the print, t the travel of the stylus
+during one revolution of the drum, and s the time required for one
+revolution of the drum. In the present instance this will be T = 4 × 90 × 3
+= 1080 seconds = 18 minutes. The number of contacts made by the stylus per
+minute is 5000, and in working at this speed the first difficulty is
+encountered in the use of the two relays. The relay R is lightly built, and
+capable of working at a fairly high speed, but R' is a heavier pattern, and
+consequently works at a slightly lower rate. This relay must necessarily be
+heavier, as more substantial contacts are needed in order to pass the heavy
+current taken by the spark-coil.
+
+Relays sensitive and accurate enough to work at this speed will in all
+probability be beyond the reach of the majority of workers, but there are
+several types of relays on the market very reasonable in price that will
+answer very well for experimental work, although the speed of working will
+no doubt be slower.
+
+For the best results the duration of the wave-trains sent out should be of
+the same duration as {26} the contact made by R, and therefore equal to the
+time taken by the stylus to trace over a conducting strip; but if the
+duration of the contact made by R is t, then that made by R' and
+consequently the duration of the groups of wave-trains would be t - v where
+v equals the extra time required by R' to complete its local circuit. The
+difference in time made by the two relays, although very slight, will be
+found to affect very considerably the quality of the received pictures.
+Renewing the platinum contacts is also a great expense, as they are soon
+burnt out where a heavy current is passed. If the distance experimented
+over is short so that the power required to operate the spark-coil is not
+very heavy, one relay will be sufficient providing the contacts are massive
+enough to carry the current safely. It is useless to expect any of the
+ordinary relays in general use to work satisfactorily at such a high speed,
+and in order to compensate for this we must either increase the time of
+transmitting, or, as already suggested, make use of a coarser line screen
+in preparing the photographs.
+
+For reasons already explained, all points of make and break should be
+shunted by a condenser. The effective working speed of an ordinary type of
+relay may be anything from 1000 to 2500 dots a minute, depending upon
+accuracy of design and construction.
+
+In the wireless transmission of photographs it {27} is absolutely essential
+to use some form of rotary spark-gap, as where sparks are passed in rapid
+succession the ordinary type of gap is worse than useless. When a spark
+passes between the electrodes of an ordinary spark-gap, Fig. 12, we find
+that for a fraction of a second after the first spark has passed, the
+normally high resistance of the gap has been lowered to less than one ohm.
+If the column of hot gas which constitutes the spark is not instantly
+dispersed, but remains between the electrodes, it will provide an easy path
+for any further discharges, and if sparks are passed at all rapidly, what
+was at first a disruptive and oscillatory discharge will degenerate into a
+hot, non-oscillatory arc.[4]
+
+[Illustration: FIG. 12.]
+
+Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are
+known as "synchronous" and "non-synchronous" gaps respectively. In the
+synchronous gap the cog-wheel is mounted on the shaft of the alternator,
+and a cog comes opposite the fixed electrode when the maximum of potential
+is reached in the condenser, thus ensuring a discharge at every alternation
+of current. With this type of gap a spark of pure tone is obtained which
+{28} [Illustration] [Illustration] is of great value where the signals are
+received by means of a telephone, but where the signals are to be
+mechanically recorded the tone of the spark is of little consequence. In a
+non-synchronous gap a separate motor is used for driving the toothed wheel,
+and can either be mounted on the motor shaft or driven by means of a band,
+there being no regard given to synchronism with the alternator. The fixed
+electrode is best made long enough to cover about two of the teeth, as this
+ensures regular sparking and a uniform sparking distance; the {29} spark
+length is double the length of the spark-gap. The toothed wheel should
+revolve at a high speed, anything from 5000 to 8000 revolutions per minute,
+or even more being required. The shaft of the toothed wheel is preferably
+mounted in ball-bearings.
+
+Owing to the large number of sparks that are required per minute in order
+to transmit a photograph at even an ordinary speed, it is necessary that
+the contact breaker be capable of working at a very high speed indeed. The
+best break to use is what is known as a "mercury jet" interrupter, the
+frequency of the interruptions being in some cases as high as 70,000 per
+second. No description of these breaks will be given, as the working of
+them is generally well understood.
+
+In some cases an alternator is used in place of the battery B, Fig. 4, and
+when this is done the break M can be dispensed with. In larger stations the
+coil H is replaced with a special transformer.
+
+The writer has designed an improved relay which will respond to currents
+lasting only 1/100th part of a second, and capable of dealing with rather
+large currents in the local circuit.[5] This relay has not yet been tried,
+but if it is successful the two relays R and R' can be dispensed with, and
+the result will be more accurate and effective transmission.
+
+{30}
+
+[Illustration: FIG. 15.]
+
+The connections for a complete experimental station, transmitting and
+receiving apparatus combined, are given in Fig. 15. The terminals W, W are
+for connecting to the photo-telegraphic receiving apparatus Q, being a
+double pole two-way switch for throwing either the transmitting or
+receiving apparatus in circuit. There is another system of transmitting
+devised by Professor Korn, which employs an entirely different method from
+the foregoing. By using the apparatus just described, the waves generated
+are what are known as "damped waves," and by using these damped waves,
+tuning, which is so essential to good commercial working, can be made to
+reach a fairly high degree of efficiency. {31}
+
+The question of damped _versus_ undamped waves is a somewhat burning one,
+and no attempt will be made here to deal with the merits or demerits of the
+claims made for the respective systems. A series of articles describing the
+production of undamped waves and their efficiency in working compared with
+damped waves will be found in the _Wireless World_, Nos. 3 and 4, 1913, and
+are well worth reading by any one interested in the subject.
+
+[Illustration: FIG. 16.]
+
+A diagrammatic representation of the apparatus as arranged by Professor
+Korn is given in Fig. 16. The undamped or "continuous" waves are generated
+by means of a high-frequency alternator or Poulsen arc. In Fig. 16, X is
+the generator, F inductance, C condenser; the aerial inductance T is
+connected by the aerial A and earth E. By this means the waves are tuned to
+a certain period. {32} A metal print, similar to what has already been
+described, is wrapped round the drum D of the machine, and when the stylus
+Z traces over an insulating strip the waves generated are in tune with the
+receiving station, but when it traces over a conducting strip, a portion of
+the inductance T is short-circuited, the period of the oscillations is
+altered, and the two stations are thrown out of tune.
+
+The receiving station is provided with an aperiodic circuit, which consists
+of an inductance F', condenser C', and a thermodetector N. A string
+galvanometer H (described in Chapter III.), and the self-induction coils B,
+B' are connected as shown, the coils B, B' preventing the high-frequency
+currents, which change their direction, from flowing through the
+galvanometer. The manner in which the string galvanometer is arranged to
+reproduce a transmitted picture is shown in Fig. 24.
+
+The connections adopted by the Poulsen Company for photographically
+recording wireless messages are given in Fig. 17, a string galvanometer of
+the Einthoven type being used. The two self-induction coils S and S' are in
+circuit with the detector D and the galvanometer G. The condenser C'
+prevents the continuous current produced by the detector from flowing
+through the high frequency circuit; P is the primary of the aerial {33}
+inductance and F the secondary. The method of transmitting adopted by
+Professor Korn appears to be a simple and reliable arrangement, provided
+that an equally reliable method of producing the undamped waves can be
+found. Owing to the absence of mechanical inertia it should be capable of
+working at a good speed, while the absence of a number of pieces of
+delicate apparatus all requiring careful adjustment add greatly to its
+reliability.
+
+[Illustration: FIG. 17.]
+
+In any spark system with a properly designed aerial a coil taking ten
+amperes is capable of transmitting signals over a distance of thirty to
+fifty miles, but where the number of interruptions of the break required
+per second is very high, as in radio-photography, it must be remembered
+that a much higher voltage is needed to drive the requisite amount of
+current through the primary winding of the coil than would be the case if
+the interruptions were slower. It is possible to use platinum {34} contacts
+for the relays, for currents up to ten amperes, but for heavier currents
+than this some arrangement where contact is made with mercury will be found
+to be more economical and reliable.
+
+In the transmitter already described and given in Fig. 11, the best results
+would be obtained by finding the speed at which the relay R' works best,
+and regulating the number of contacts made by the stylus accordingly.
+
+The method employed by De' Bernochi (see Chapter I.) of varying the
+intensity of a beam of light by passing it through a photographic film,
+which in turn alters the resistance of a selenium cell, has been very
+successfully employed in at least one system of photo-telegraphy. Its
+application has also been suggested for wireless transmission, and although
+with any system using continuous waves this would not be very difficult, it
+could hardly be adapted to work with the ordinary spark system. The
+apparatus for receiving from this type of transmitter would, on the other
+hand, necessarily be more elaborate than the methods that are described in
+the next chapter, and as far as the writer's experience goes, experiments
+along these lines would not prove very profitable, as simplicity is the
+keynote of success in any radio-photographic system.
+
+It has been suggested that in order to decrease the time of transmission a
+cylinder capable of {35} taking a print 7 inches by 5 inches be employed,
+the print being prepared from rather a coarse line screen--say 35 to the
+inch--and a traverse of about 1/50 inch given to the stylus, thus reducing
+the time of transmission to about twelve minutes. It is questionable,
+however, whether the increase in speed would compensate for the loss of
+detail, as only very bold subjects could be transmitted. As already pointed
+out, wireless transmission would only be employed for fairly long
+distances, and the extra time and expense required to receive a fairly good
+detailed picture is negligible when compared with the enormous time it
+would take to receive the original photograph by any ordinary means of
+transit.
+
+The public much prefer to have passable pictorial illustrations of current
+events than wait several days for a more perfect picture--the original, and
+the advantage of any newspaper being able to publish photographs several
+days before its rivals is obvious. There can also be no doubt but that a
+system of radio-photography, if fairly reliable and capable of working over
+a distance of say thirty miles, would be of great military use for
+transmitting maps and written matter with a great saving of time and even
+life. Written matter could be transmitted with even greater safety than
+messages which are sent in the ordinary way in Morse Code, as the signals
+received in the receiver {36} of an hostile installation would be but a
+meaningless jumble of sounds, and even were they possessed of
+radio-photographic apparatus the received message would be unintelligible,
+unless they knew the exact speed at which the machines were running and
+could synchronise accurately.
+
+       *       *       *       *       *
+
+
+{37}
+
+CHAPTER III
+
+RECEIVING APPARATUS
+
+There are only two methods available at present for receiving the
+photographs, and both have been used in ordinary photo-telegraphic work
+with great success. They have disadvantages when applied to wireless work,
+however, but these will no doubt be overcome with future improvements. The
+two methods are (1) by means of an ordinary photographic process, and (2)
+by means of an electrolytic receiver.
+
+In several photo-telegraphic systems the machine used for transmitting has
+the cylinder twice the size of the receiving cylinder, thus making the area
+of the received picture one-quarter the area of the picture transmitted.
+The extra quality of the received picture does not compensate for the
+disadvantage of having to provide two machines at each station, and in the
+writer's opinion results, quite good enough for all practical purposes, can
+be obtained by using a moderate size cylinder so that one machine answers
+for both transmitting {38} and receiving, and using as fine a line screen
+as possible for preparing the photographs.
+
+[Illustration: FIG. 18.]
+
+The writer, when first experimenting in photo-telegraphy, endeavoured to
+make the receiving apparatus "self-contained," and one idea which was
+worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and is
+placed just within the focus of a lens which has a focal length of 3/4
+inch. When a source of light is placed at some point between a lens and its
+principal focus, the light rays are not converged, but are transmitted in a
+parallel beam the same size as the lens. It has been found that this
+arrangement gives a sharper line on the drum than would be the case were
+the light focussed direct upon the hole in the cone A. An enlarged drawing
+of the cone is given in Fig. 19. The hole in the tip of the cone A is a
+bare 1/90 inch in diameter--the size of this hole depends upon the travel
+per revolution of the drum or table of the machine used--and in working,
+the cone is run as close as possible to the {39} drum without being in
+actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and the
+armature is made as light as possible. The spring to which the armature is
+attached should be of such a length that its natural period of vibration is
+equal to the number of contacts made by the transmitting stylus. The spring
+must be stiff enough to bring the armature back with a fairly crisp
+movement. The spring and armature is shown separate in Fig. 20.
+
+[Illustration: FIG. 19.]
+
+[Illustration: FIG. 20.]
+
+The shutter C is about 1/4 inch square and made from thin aluminium. The
+hole in the centre is 1/16 × 1/8 inch, and the movement of the armature is
+limited to about 3/32 inch. In all arrangements of this kind there is a
+tendency for the armature spring to vibrate, as it were, sinusoidally, if
+the coil is magnetised and demagnetised at a higher rate than the natural
+period of vibration of the spring. {40} This causes an irregularity in the
+rate of the vibrations which affects the received image very considerably.
+A photographic film is wrapped round the drum of the machine, being
+fastened by means of a little celluloid cement smeared along one edge.
+
+This device, although it will work well over artificial conductors, is not
+suitable for wireless work, as it is too coarse in its action; it can be
+made sensitive enough to work at a speed of 1000 to 1500 contacts per
+minute, with a current of .5 milliampere. It is impossible to obtain a
+current of this magnitude from the majority of the detectors in use, so
+that if any attempt is made to use this device for radio-photography it
+will be necessary to employ a Marconi coherer (filings), as this is
+practically the only coherer from which so large a current can be obtained.
+
+There have been many attempts made to receive with an ordinary filings
+coherer, but as was pointed out in Chapter I. these have now been discarded
+in serious wireless work, being only used in small amateur stations or
+experimental sets. As the reasons for this are well known to the majority
+of wireless workers there is no need to enumerate them here.
+
+A method whereby a filings coherer can be decohered, the act of decohering
+closing a local circuit which contains the photographic {41} receiving
+apparatus, is given in the diagram Fig. 21.
+
+[Illustration: FIG. 21.]
+
+In the figure, the coherer C is fixed in rigid supports, one support being
+provided with a platinum pin F. To the coherer is connected the sensitive
+electro-magnet M, which becomes magnetised as soon as the incoming waves
+act upon the coherer. To the armature B is attached a light aluminium arm
+S, pivoted at K, and carrying at the other end the striker G, which is
+fitted with a platinum contact. When the armature B is attracted the
+coherer is decohered by the force of the impact between the contacts F and
+G. To prevent damage to the coherer the force of the blow is taken off by
+the ability of the striker to work back through a hole in the arm S, the
+spring {42} N keeping it normally in a fixed position. T and P are
+adjusting screws, and the terminals J are for connecting to the receiving
+apparatus. With this arrangement a very short wave-train causes only one
+tap of the contacts, so that only one mark is registered on the receiving
+drum for every contact made on the transmitter.
+
+[Illustration: FIG. 22.]
+
+The drawing, Fig. 22, gives a diagrammatic representation of apparatus
+arranged for another photographic method of receiving. The machine shown in
+Fig. 6 is used in this case. A is the aerial, E earth, P primary of
+oscillation-transformer, S secondary of transformer, C variable condenser,
+C' block condenser, D detector, X two-way switch, T telephone.
+
+A De' Arsonval galvanometer H is also connected to the switch X, so that
+either the telephone or the galvanometer can be switched in. The {43}
+galvanometer can be made sensitive enough to work with a current as small
+as 10^{-7} of an ampere, with a period of about 1/150th of a second. The
+screen J has a small hole about 1/8 inch diameter drilled in the centre.
+Under the influence of the brief currents which pass through the detector
+every time a group of waves is received, the mirror of the galvanometer
+swings to-and-fro in front of the screen J, and allows the light reflected
+from the source of light M to pass through the aperture in the screen, on
+to the lens N.
+
+Round the drum V of the machine is wrapped a sensitive photographic film,
+and this records the movements of the mirror which correspond to the
+contacts on the half-tone print used in transmitting. Every time current
+passes through the galvanometer, the light that is received from M,[6]
+passes through the aperture in the screen J, and is focussed by the lens N
+to a point upon the revolving film. As soon as the current ceases, the
+mirror swings back to its original position, and the film is again in
+darkness. Upon being developed a photograph, similar to the negative used
+for preparing the metal print is obtained. If desired the apparatus can be
+so arranged that the received picture is a positive instead of a negative.
+
+{44}
+
+The detector used should be a Lodge wheel-coherer or a Marconi
+valve-receiver, as these are the only detectors that can be used with a
+recording instrument. If the swing of the galvanometer mirror is too great,
+a small battery with a regulating resistance can be inserted in order to
+limit the movement of the mirror to a very short range; the current of
+course flowing in an opposite direction to the current flowing through the
+coherer.
+
+In this, as in all other methods of receiving, the results obtained depend
+upon the fineness of the line screen used in preparing the metal prints;
+and as already shown the fineness of the screen that can be used is
+dependent upon the mechanical efficiency of the entire apparatus.
+
+Another system, and one that has been tried as a possible means of
+recording wireless messages, is as follows. The wireless arrangements
+consist of apparatus similar to that shown in Fig. 22, but instead of a
+Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is
+substituted for the reflecting galvanometer. The Einthoven galvanometer
+consists of a very powerful electro-magnet, the pole pieces of which
+converge almost to points. A very fine silvered quartz thread is stretched
+between the pole pieces, as shown in Fig. 23, the tension being adjustable.
+The period of swing is about 1/250th of a second. A hole is bored through
+the poles, and one of them is fitted {45} [Illustration] with a sliding
+tube which carries a short focus lens N. The light from M passes through
+the magnets, and a magnified image of the quartz thread is thrown upon the
+ebonite screen J. This screen is provided with a fine slit, and when the
+galvanometer is at rest the shadow of the thread just covers the slit in
+the screen and prevents any light from M reaching the photographic film.
+Upon signals being received the shadow of the thread moves to one side for
+a long or short period, uncovering the slit, and allowing light to pass
+through. The lens R concentrates the collected light to a point upon the
+revolving film. The connections for the complete receiver are given in Fig.
+24.
+
+The modified form of the Einthoven galvanometer, as arranged by Professor
+Korn for use with his selenium machines for photo-telegraphy over ordinary
+land lines, consists of two fine silver wires which are displaced in a
+lateral direction between the pole pieces when traversed by a current; the
+current passing through both wires in the same {46} direction. A small
+shutter of aluminium foil is attached to the wires at the optical centre.
+The silver wires used are 1/1000 inch in diameter, with a natural period of
+about 1/120th of a second; the length of wires free to swing being usually
+about 5 cm.
+
+[Illustration: FIG. 24.]
+
+The period of the wires depends to a great extent upon their length and
+diameter, and also upon their tension. By using short fine wires the period
+can be made much smaller, but a greater current is required to produce a
+similar displacement. Where the current available, as in wireless
+telegraphy, is very small, and a definite displacement of the wires is
+required, it is at once apparent that with wires of a given diameter there
+is a limit to their length and therefore to the period. Finer wires can be
+used, but here again there is a practical limit to their fineness, although
+galvanometers have been constructed with a single silvered quartz thread
+1/12000th of an inch diameter, which, when placed in a powerful field, will
+give a good displacement with a current as small as 10^{-8} ampere. {47}
+
+With the apparatus arranged by the Poulsen Company, given in the diagram,
+Fig. 17, for photographically recording wireless signals, the current
+required to operate the galvanometer for signals transmitted at the rate of
+1500 a minute is 1 × 10^{-6} ampere, while for signals up to 2500 a minute
+a current about 5 × 10^{-6} ampere is necessary.
+
+Another very sensitive instrument, employed by M. Belin, and known as
+Blondel's oscillograph, consists of two fine wires stretched between the
+poles of a powerful electro-magnet, a small and very light mirror being
+attached to the centre of the wires. The current passes down one wire and
+up the other, and the wires, together with the mirror, are twisted to a
+degree depending upon the strength of the received current. In order to
+render the instrument dead-beat the moving parts are arranged to work in
+oil. The light reflected from the mirror is made use of in a manner similar
+to that shown in Fig. 22.
+
+In all photographic methods of receiving, the apparatus must be enclosed in
+some way to prevent any extraneous light from reaching the film, or better
+still placed in a room lighted only by means of a ruby light.
+
+The following method is given more as a suggestion than anything else, as I
+do not think it has been tried for wireless receiving, although it is
+stated to have given some good results over {48} ordinary land lines. It is
+the invention of Charbonelle, a French engineer, and is quite an original
+idea. His method consists of placing a sheet of carbon paper between two
+sheets of thin white paper, and wrapping the whole tightly round the drum
+of the machine. A hardened steel point is fastened to the diaphragm of a
+telephone receiver, and this receiver is placed so that the steel point
+presses against the sheets of paper. As the diaphragm and steel point
+vibrates under the influence of the received currents marks are made by the
+carbon sheet on the bottom paper.
+
+Over a line where a fair amount of current is available at the receiver,
+the diaphragm would have sufficient movement to mark the paper, but the
+movement would be very small with the current received from a detector.
+This difficulty could no doubt be overcome to a certain extent by making a
+special telephone receiver, with a large and very flexible diaphragm, and
+wound for a very high resistance. The movement of an ordinary telephone
+diaphragm for a barely audible sound is, measured at the centre, about
+10^{-6} of a c.m. With a unit current the movement at the centre is about
+1/700th of an inch. Greater movement of the diaphragm could be obtained by
+connecting a _Telephone relay_ to the detector, and using the magnified
+current from the relay to operate the telephone. {49}
+
+[Illustration: FIG. 25.]
+
+The telephone relay consists of a microphone C, Fig. 25, formed of the two
+pieces of osmium iridium alloy. The contact is separated to a minute degree
+partly by the action of the local current from F, which flows through it
+and also through the winding W of the two magnet coils. The local current
+from F assists in forming the microphone by rendering the space between the
+contacts conductive. The vibrating reed P is fastened to the metal frame
+(not shown) which carries a micrometer screw by which the distance between
+the contacts can be accurately regulated. It will be seen from Fig. 25 that
+the local circuit consists of a battery F (about 1.5 volts), the microphone
+contacts C, the windings W, milliampere meter B, and the terminals T, for
+connecting to the galvanometer or telephone, all in {50} series. On the top
+of the magnet cores N, S is a smaller magnet D, wound with fine wire for a
+resistance of about 4935 ohms, the free ends of the coils being connected
+to the detector terminals. The working is as follows. Supposing the current
+from the detector flows through D in such a way that its magnetism is
+increased, the reed P will be attracted, the contacts opened, and their
+resistance increased. It will be seen that the current from F is passed
+through the coils W, in such a way as to increase the magnetism of the
+permanent magnet, so that any opening of the microphone contact increases
+their resistance, causes the current to fall, and weakens the magnets to
+such an extent that the reed P can spring back to its normal position. On
+the other hand, if the detector current flows through D in such a direction
+as to decrease the magnetism in the permanent magnets, the reed P will rise
+and make better contact owing to the removal of the force opposing the
+stiffness of the reed. Owing to the decrease in the resistance of the
+microphone, the strength of the local current will be increased, the
+magnets strengthened, and the reed P will be pulled back to its original
+position. This relay gives a greatly magnified current when properly
+adjusted, the current being easily increased from 10^{-4} to 10^{-2}
+amperes. It is also very sensitive, but needs careful adjustment in order
+that the best results may {51} be obtained. A greater range of
+magnification can be obtained by placing two or more relays in series.
+
+[Illustration: FIG. 26.]
+
+A very sensitive receiver designed by the writer is given in the figures 26
+and 27. To the centre of a telephone diaphragm is fastened a light steel
+point P, and the movement of this point is communicated to the aluminium
+arm D, which is pivoted at C. As will be seen the telephone receiver is of
+special construction, it containing only one coil and therefore only one
+core; by this means the movement of the diaphragm is centralised. The coil
+is wound for a resistance of about 200 ohms, and the diaphragm should be
+fairly thin but very resillient.
+
+[Illustration: FIG. 27.]
+
+To the free end of D is fastened the mirror T, made from thin diaphragm
+glass about 1-1/2 centimetres diameter, and having a focal length of 40
+inches. Light from the lamp L is transmitted by the lens N in a parallel
+beam to the mirror which {52} concentrates it to a point upon a hole
+1/100th of an inch in diameter in the screen J. As the telephone diaphragm
+vibrates under the influence of the received signals the arm, and
+consequently the mirror, vibrates also, and the hole in the screen J is
+constantly being covered and uncovered by the spot of light. It will be
+seen from Fig. 27 that the ratio between the centre of the mirror and the
+pivot C, and C and the steel point P is 10:1, so that if a movement of
+1/20000th of an inch is obtained at the centre of the diaphragm the mirror
+will move 1/2000th of an inch; and as the focal length of the mirror is 40
+inches a movement of 1/50th inch is given to the spot of light.
+
+This receiver is capable of working at a fairly high speed, as the inertia
+of the moving parts is practically negligible; the weight of the arm and
+mirror being less than 20 grains. The hole in the screen is made slightly
+less in diameter than the traverse of the revolving cylinder, the slight
+distance between the cylinder and the screen allowing the light to disperse
+sufficiently to produce a line on the film of about the right thickness.
+
+There are two other possible means of photographically receiving the
+picture that upon investigation may yield some results; but it is doubtful
+whether the current available, even that obtained from a telephone relay,
+will be sufficient to produce the desired magnetic effect, and the {53}
+insertion of a second relay would detract greatly from the efficiency by
+decreasing the speed of working. If rays of monochromatic light from a lamp
+L, Fig. 28, pass through a Nicol prism P (polarising prism), then through a
+tube containing CS_2 (carbon bisulphide), afterwards passing through the
+second prism P' (analysing prism), and if the two Nicol prisms are set at
+the polarising angle, no light from L would reach the photographic film
+wrapped round the drum V of the machine. Upon the tube being subjected to a
+field produced by a current passing through the coil C, the refractive
+index of the liquid will be changed, and light from L will reach the
+photographic film.[7]
+
+[Illustration: FIG. 28.]
+
+The second method is rather more complicated, and is based upon the fact
+that the kathode rays in a Crookes' tube can be deflected from their course
+by means of a magnet. In Fig. 29 the kathode K of the X-ray tube sends a
+kathode ray discharge through an aperture in the anode A, through a small
+aperture in the ebonite screen J {54} on to the drum V of the machine,
+round which is wrapped a photographic film; A and K being connected to
+suitable electrical apparatus. Upon the coil M being energised, the
+kathode-ray is deflected from its straight-line course, and the drum V is
+left in darkness.
+
+[Illustration: FIG. 29.]
+
+The method which is now going to be described is very ingenious, as it
+makes use of what is known as an electrolytic receiver. This method of
+receiving has proved to be the most practical and simple of all the
+photo-telegraphic systems that have been devised.
+
+The application of this system to wireless reception is as follows. The
+aerial A, and the earth E, are joined to the primary P of a transformer,
+the secondary S being connected to a Marconi valve receiver C. The valve
+receiver is connected to the battery B and silvered quartz thread K of an
+Einthoven galvanometer (already described). The thread is 1/12000th of an
+inch in diameter, and will respond to currents as small as 10^{-8} of {55}
+an ampere. The light from M throws an enlarged shadow of the thread over a
+slit in the screen J, and as the thread moves to one side under the
+influence of a current, the slit in J is uncovered, and the light from M is
+thrown upon a small selenium cell R. In the dark the selenium cell has a
+very high resistance, and therefore no current can flow from the battery D
+to the relay F. When the string of the galvanometer moves to one side and
+uncovers the slit in the screen J, a certain amount of light is thrown upon
+the selenium cell lowering its resistance, allowing sufficient current to
+pass through to operate the relay.
+
+Round the drum of the machine (shown in Fig. 7) is wrapped a sheet of paper
+that has been soaked in certain chemicals that are decomposed on the
+passage of an electric current through them. As soon as the local circuit
+of the relay is closed, the current from the battery Z (about 12 volts)
+flows through the paper and produces a coloured mark. The picture,
+therefore, is composed of long or short marks which correspond to the
+varying strips of conducting material on the single line print. In order to
+render the marks short and crisp, a small battery Y, and regulating
+resistance L, is placed across the drum and stylus. The diagram, Fig. 30,
+gives the connections for the complete receiver. {56}
+
+The paper used is soaked in a solution consisting of
+
+  Ferrocyanide of potassium             1/4 oz.
+  Ammoniac Nitrate                      1/2 oz.
+  Distilled water[8]                      4 oz.
+
+[Illustration: FIG. 30.]
+
+The paper has to be very carefully chosen, as besides being absorbent
+enough to remain moist during the whole of the receiving, the surface must
+also remain fairly smooth, as with a rough paper the grain shows very
+distinctly, and if there is an excess of solution the electrolytic marks
+are inclined to spread and so cause a blurred image. The writer tried
+numerous specimens of paper before one could be found that gave really
+satisfactory results. It was also found that when working in a warm room
+the paper became nearly {57} dry before the receiving was finished, and the
+resistance of the paper being greatly increased (this may be anything up to
+1000 ohms), the marking became very faint. A sponge moistened with the
+solution and applied to the undecomposed portion of the paper, while still
+revolving, was found to help matters considerably.
+
+Another experience which happened during the writer's early experiments,
+the cause of which I am still unable to explain, occurred in connection
+with the stylus. The stylus used consisted of a sharply pointed steel
+needle, and after working for about three minutes it was noticed that the
+lines were becoming gradually wider, finally running into each other. Upon
+examination it was found that the point of the needle had worn away
+considerably, becoming in fact, almost a chisel point. Almost every needle
+tried acted in a similar manner, and to overcome this difficulty the stylus
+shown in Fig. 31 was devised.
+
+It will be seen that it consists of a holder A, somewhat resembling a drill
+chuck, fastened to the flat spring B in such a manner that the angle the
+stylus makes to the drum can be altered. The needle consists of a length of
+36-gauge steel wire, and as this wears away slowly the jaws of the holder
+can be loosened and a fresh length pushed through. The wire should not
+project beyond the face of the holder more than 1/8th inch. The gauge {58}
+of wire chosen would not suit every machine, the best gauge to use being
+found by trial, but in the writer's machine the pitch of the decomposition
+marks is much finer than of those made by the commercial machines, and this
+gauge, with the slight but unavoidable spreading of the marks, will produce
+a mark of just the right thickness. As already mentioned, no explanation of
+this peculiarity on the part of the stylus can be given, as there is
+nothing very corrosive in the solution used, and the pressure of the stylus
+upon the paper is so slight as to be almost negligible.
+
+[Illustration: FIG. 31.]
+
+No special means are required for fastening the paper to the drum, the
+moist paper adhering quite firmly. Care should be taken, however, to fasten
+the paper--which should be long enough to allow for a lap of about 1/4
+inch--in such a manner that when working the stylus draws away from the
+edge of the lap and not towards it.
+
+The current required to produce electrolysis is very small, about one
+milliampere being sufficient. {59} Providing that the voltage is
+sufficiently high, decomposition will take place with practically "no
+current," it being possible to decompose the solution with the discharge
+from a small induction coil. The quantity of an element liberated is by
+weight the product of time, current, and the electro-chemical equivalent of
+that element, and is given by the equation W = zct, where
+
+  W = quantity of element liberated in grammes.
+  z = electro-chemical equivalent,
+  c = current in amperes,
+  t = time in seconds.
+
+The chemical action that takes place is therefore very small, as the
+intermittent current sent out from the transmitter in some cases only lasts
+from 1/50th to 1/100th a second.
+
+The decomposed marks on the paper are blue, and, as photographers know,
+blue is reproduced in a photograph as a white, so that a photograph taken
+of our electrolytic picture, which will of course be a blue image upon a
+white ground, will be reproduced almost like a blank sheet of paper. If,
+however, a yellow contrast filter is placed in front of the camera lens,
+and an orthochromatic plate used, the blue will be reproduced in the
+photograph as a dead black.
+
+There is one other point that requires attention. It will be noticed that
+the metal print used for {60} transmitting is a positive, since it is
+prepared from a negative. The received picture will therefore be a
+negative, making the final reproduction, if it is to be used for newspaper
+work, a negative also. Obviously this is no good. The final reproduction
+must be a positive, therefore the received picture must be also a positive.
+To overcome this difficulty matters must be so arranged at the receiving
+station that in the cases of Figs. 17, 18, 22, and 24, the film is kept
+permanently illuminated while the stylus on the transmitter is tracing over
+an insulating strip, and in darkness when tracing over a conducting strip.
+In Fig. 30 the relay F should allow a continuous current from Z to flow
+through the electrolytic paper, and only broken when the resistance of the
+selenium cell is sufficiently reduced to allow the current from D to
+operate the relay.
+
+The author has endeavoured to make direct positives on glass of the picture
+to be transmitted, so that a negative metal print could be prepared. The
+results obtained were not very satisfactory, but the method tried is given,
+as it may perhaps be of interest. The plate used in the camera has to be
+exposed three or four times longer than is required for an ordinary
+negative. The exposed plate is then placed in a solution of protoxalate of
+iron (ferrous oxalate) and left until the image shows plainly through the
+back of the plate. It {61} is then washed in water and placed in a solution
+consisting of
+
+  Distilled water                      1000 cc.
+  Nitric acid                             2 cc.
+  Sulphuric acid                          3 cc.
+  Bichromate of potash                  105 grammes.
+  Alum                                   80    "
+
+After being in this bath for about fifteen minutes the plate is again well
+washed in water, and developed in the ordinary way. The first two
+operations should be performed in the dark room, but the remaining
+operations can be performed in daylight, once the plate has been placed in
+the bichromate bath. As already stated, the results obtained were not very
+satisfactory, and such a method is not now worth following up, as it is
+comparatively easy so to arrange matters at the receiving station that a
+positive or negative image can be received at will.
+
+It is necessary to connect the stylus of the receiving machine to the
+positive pole of the battery Z, otherwise the marks will be made on the
+underside of the paper. The electrolytic receiver, owing to the absence of
+mechanical and electro-magnetic inertia, is capable of recording signals at
+a very high speed indeed.
+
+"Atmospherics," which are such a serious nuisance in long-distance wireless
+telegraphy, will also prove a nuisance in wireless photography, {62} but
+their effects will not be so serious in a photographic method of receiving
+as they would be in the electrolytic system. In a photographic receiver
+where the film is, under normal conditions, constantly illuminated, the
+received signals (both the transmitted signals and the atmospheric
+disturbances) will be recorded, after development, as transparent marks
+upon the film, the remainder of the film being, of course, perfectly
+opaque. By careful retouching the marks due to the disturbances can be
+eradicated, a print upon sensitised paper having been first obtained to act
+as a guide during the process.
+
+       *       *       *       *       *
+
+
+{63}
+
+CHAPTER IV
+
+SYNCHRONISING AND DRIVING
+
+Clockwork and electro-motors are the source of driving power that are most
+suitable for photo-telegraphic work, and each has its superior claims
+depending on the type of machine that is being used. For general
+experimental work, however, an electro-motor is perhaps the most
+convenient, as the speed can be regulated within very wide limits. For a
+constant and accurate drive a falling weight has no equal, but the
+apparatus required is very cumbersome and the work of winding both tedious
+and heavy. This method of driving was at one time universally employed with
+the Hughes printing telegraph, but it has now been discarded in favour of
+electro-motors, which are more compact, besides being cheaper to instal in
+the first instance.
+
+Synchronising and isochronising the two machines are the most difficult
+problems that require solving in connection with wireless photography, and
+as previously mentioned, the {64} synchronising of the two stations must be
+very nearly perfect in order to obtain intelligible results. The limit of
+error in synchronising must be about 1 in 500 in order to obtain results
+suitable for publication.
+
+The electrolytic system is perhaps the easiest to isochronise, as the
+received picture is visible. On the metal print used for transmitting, and
+at the commencing edge a datum line is drawn across in insulating ink. The
+reproduction of this line is carefully observed by the operator in charge
+of the receiving instrument, and the speed of the motor is regulated until
+this line lies close against a line drawn across the electrolytic paper.
+Although this may seem an ideal method there are one or two considerations
+to be taken into account. Unless the decomposition marks are made the
+correct length and are properly spaced, however good the isochronising may
+be, the result will be a blurred image. Any one who has worked with a
+selenium cell, will know that it cannot change from its state of high
+resistance to that of low resistance with infinite rapidity, and the
+effects of this inertia, or "fatigue" as it has been called, are more
+pronounced when working at a high speed. In working, the effects of this
+inertia would be to increase the time of contact of the relay F (Fig. 30)
+as the current from D would flow for a slightly longer period through R to
+F than the period of {65} illumination allowed by K. This, of course, would
+mean a lengthening of the marks on the paper; results would also differ
+greatly with different selenium cells. There is a method of compensation by
+which the inertia of a cell can almost entirely be overcome, but it would
+add greatly to the complicacy of the receiving apparatus.
+
+In using an electro-motor with any optical method of receiving there are
+two methods available. The first is an arrangement similar to that used by
+Professor Korn in his early experiments with his selenium machines. The
+motor used for driving has several coils in the armature connected with
+slip rings, from which an alternating current may be tapped off; the motor
+acting partially as a generator, besides doing good work as a motor in
+driving the machine. This alternating current is conducted to a frequency
+meter, which consists of a powerful electro-magnet, over which are placed
+magnetised steel springs, having different natural periods of vibration. By
+means of a regulating resistance the motor is run until the spring which
+has the same period as the desired armature speed vibrates freely. The
+speed of the motors at both stations can thus be adjusted with a fair
+amount of accuracy. Another method is to make use of a governor similar to
+those employed in the Hughes printing telegraph system. A drawing of the
+governor is given in Fig. 32. It consists of a
+
+[Illustration] {67} metal frame which supports an upright steel bar S,
+whose ends turn on pivots. This bar is rectangular in section. The
+gear-wheel G is fastened near the bottom of this rod and gears with a
+similar wheel on the shaft of the driving motor (not shown). Suspended from
+the broader sides of S are the two flexible arms D, each carrying a brass
+ball T. These balls are not fastened to the arms, but can slide up and
+down, being held in position by the wire springs M, one end of each spring
+being fastened to the screws C. These screws work in a slot cut in the
+upper part of S, and are connected to the adjusting screw E. When E is
+turned the screws are raised or lowered accordingly, and also the balls on
+the arms D.
+
+Fastened to the arms are two brushes of tow B, and these revolve inside but
+just clearing the inner surface of the steel ring Z. Upon the motor speed
+increasing above the normal the arms D, and consequently the balls T, swing
+out, making a larger circle, causing the brushes B to press against the
+steel ring Z, setting up friction which, however, is reduced as soon as the
+motor regains its ordinary working speed. By careful adjustment the speed
+of the motors can be kept perfectly constant. The object of having the
+balls T adjustable on D, is to provide a means of altering the motor speed,
+as the lower the balls on D the slower the mechanism runs, and _vice
+versa_. {68}
+
+[Illustration]
+
+A simple and effective speed regulator devised by the writer is given in
+drawings 33 and 34. It comprises two parts, A and B, the part A being
+connected to the driving motor, and the part B working independently. The
+independent portion B consists of an ordinary clock movement M, a steel
+spindle J being geared to one of the slower moving wheels, so that it makes
+just one revolution in two seconds. This spindle, which runs in two coned
+bearings, carries at its outer end a light [Illustration] pointer D, about
+two inches long, to the underside of which is fastened the thin brass
+contact spring S, which presses lightly upon the ebonite ring N. {69} The
+portion A comprises a spindle, pointer, and contact spring similar to those
+employed in B, the spindle J' being geared to the driving motor by means of
+F, so that the pointer D' makes a little more than one revolution in two
+seconds. By means of a special form of brake on the driving motor, the
+speed is reduced, so that both pointers travel at the same rate, viz. one
+revolution in two seconds. By careful adjustment the two pointers can be
+made to revolve in synchronism,[9] and when this is obtained the contact
+springs S, S', pass over the contacts C, C', completing the circuit of the
+battery B and lamp L. When working properly the lamp L lights up regularly
+once every second. This regulator is an excellent one to use for
+experimental work, although it depends a great deal upon the skill of the
+operator, but good adjustment should be obtained in about two minutes. It
+is a good plan to insert a clutch of some description between the driving
+motor and the machine, so that the regulator can be adjusted prior to the
+act of receiving or transmitting, the machine being prevented from
+revolving by means of a catch. The motor used should be powerful enough to
+take up the work of driving the machine without any reduction in speed. The
+clocks M can be regulated so that they only gain or lose a few seconds in
+{70} twenty-four hours, which gives an accuracy in working sufficient for
+all practical purposes.
+
+Connection is made with the contact springs S, S', by means of the springs
+T, T', which press against the spindles J, J'.
+
+Another important point is the correct placing of the picture upon the
+receiving drum. It is necessary that the two machines besides revolving in
+perfect isochronism should synchronise as well, _i.e._ begin to transmit
+and record at exactly the same position on the cylinders, viz. at the edge
+of the lap, so that the component parts of the received image shall occupy
+the same position on the paper or film as they do on the metal print. If
+the receiving cylinder had, let us suppose, completed a quarter of a
+revolution before it started to reproduce, the reproduction when removed
+from the machine and opened out will be found to be incorrectly placed; the
+bottom portion of the picture being joined to the top portion, or _vice
+versa_, and this means that perhaps an important piece of the picture would
+be rendered useless even if the whole is not spoilt. It is evident,
+therefore, that some arrangement must be employed whereby synchronism, as
+well as isochronism of the two instruments can be maintained.
+
+There are several methods of synchronising that are in constant use in
+high-speed telegraphy, in which the limit of error is reduced to a minimum,
+{71} and some modification of these methods will perhaps solve the problem,
+but it must be remembered that synchronism is far easier to obtain where
+the two stations are connected by a length of line than where the two
+stations are running independently.
+
+In one system of ordinary photo-telegraphy synchronism is obtained in the
+following manner. The receiving cylinder travels at a speed slightly in
+excess of the transmitting cylinder, and as its revolution is finished
+first is prevented from revolving by a check, and when in this position the
+receiving apparatus is thrown out of circuit and an electro-magnet which
+operates the check is switched in. When the transmitting cylinder has
+completed its revolution (about 1/100th of a second later) the transmitting
+apparatus, by means of a special arrangement, is thrown out of circuit for
+a period, just long enough for a powerful current to be sent through the
+line. This current actuates the electro-magnet. The check is withdrawn and
+the receiving cylinder commences a fresh revolution in perfect synchronism
+with the transmitting cylinder. As soon as the check is withdrawn the
+receiving apparatus is again placed in circuit until another revolution is
+completed. As the receiver cannot stop and start abruptly at the end of
+each revolution a spring clutch is inserted between the driving motor and
+the machine. {72}
+
+Although a method of synchronising similar to this may later on be devised
+for wireless photography, the writer, from the result of his own
+experiments, is led to believe that results good enough for all practical
+purposes can be obtained by fitting a synchronising device whereby the two
+machines are started work at the same instant, and relying upon the perfect
+regulation of the speed of the motors for correct working.
+
+The method of isochronism must, however, be nearly perfect in its action,
+as it is easy to see that with only a very slight difference in the speed
+of either machine this error will, when multiplied by 40 or 50 revolutions,
+completely destroy the received picture for practical purposes.
+
+From what has been written in this and in the preceding chapters it will be
+evident that the successful solution of transmitting photographs by
+wireless methods will necessitate the use of a great many pieces of
+apparatus all requiring delicate adjustment, and depending largely upon
+each other for efficient working. As previously stated, there is at present
+no real system of wireless photography, the whole science being in a purely
+experimental stage, but already Professor Korn has succeeded in
+transmitting photographs between Berlin and Paris, a distance of over 700
+miles. If such a distance could be worked over successfully, there is no
+reason to doubt that before long {73} we shall be able to receive pictures
+from America with as great reliability and precision as we now receive
+messages.
+
+In nearly all wireless photographic systems devised up to the present the
+chief portion of the receiver consists of a very sensitive galvanometer,
+and although very good results have been obtained by their use they are
+more or less a nuisance, as the extreme delicacy of their construction
+renders them liable to a lot of unnecessary movement caused by external
+disturbances. A galvanometer of the De' Arsonval pattern, used by the
+writer, was constantly being disturbed by merely walking about the room,
+although placed upon a fairly substantial table; and for the same reason it
+was impossible to attempt to place the driving motor of the machine on the
+same table as the galvanometer. For ship-board work it will be evident that
+the use of such a sensitive instrument presents a great difficulty to
+successful working, and a good opening exists for some piece of
+apparatus--to take the place of the galvanometer--that will be as sensitive
+in its action but more robust in its construction.
+
+       *       *       *       *       *
+
+
+{74}
+
+CHAPTER V
+
+THE "TELEPHOGRAPH"
+
+In the present chapter it is proposed to give a brief description of a
+system of radio-photography devised by the author, and which includes a
+greatly improved method of transmitting and receiving, as well as an
+ingenious arrangement for synchronising the two stations; the whole being
+an attempt to produce a system that would be capable of working
+commercially over fairly long distances.
+
+The system about to be described, and which I have designated the
+"telephograph," is the outcome of several years' original experimental
+work, many difficulties that were manifest in the working of the earlier
+systems having been overcome by apparatus that has been expressly designed
+for the purpose.
+
+In any practical system of radio-photography the following points are of
+great importance: (1) the speed of transmission; (2) the quality of the
+received picture; (3) the method of synchronising {75} the two machines so
+that transmission and reception begin simultaneously; (4) the correct
+regulation of the speed of the driving motors; (5) the simplicity and
+reliability of the entire arrangement. Points 1 and 2 are dependent upon
+several factors; the number of contacts made by the stylus per minute; the
+size of the metal print used; the number of lines per inch on the screen
+used in preparing the print; and the accurate and harmonious working of the
+various pieces of apparatus employed.
+
+In the system under discussion the size of the metal print used is 5 inches
+by 7 inches, and a screen having 50 lines to the inch is used for preparing
+it. With the drum of the machine making one revolution in four seconds, the
+stylus makes 87 contacts per second, or 5220 a minute, the time for
+complete transmission being twenty-five minutes. By the use of ordinary
+relays not more than 2000 contacts a minute can be obtained, and in the
+present system it is only by means of a specially designed relay that such
+a high rate of working has been made possible. Similarly, too, with the
+receiving of such a large number of signals transmitted at such a high
+speed, a special instrument has been devised that can record this number of
+signals without any trouble, and could even record up to 8000 signals a
+minute, provided that a suitable transmitter could be designed. {76}
+
+In the present system the writer does not claim to have completely solved
+the problem of the wireless transmission of photographs, but it is a great
+advance on any system previously described, and the following advantages
+are put forward for recognition: (1) a greatly improved method of
+transmitting and receiving; (2) a simple method of regulating the speed of
+the driving motors and maintaining isochronism with a limit of error of
+less than 1 in 800; (3) an arrangement for synchronising the two machines
+whereby transmitting and receiving begin simultaneously; (4) the use of one
+machine only at each station.
+
+TRANSMITTING APPARATUS
+
+A diagrammatic representation of the apparatus required for a complete
+station, transmitting and receiving combined, is given in Fig. 35, the
+usual wireless equipment having been omitted from the diagram to avoid
+confusion.
+
+_The Machine._--This, as will be seen from Fig. 36, consists of a
+base-plate M, to which are attached the two bearings B and B'. The bearing
+B' is fitted with an internal thread to correspond with the threaded
+portion of the shaft D. The drum V is a brass casting, being fastened to
+the shaft by set screws. The shaft is threaded 75 to the inch. The bearings
+are preferably of the concentric type. The circuit breaker C is so arranged
+that when {77} the drum has traversed the required distance, the end of the
+shaft pushes back the spring M, breaking the circuit of the driving gear
+and stopping the machine. The machine is connected to the driving gear by
+the flexible coupling A.
+
+[Illustration: FIG. 35.
+
+M, motor; Y, isochroniser; F, clutch; A, machine; R, stylus; S, relay; X,
+gearing; O, circuit breaker; T, receiver; C, condenser; U, telephone relay;
+K, polarised relay; L, contact breaker; D, D^1, D^2, D^3, batteries; P,
+friction brake; B, B^1, double-pole two-way switches; N, N^1, N^2, single
+switches; W, key; E, electric clock; J, telephones.]
+
+The drum measures 5 inches long by 2-1/8 inches diameter, and this takes a
+metal print 5 inches by 7 inches, which allows for a lap of about 1/4 inch.
+In working, the print is wrapped tightly round the drum, being secured by
+means of a little seccotine smeared along one edge. Care must be taken that
+the edge of the lap draws away from the point of {78} the stylus and not
+towards it. A margin of bare foil, about 1/8 inch wide, should be left on
+the print at the commencing edge, the purpose of which will be explained
+later.
+
+[Illustration: FIG. 36.]
+
+_The Stylus._--As the drum of the machine travels laterally, by reason of
+the threaded shaft and bearing, the stylus must necessarily be a fixture.
+It consists of a holder B, drilled to take a hardened steel point S,
+attached to the spring M. The spring is arranged to work in the guide F,
+which is provided with an adjusting screw W for regulating the pressure of
+the stylus upon the print; the pressure being sufficient to enable good
+contact to be made, but must not be heavy enough to scratch the soft foil.
+The needle should present an angle of about 60° to the surface of the
+print, as this angle has been found to give the best results in working.
+
+To eliminate any sparking that may take place at the point of make and
+break, due to the self-induction of the relay coils, a condenser C, about 1
+microfarad capacity, should be connected across {79} the drum and stylus.
+The complete stylus is given in the drawings, Figs. 37, 37_a_, and also in
+the diagrams Figs. 8 and 9.
+
+[Illustration: FIG. 37.
+
+Showing the arrangement for sliding the stylus to or from the machine.]
+
+[Illustration: FIG. 37a.]
+
+_The Relay._--As will be seen from the diagram, Fig. 38, this consists of
+two electro-magnets having very soft iron cores, the magnet M being wound
+in the usual manner, while the magnet N is wound differentially. The
+armature A is made as light as possible, and is pivoted at P, and when
+there is no current flowing through any of the coils, is held midway
+between the magnet cores by the two spiral springs S and T, which are under
+slight but equal tension. The connections are as follows. The wires from
+the winding on M are connected directly to the relay terminals F and H, as
+are also the wires from one winding on N. The other winding on N is
+connected in series with the battery C, ammeter B, and regulating
+resistance R. {80}
+
+[Illustration: FIG. 38.]
+
+When the circuit of the battery C is completed, the coil of N, to which it
+is connected, is energised, and the armature A is attracted against the
+stop V. When in this position the tension of the spring S is released,
+while the tension of the spring T is increased. As soon as the circuit of
+the battery D is completed by means of the metal line print on the
+transmitting machine, the current divides at the terminals F and H, a
+portion flowing through the magnet coil M, and a portion through the
+remaining winding on N. The current which flows through the winding on N
+produces a magnetising effect equal to that caused by the other winding on
+N, but since the two windings are of equal length and resistance, and since
+the current flowing through the two windings is of equal strength but in
+opposite directions, the result is to neutralise {81} the magnetising
+effects produced by each winding, and consequently no magnetism is produced
+in the cores.
+
+The other portion of the current from D flows through the coil M, and it
+becomes magnetised at the same time that the coil N becomes demagnetised.
+The armature A is attracted by M against the stop X, and this attraction is
+assisted by the spring T, which was under increased tension. The conditions
+of the springs are now reversed, the spring S being under increased
+tension, while the tension of the spring T is released.
+
+As soon as the current from D is broken, the magnetism disappears from M,
+the neutralising current in N ceases, and N once more becomes magnetised,
+owing to the current which still flows through one winding from C; the
+armature is therefore again attracted by N, assisted by the spring S. The
+current flowing through the two windings of N must be perfectly equal, and
+the regulating resistance R, and ammeters B and B', are inserted for
+purposes of adjustment. The current from C must flow in a direction
+opposite to that which flows from D.
+
+[Illustration: FIG. 39.
+
+H, H', containers; M, mercury; E, paraffin oil; T, T', terminals; C,
+suspending rod; D, base; F, F', dipping rods.]
+
+The local circuit of the relay is completed by means of a copper dipper in
+mercury, somewhat resembling an ordinary mercury break, but modified to
+suit the present requirements. The arrangement will be seen from Fig. 39.
+The whole of the {82} moving parts are made as light as possible, and for
+this reason the rod C and the dippers F, F' should be made as short as
+convenient. The containers H, H' are separate, of cast iron, and
+rectangular in shape. The dipper is of very thin copper tube--an advantage
+where alternating current is to be used--and is made adjustable for height
+on the suspending rod C. The leg F is of such a length that permanent
+contact is made with the mercury in the container H, while the leg F'
+clears the surface of the mercury by about 1/4 inch, when the armature of
+the relay is in its normal position. To prevent undue churning of the
+mercury, which would necessarily take place if the dipper entered and left
+the mercury at each movement of the armature, a pointed ebonite plug is
+inserted in the end of the tube. This will be found to give good results at
+a high speed, the mercury being practically undisturbed, and the production
+of "sludge" reduced to a minimum. To prevent oxidation of the mercury, and
+to prevent arcing, the surface is covered with paraffin oil. If this is not
+sufficient to prevent arcing a condenser should be shunted across the {83}
+containers. The volume of mercury, and the area of the dippers, should be
+sufficient to carry the current used for a considerable period without
+heating up to any extent. An adjustable weight J is provided in order to
+balance the armature and dipping rod.
+
+The remaining transmitting apparatus consists of the battery D^2 and the
+usual wireless apparatus. The double-pole two-way switch B' is to enable
+the photo-telegraphic set to be switched out and the hand key W switched in
+for ordinary signalling purposes. The battery D^2 should be about 12 volts.
+
+RECEIVING APPARATUS
+
+The wireless portion of the receiver is similar to that given in Fig. 22,
+is of the usual syntonic type, and comprises an oscillation transformer, S
+being the secondary, and P the primary; C' is a block condenser, and C a
+variable condenser. The detector D is of the carborundum crystal or
+electrolytic pattern. A two-way switch B is provided so that the relay U
+can be switched out and the telephones J switched in for ordinary receiving
+purposes. The relay U is a Brown's telephone relay.
+
+[Illustration: FIG. 40.]
+
+_The Receiver._--The magnified current from the relay U is taken to a
+special telephone receiver, the construction of which is given in Fig. 40.
+The diaphragm F is about 2-1/2 inches diameter, and should be fairly thin
+but very resilient. Only one {84} [Illustration] [Illustration] coil is
+provided, and this should be wound with No. 47 S.S.C. copper wire for a
+resistance of about 2000 ohms. By using only one coil and therefore only
+one core, the movement of the diaphragm is centralised. To the centre of
+the diaphragm a light steel point is fastened, about 1/2 inch long, and
+provided with a projecting hook H. An enlarged view of this pin is given in
+Fig. 41. The movement of the diaphragm and consequently of the steel point
+P is communicated to a pivoted rod R, which is of special construction. A
+piece of aluminium tube 3-3/4 inches long, and of the section given at B,
+is bushed at one end with a piece of brass of the shape shown in Fig. 41a.
+A stiff steel wire T about 1 inch long (20 gauge) is screwed into the end
+of Z, and carries a counterbalance weight C. A hardened {85} steel spindle,
+pointed at both ends, is fastened at D, and runs between two coned
+bearings, one of which is adjustable. The underside of Z is flattened, and
+a small coned depression is made for the reception of the pointed end of
+the pin. By means of the spring J the two pieces, Z and P, are held firmly
+together, at the same time allowing perfect freedom of movement. The bridge
+G is made from a piece of sheet aluminium placed in a slot cut in the tube
+R, the end of the tube being pressed tight upon G, and secured by means of
+a small rivet.
+
+The optical arrangements are as follows. By means of the Nernst lamp L, and
+the lenses B and B', Figs. 42 and 43, a magnified shadow of G is thrown
+upon the screen J. When the shutter G is in its normal position (_i.e._ at
+rest), its shadow is just above the small hole in J, and light from L
+reaches the photographic film wrapped round the drum V of the machine.
+
+[Illustration: FIG. 42.
+
+J, screen; L, Nernst lamp; G, shutter; B, condensing lens; B_1, focussing
+lens.]
+
+When, however, signals are sent out from the transmitting apparatus, the
+magnified current from the relay U energises the coil of the special
+telephone S, attracting the diaphragm F, and consequently giving movement
+to the pivoted rod R. As by means of the optical arrangements a {86}
+magnified movement as well as a magnified image of G is thrown upon the
+screen J, the shadow of G will, when the telephone S is actuated, cover the
+hole in the screen, and prevent any light from reaching the film on V,
+until current from the relay U ceases to flow. Therefore, when the stylus
+of the transmitter traces over a conducting strip on the metal print, no
+light reaches the film on V, but when tracing over an insulating strip the
+shadow of G on the screen J rises, and the light from L reaches the film.
+By this means a positive picture is received, which is a great advantage
+where the photographs are required for reproduction. Atmospherics would be
+represented by irregular transparent marks on the film after development,
+and these can be easily eradicated by retouching.
+
+[Illustration: FIG. 43. E, ebonite screen; F, focussing lens; G, shutter;
+O, condensing lens; L, Nernst lamp.]
+
+The drum of the machine moves laterally 1/75th of an inch per revolution,
+and the hole in the screen is 1/90th of an inch in diameter. As the screen
+J is not in direct contact with the film, the slight diffusion of the light
+that takes place will produce {87} a mark of about the right thickness.
+With a movement of the diaphragm of only 1/40000th of an inch, the actual
+movement of G will be 1/4000th of an inch. If the optical arrangements have
+a magnifying power of 100, then the movement of the shadow upon the screen
+will be 1/40th of an inch, which will be ample to cover the aperture.
+
+The aluminium rod R, minus the counter-weight, can be made to weigh not
+more than 12 grains. It is necessary to enclose the optical parts in a
+light tight box, indicated by the dotted lines in Fig. 43, in order to
+prevent any extraneous light from reaching the film.
+
+_The Contact Breaker._--The contact breaker (L, Fig. 35), as will be seen
+from Fig. 44, consists of an electro-magnet N, the windings of which are
+connected with the battery B and the polarised relay K. The armature which
+is supported by the spring G carries a contact arm A, which in its normal
+position makes permanent contact with the contact screw T, and completes
+the circuit between the relay K and the telephone relay U (Fig. 35). As
+soon as the transmitter sends out the first signal, the magnified current
+from the telephone relay actuates the relay K, which in turn completes the
+circuit of the contact breaker. Directly the armature M has been attracted,
+the contact with T is broken, and A makes fresh contact with the screw H,
+by means of the spring Z {88} fastened to the underside of A. The armature,
+once it has been attracted, is held in permanent contact with H by the
+catch S, independent of the magnets N. As soon as contact is made with H,
+the clutch (F, Fig. 35) circuit is completed, and the circuit of the relay
+K is broken. When the circuit of the clutch F is broken by means of the
+circuit breaker C on the machine (Fig. 36), the stop S is pulled back by
+hand, allowing the contact arm A to rise, and again make fresh contact with
+the contact screw T.
+
+[Illustration: FIG. 44.]
+
+DRIVING APPARATUS
+
+_The Friction Brake._--This consists of a steel disc A, Fig. 45, about
+2-1/2 inches diameter and 3/8 inch or 1/2 inch wide on the face, secured to
+the main shaft of the driving motor. The arm H, pivoted at C, carries at
+one end the curved block B, which is faced with a pad of tow F. The other
+extremity is pivoted to the steel rod P, which slides {89} [Illustration]
+in holes bored in the standards J. One end of the rod P is screwed with a
+fine thread, about 75 to the inch, and is fitted with a regulating wheel T,
+by means of which the block B can be made to press upon the disc A with any
+required degree of pressure. A fairly stiff steel spring R is placed upon
+the rod P, between one standard J and the collar N. As the speed of the
+driving motor is slightly in excess of that required by the machine, the
+block B, by means of the wheel, is made to press upon the disc A, setting
+up friction which reduces the motor speed until the isochroniser indicates
+that the correct working speed has been attained.
+
+_The Clutch_.--The details of this will be seen from Figs. 46 and 47. It
+consists of a steel shaft coned at both ends running between two
+countersunk bearings, one of which is adjustable. This shaft carries the
+two portions of the clutch A and B, the portion A being a fixture on the
+shaft, and the portion B running free upon it. The portion B is a gun-metal
+casting bored to run accurately upon the steel shaft. A soft iron annular
+ring is fastened to the face.
+
+[Illustration: FIG. 46.
+
+E, spindle; R, bobbins; P, iron cores; D, copper rings; T, brushes; N, back
+plate; V, front plate; J, gearing; S, spring; H, collar; Z, iron ring; F,
+fixed bearing; C, insulating bush.]
+
+The portion A consists of a gun-metal casting {90} [Illustration] bored a
+tight fit for the shaft E, secured by means of a set screw. The two magnet
+cores P are screwed into the front plate V, which is also of gun-metal, and
+after the bobbins R have been slipped on, the shanks of the cores are
+passed through holes drilled in the flange N of the main casting and held
+in place with nuts. The faces of both A and B must be turned perfectly
+square with the shaft, so that they run accurately together. The portion B
+is {91} kept in contact with A by means of a spring S, the pressure being
+regulated by the collar H. Current is taken to the magnets by means of the
+two insulated copper rings D mounted upon the body of A. The gear-wheels on
+both portions have teeth of very fine pitch, the number of teeth on each
+being regulated by the speed of the driving motor and the required machine
+speed. Connection with the circuit breaker L and the battery B^2 is made
+with the collecting rings D by the brushes T. The complete connections are
+given in the diagram Fig. 51.
+
+_The Isochroniser._--This is a device for ensuring the correct speed
+regulation of the driving motors, and is shown in detail in Fig. 48. It
+comprises two portions, one portion being rotated at a definite speed by
+electrical means, and the other portion rotated by the driving motor.
+
+The main portion consists of a metal tube N, bushed at both ends, the
+bottom end of the tube being arranged to work on ball-bearings. An ebonite
+bush C carries three copper rings T, T^1, T^2, and the brushes R, R^1, R^2
+are in electrical contact with them. The ebonite plate J, 3-1/2 inches
+diameter, is secured to the top end of N, and carries a contact piece Q,
+shown separate at E. As will be seen this is a block of ebonite with three
+contacts arranged on the top surface. The middle contact P is 1/64th of an
+inch wide, and the contacts P^1 {92} and P^2 are placed on either side at a
+distance of 1/16 inch; the contact strips P^1, P^2 carry the brass pins D,
+which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting
+wire is carried from the contact P to the copper ring T, another from P^1
+to T^1, and one from P^2 to T^2.
+
+[Illustration: FIG. 48.
+
+N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T^1, T^2,
+copper rings; C, insulating block; R, R^1, R^2, brushes; J, ebonite disc;
+Q, contact block; D, metal pins; O, pulley, P, P^1, P^2, contact plates; K,
+needle; Z, spring; W, steel rod; E, countersunk bearing.]
+
+The bushes S are bored a running fit for the steel rod W (shown separate at
+A), which is coned at both ends, and runs between two countersunk bearings,
+the bottom bearing E being fixed while {93} the top bearing (not shown) is
+adjustable. A needle K is fastened near the end of the rod W, and attached
+to this needle is the spring Z, which presses lightly but firmly upon the
+contact block Q. To provide a level surface for Z to work over, the spaces
+between the contact pieces are filled in with an insulating material, and
+the whole surface finished off perfectly smooth. The spring Z is 1/8 inch
+wide for portion of its length, but at the point where it presses upon Q it
+is reduced in width to 1/64th of an inch (see Fig. 48). The driving
+arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a
+grooved pulley, is run in bearings parallel with the shaft W, and is
+connected by suitable gearing to the shaft of the driving motor, so that
+the needle K makes one revolution in about 2-1/2 seconds. A belt passing
+over the pulleys connects the two shafts, and the tension of the belt is
+regulated by means of an adjustable jockey pulley.
+
+The tube N, carrying the disc J, must be rotated at a fixed speed, and this
+is accomplished in the following manner. An ordinary electric clock impulse
+dial, actuated from a master clock, is connected by suitable gearing H, so
+that the tube N makes exactly one revolution in 2 seconds; it being
+possible to adjust an electric clock of the "Synchronome" type, so that it
+only gains or loses about 1 second in 24 hours, and this provides {94} an
+accuracy sufficient for all practical purposes. The connections are given
+in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that
+a connecting wire is run from the steel spindle W to one terminal each of
+the lamps L, L^1, L^2, and from the other terminal of the lamps to one
+terminal of the batteries J, the battery comprising a set of three 4-volt
+accumulators. The other terminals of the batteries are joined one to each
+of the brushes R, R^1, R^2.
+
+[Illustration: FIG. 49.]
+
+[Illustration: FIG. 50.
+
+M, terminals for connecting to electric clock; L, white lamp; L^1, blue
+lamp; L^2, red lamp.]
+
+The lamps are coloured, the lamp L being white, and the lamps L^1 and L^2
+blue and red respectively, and care must be taken in connecting up that
+when the needle K makes contact with the stud P the white lamp L is in
+circuit. When the machines are working, the operator, by means of the brake
+(already described), reduces the speed of the driving motor until the
+needle K travels in unison with the disc J, making permanent contact with P
+on the contact {95} block Q, which is evidenced by the lamp L remaining
+alight. If, however, the needle travels faster than the disc J, contact
+with P is broken and fresh contact is made with P^2, the lamp L is
+extinguished and the red lamp L^2 lights up, and remains alight until the
+operator reduces the speed. Similarly, too, if the needle travels slower
+than J, contact is made with P^1, and the circuit of the blue lamp L^1 is
+completed. When the speed is either above or below the normal, the needle K
+engages with one or the other of the pins D, and as the tension of the
+driving belt is only such as is required to drive the needle, the belt
+slips on the pulleys until the normal speed is regained.
+
+METHOD OF WORKING
+
+The clockwork motor M, Fig. 51, should be capable of running for several
+hours with one winding, and powerful enough to take up the work of driving
+the machine without any appreciable effort. The main spindle of the motor
+is so arranged that it makes one revolution in two minutes, and the
+reduction in speed between the motor shaft and the shaft to which the
+coupling A is attached is 30:1. The metal line print having been wrapped
+round the drum of the machine, the stylus is put into position, at the edge
+of the lap, and with the needle resting about half-way on {96} the margin
+of the bare foil left at the commencing edge of the print. Now, when the
+two stations are in perfect readiness for work, the motors are started and
+the speed adjusted; the speed of the machine being just under one
+revolution in four seconds.
+
+[Illustration: FIG. 51.
+
+M, clockwork motor; S, isochroniser; E, friction break; T, brushes; F,
+electric clutch; X, gearing; D, D^1, switches; A, flexible coupling; K,
+polarised relay; L, circuit breaker; B_1, B_2, B_3, batteries; P, electric
+clock; W, terminals for connection to telephone relay; H, terminals for
+connection to terminals J, on transmitting machine.]
+
+The switch D is then closed, and the arm of the switch D^1 placed on the
+contact stud (1), at the transmitting station only. As soon as the switches
+are closed the clutch F comes into action, and the transmitting machine
+begins to revolve. When the whole of the line print wrapped round the drum
+of the machine has passed under the stylus, the end of the shaft D, Fig.
+36, engages {97} with the spring _m_, breaking the clutch circuit and
+allowing the motor to run free. As soon as the machine stops, the switch D
+is opened and the machine run back to its starting position by hand.
+
+At the receiving station the switch D is also closed, and the arm of the
+switch D^1 placed on the contact stud (2). The closing of these switches
+does not bring the clutch F into operation until current from the telephone
+relay U connected to the wireless receiving apparatus works the sensitive
+polarised relay K, which in turn completes the circuit of the
+circuit-breaker L. When the armature of L is attracted, the circuit of the
+relay K is broken, the circuit of the clutch F is completed, and the
+machine starts revolving.
+
+[Illustration: FIG. 52.]
+
+The current from the relay U, due to the transmitting stylus passing over
+_one_ contact strip on the metal print, is too brief to actuate the heavier
+mechanism of the relay K, hence the need of the margin of bare foil at the
+commencing edge of the metal print, so that a practically continuous
+current will flow to the relay K until the armature is attracted. As,
+however, the relay is not actuated at the receipt of the first signal, and
+as it is necessary for the machine to start recording at a certain point on
+the film, viz. {98} at the edge of the lap--the reason for this was given
+in Chapter IV.--the starting position of the receiving drum will be similar
+to that given in the diagram Fig. 52, where X indicates the lap of the
+photographic film, and the arrow the direction of rotation.
+
+It is, of course, obvious that a somewhat similar adjustment must be made
+with regard to the position of the stylus on the metal print at the
+transmitting machine.
+
+In the present system, as in almost every photographic method of receiving
+that has been described, the Nernst lamp is invariably mentioned as the
+source of illumination. Since the advent of the high-voltage metal-filament
+lamps the Nernst lamp has fallen somewhat into disuse for commercial
+purposes, but it possesses certain characteristics that render it eminently
+suitable for the purpose under discussion.
+
+The main principle of this type of lamp depends upon the discovery made by
+Professor Nernst in 1898, after whom the lamp is named, that filaments of
+certain earthy bodies when raised to a red heat became conductive
+sufficiently well to pass a current which raised it to a white heat, and
+furthermore that the glowing filament emitted a brighter light for a given
+amount of current than carbon filaments.
+
+[Illustration: FIG. 52a.]
+
+Nernst lamps are made in two sizes, the larger {99} being intended for the
+same work as usually done by arc lamps, and the smaller to replace
+incandescent lamps; the smaller type being made to fit into the ordinary
+bayonet lampholders. The principal parts of a Nernst lamp consist of the
+filament, the heater, the automatic cut-out, and the resistance, and their
+arrangement in the smaller type of lamp is given in the diagram, Fig. 52a.
+The current enters at the positive terminal, passes through the heater M,
+and out through the negative terminal. The filament B, which consists of a
+short length of an infusible earth made of the oxides of several rare
+minerals, of which zirconia is one, is a non-conductor at first, but
+becomes a conductor upon being raised to a high temperature by means of the
+heater M. As soon as the filament becomes conductive the current then
+passes through the automatic cut-out H, and the armature D is attracted,
+thus breaking the heater circuit. The current then flows from the positive
+terminal {100} [Illustration] through the cut-out H, resistance J, and
+filament B, and from thence out of the lamp. Since the resistance of the
+filament decreases the hotter it gets, it is necessary to insert a
+ballasting resistance in series with it which has the opposite property of
+increasing its resistance as it gets hotter, to prevent the filament taking
+too much current and destroying itself. Such a resistance, J, consists of a
+filament of fine iron wire, which, to prevent oxidation from exposure to
+the air, is enclosed in a glass bulb filled with hydrogen gas. Fig. 52_b_
+shows the form of ballast resistance used in the small and large type of
+lamp respectively.
+
+Either direct or alternating current can be used with these lamps, and with
+direct current the polarity must be strictly observed, and that the
+positive wire is connected to the positive and the {101} negative wire to
+the negative terminal. With the smaller type of lamp once it has been
+correctly placed in its holder it is essential that it should not be
+turned, as a change in the direction of the current will rapidly destroy
+the filament.
+
+[Illustration: FIG. 52c.]
+
+The arrangement of the larger type of Nernst lamp can be readily seen from
+the drawing, Fig. 52c.
+
+Care must be taken to see that the voltage required by the burner and
+resistance equals the voltage of the supply circuit, and that only parts of
+the same amperage are used together on the same lamp. No advantage is
+obtained by over-running a Nernst lamp, this only shortening its life
+without increasing the light. Under normal conditions the average life of
+the burner is about 700 hours.
+
+The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75
+watts per c.p. The light given is remarkably steady, and the lamps are
+adaptable for all voltages from 100 to 300. In one of the large type of
+lamps for use on a 235-volt {102} circuit the burner takes 0.5 ampere at
+215 volts, and the resistance 0.5 ampere at 20 volts, while one of the
+smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts
+and 0.25 ampere at 20 volts for the burner and resistance respectively. The
+burner and heater are very fragile, and should never be handled except by
+the porcelain plate to which they are attached. The lamps burn in air and
+emit a brilliant white light of high actinic power, the intrinsic
+brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with
+1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon
+filament lamps.
+
+The chief advantage of the Nernst lamp from a photographic point of view
+lies in the fact that it produces abundantly the blue and violet rays which
+have the greatest chemical effect upon a photographic plate or film. These
+rays are known as chemical or actinic rays, and are only slightly produced
+in some types of incandescent electric lamps. Carbon-filament lamps are
+very poor in this respect.
+
+Because a light is visually brilliant it must by no means be assumed that
+it is the best to use for purposes of photography, and this is a point over
+which many photographers stumble when using artificial light. Many sources
+of light, while excellent for illumination, have very low actinic powers,
+while others may have low illuminating but high {103} actinic powers. A
+lamp giving a light yellowish in colour has usually low actinic power,
+while all those lamps giving a soft white light are generally found to be
+highly actinic.
+
+In addition to the actinic value of the source of illumination, the
+photographic film used must be very carefully chosen, as the chemical
+inertia of the sensitised film plays an important part in the successful
+reproduction of the picture, and also, to a certain extent, affects the
+speed of transmission. The length of exposure, the amount of light admitted
+to the film, and the characteristics of the film itself, are all factors
+which have a decided bearing upon the quality of the results obtained, and
+the film found to be most suitable in one case will perhaps give very
+unsatisfactory results in another.
+
+In photo-telegraphy the length of exposure is determined by the time taken
+by the transmitting stylus to trace over a conducting strip on the metal
+print, and this time, of course, varies with the density of the image and
+also with the speed of transmission.
+
+The film in ordinary photography is chosen with regard to the subject and
+the existing light conditions, and the amount of light admitted to the film
+and the length of exposure are regulated accordingly. No such latitude is,
+however, possible in photo-telegraphy. With each set of apparatus {104} the
+various factors, such as the light value, the amount of light admitted to
+the film, and the length of exposure, will be practically fixed quantities,
+and the film that will give the most satisfactory results under these fixed
+conditions can only be found by the rough-and-ready method of "trial and
+error."
+
+The films in common use are manufactured in four qualities, namely,
+ordinary, studio, rapid, and extra rapid. These terms should really relate
+to the light sensitiveness of the film (or, as it is technically termed,
+the speed), but at the best they are a rough and very unsatisfactory guide,
+for the reason that some unscrupulous makers, purely for business purposes,
+do not hesitate to label their films and plates as slow, rapid, etc.,
+without troubling to make any tests for correct classification.
+
+The speed of photographic films and plates is generally indicated by a
+number, and the system of standardisation adopted by the majority of makers
+in this country is that originated by Messrs. Hurter & Driffield,
+abbreviated H. & D. In their system the speed of the film and the exposure
+varies in geometrical proportion, a film marked H. & D. 50 requiring double
+the exposure of one marked H. & D. 100. The highest number always denotes
+the highest speed, and the exposure varies inversely with the speed.
+
+Besides the Hurter & Driffield method of {105} obtaining the speed numbers
+of plates and films adopted by a large number of makers in this country,
+there are also two standard English systems known as the W.P. No. (Watkin's
+power number) and Wynne F. No., both of which are used to a fair extent.
+
+The "Actinograph" number or speed number of a plate in the H. & D. system
+is found by dividing 34 by a number known as the Inertia, the Inertia,
+which is a measure of the insensitiveness of the plate, being determined
+according to the directions laid down by Hurter & Driffield--that is, by
+using pyro-soda developer and the straight portion only of the density
+curve. If, for instance, the Inertia was found to be one-fifth, then the
+speed number would be 34 ÷ 1/5 = 170, and the plate is H. & D. 170. The
+W.P. No. is found by dividing 50 by the Inertia. Thus 50 ÷ 1/5 = 250, and
+the plate is W.P. 250, but for all practical purposes the W.P. No. can be
+taken as one and a half times H. & D. The Wynne F. numbers may be found by
+multiplying the square root of the Watkins number by 6.4. Thus
+
+  [sqrt]250 = 15.81, and 15.81 × 6.4 = W.F. 101.
+
+For those photographers who are in the habit of using an actinometer giving
+the plate speeds in H. & D. numbers, the following table, taken from the
+_Photographer's Daily Companion_, is given, {106} which shows at a glance
+the relative speed numbers for the various systems. The Watkins and Wynne
+numbers only hold good, however, when the inertia has been found by the H.
+& D. method.
+
+TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS
+
+  ------------------------------------------------------
+  |H. & D.|W.P. No.|W.F. No.||H. & D.|W.P. No.|W.F. No.|
+  --------+--------+-----------------+--------+---------
+  |   10  |   15   |   24   ||  220  |  323   |  114   |
+  |   20  |   30   |   28   ||  240  |  352   |  120   |
+  |   40  |   60   |   49   ||  260  |  382   |  124   |
+  |   80  |  120   |   69   ||  280  |  412   |  129   |
+  |  100  |  147   |   77   ||  300  |  441   |  134   |
+  |  120  |  176   |   84   ||  320  |  470   |  138   |
+  |  140  |  206   |   91   ||  340  |  500   |  142   |
+  |  160  |  235   |  103   ||  380  |  558   |  150   |
+  |  200  |  294   |  109   ||  400  |  588   |  154   |
+  ------------------------------------------------------
+
+Although theoretically the higher the speed of the film the less the
+duration of exposure required, there is a practical limit, as besides the
+intensity and actinic value of the light admitted to the film a definite
+time is necessary for it to overcome the chemical inertia of the sensitised
+coating and produce a useful effect. With every make of film it is possible
+to give so short an exposure that although light does fall upon the film it
+does no work at all--in other words, we can say that for every film there
+is a minimum amount of light action, and anything below this is of no use.
+The exposure that enables the smallest amount of light action to take place
+is termed the limit of the smallest useful exposure. {107}
+
+There is also a maximum exposure in which the light affects practically all
+the silver in the film, and any increased light action has no increased
+effect. This is the limit of the greatest useful exposure.
+
+In photo-telegraphy the duration of exposure, as already pointed out, is
+determined by certain conditions connected with the transmitting apparatus,
+and with conditions similar to those mentioned on page 75 the length of
+exposure will vary roughly from 1-50th to 1-150th of a second.
+
+The most suitable film to use for purposes of photo-telegraphy is one
+having a fairly slow speed in which the range of exposure required comes
+well within the limits of the film. There is no advantage in using a film
+having a speed of, say, H. & D. 300 if good results can be obtained from
+one with a speed of, say, H. & D. 200, as the use of the higher speed
+increases the risk of overexposure. With the high-speeded films the
+difficulties of development are also greatly increased, there being more
+latitude in both exposure and development with the slower speeds, and
+consequently a better chance of obtaining a good negative.
+
+Another point, often puzzling to the beginner, and which increases the
+difficulty of choosing a suitable make of film, is that, although one make
+of film marked H. & D. 100 will give good results, another make, also
+marked H. & D. 100, will give {108} very poor results. This is owing, not
+to a poor quality film, as many suppose, but to the almost insurmountable
+difficulty of makers being able to employ exactly the same standard of
+light for testing purposes, so that although various makes may all be
+standardised by the H. & D. method, films bearing the same speed numbers
+may vary in their actual speed by as much as 30 to 50 per cent.
+
+       *       *       *       *       *
+
+
+{109}
+
+APPENDIX A
+
+SELENIUM CELLS
+
+Selenium is a non-metallic element, and was first discovered by Berzelius
+in 1817, in the deposit from sulphuric acid chambers, which still continues
+the source from which it is obtained for commercial purposes, although it
+is found to a small extent in native sulphur. Its at. wt. is 79.2, and its
+sp. gr. 4.8. Symbol, Se.
+
+In its natural state selenium is practically a non-conductor of
+electricity, its resistance being forty thousand million times greater than
+copper. Its practical value lies in the property which it possesses, that
+when in a prepared condition it is capable of varying its electrical
+resistance according to the amount of light to which it is exposed, the
+resistance decreasing as the light increases.
+
+Selenium is prepared by heating it to a temperature of 120° C., keeping it
+there for some hours, and allowing it to cool slowly, when it assumes a
+crystalline form and changes from a bluish grey to a dull slate colour. A
+selenium cell in its simplest form consists merely of some prepared
+selenium placed between two or more metal electrodes, the selenium acting
+as a high resistance conductor between them. The form given by Bell and
+Tainter to the cells used in their experiments is given in Figs. 53 and
+53a. It consists of a number of rectangular brass plates P, P', separated
+by very thin sheets of mica M, the mica sheets being slightly narrower than
+the brass plates, the whole being clamped together in the frame F by the
+two bolts B. {110} By means of a sand-bath the cell is raised to the
+desired temperature, and selenium is rubbed over the surface, which melts
+and fills the small spaces between the brass plates. All the plates P are
+connected together to form one terminal, and the plates P' to form the
+other. By using very thin mica sheets, and a large number of elements, a
+very narrow transverse section of selenium, together with a large active
+surface, can be obtained.
+
+The cell used for commercial purposes is usually constructed as follows. A
+small rectangular piece of porcelain, slate, mica, or other insulator, is
+wound with many turns of fine platinum wire. The wire is wound double, as
+shown in Fig. 54, the spaces between the turns being filled with prepared
+selenium. A thin glass cover is sometimes placed over the cell to protect
+the surface from injury.
+
+[Illustration: FIG. 53.
+
+P, P', plates; M, mica; S, selenium.]
+
+[Illustration: FIG. 53a.]
+
+A strong light falling upon a cell lowers its resistance, and _vice versa_,
+the resistance of a cell being at its highest when unexposed to light; the
+light is apparently absorbed and made to do work by varying the electrical
+resistance of the selenium. Selenium cells vary very considerably as
+regards their quality as well as in their electrical resistance, it being
+possible to obtain cells of the same size for any resistance between 10 and
+1,000,000 ohms, and also, a cell may remain in good working condition for
+several months, while another will become useless in as many weeks.
+
+The ability of a cell to respond to very rapid changes in the illumination
+to which it is exposed is determined largely upon its inertia, it being
+taken as a general rule {111} that the higher the resistance of a cell the
+less the inertia, and _vice versa_, and also, that the higher the
+resistance the greater the ratio of sensitiveness. Inertia plays an
+important part in the working of a cell, slightly opposing the drop in
+resistance when illuminated, and opposing to a [Illustration] much greater
+degree the return to normal for no-illumination. The effects of inertia or
+"lag," as it is termed, can readily be seen by reference to Fig. 55. It
+will be noticed that the current value rapidly increases when the cell is
+first illuminated, but if after a short time _t_ the light is cut off, the
+current value, instead of returning at once to normal for no-illumination,
+only partially rises owing to the interference of the inertia, and some
+time elapses before the cell returns to its normal condition; the time
+varying from a few seconds to several minutes, depending upon the
+characteristics of the cell and the amount of light to which it is exposed.
+An actual curve is given in Fig. 55a. The inertia or "lag" of a cell
+produces upon an intermittent current an effect similar to that produced by
+the capacity [Illustration] of a line, as was noted in Chapter I.,
+preventing the incoming signals from being recorded separately, and
+distinctly. To obtain the best results in photo-telegraphy, the resistance
+of a cell should only be decreased to an extent sufficient to pass the
+current required to operate the recording apparatus, and the illumination
+should be regulated so that this condition of the cell takes place.
+
+The comparative slowness of selenium in responding to {112} any great
+changes in the illumination offers a serious difficulty to its use in
+photo-telegraphy, but various methods have been devised whereby the effects
+of inertia can be counteracted. In the system of De' Bernochi (see Chapter
+I.) the changes in the illumination are neither very rapid nor very great,
+and the inertia effects would therefore be very slight; but in any
+photo-telegraphic system in which a metal line print is used for
+transmitting, where the source of illumination is constant and the
+resistance of the cell is required to drop to a definite value and return
+to normal instantly, many times in succession, the inertia effects are very
+pronounced. The most successful method of counteracting the inertia is that
+adopted by Professor Korn of always keeping the cell sufficiently
+illuminated to overcome it, so that any additional light acts very rapidly.
+Another method worked out and patented by Professor Korn, and known as the
+"compensating cell" method, gives a practically dead beat action, the
+resistance returning to its normal condition as soon as the illumination
+ceases. The arrangement is given in the diagram Fig. 56.
+
+[Illustration: FIG. 55a.]
+
+Light from the transmitting or receiving apparatus, as the case may be,
+falls upon the selenium cell S^1, which is {113} placed on one arm of a
+Wheatstone bridge, a second cell S^2 being placed on the opposite arm. The
+selenium cell S^1 should have great sensitiveness and small inertia, the
+compensating cell S^2 having proportionally small sensitiveness and large
+inertia. Two batteries B, B', of about 100 volts, are connected as shown, B
+being provided with a compensating variable resistance W; W' is also a
+regulating resistance. When no light is falling upon the cell S^1, light
+from L is prevented from reaching the second cell S^2 by a small shutter
+which is fastened to the strings of the Einthoven galvanometer (described
+in Chapter III.), and the piece of apparatus C--relay or galvanometer as
+the case may be--remains in a normal condition. When, however, light falls
+upon the cell S^1, the balance of the bridge is upset, and light from L
+falls a fraction of a second later upon the second cell S^2, and the
+current flowing through C completes the circuit. Needless to say it is
+necessary that the two cells be well matched, as it is very easy to have
+over-compensation, in which case the current is brought below zero.
+
+[Illustration: FIG. 56.]
+
+It is also stated that by enclosing the cells in exhausted glass tubes,
+their inertia can be greatly reduced and their life considerably prolonged.
+The sensitiveness of a cell is the ratio between its resistance in the dark
+and its resistance when illuminated. The majority of cells have a ratio
+between 2:1 and 3:1, but Professor Korn has shown mathematically that by
+conforming to certain conditions regarding the construction the ratio of
+sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms
+can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only
+a short distance away; the resistance may be still {114} further decreased
+by continuing the illumination, but this produces a permanent defect in the
+cells termed "fatigue," the cells becoming very sluggish in their action
+and their sensitiveness gradually becoming less, the ratio between their
+resistance in the dark and their resistance when illuminated being reduced
+by as much as 30 per cent.
+
+Excessive illumination will also produce similar results. The inertia of a
+cell is practically unaffected by the wavelength of the light used, but the
+maximum sensitiveness of a cell is towards the yellow-orange portion of the
+spectrum.
+
+In addition to light, heat has also been found to vary the electrical
+resistance of selenium in a very remarkable manner. At 80° C. selenium is a
+non-conductor, but up to 210° C. the conductivity gradually increases,
+after which it again diminishes.
+
+       *       *       *       *       *
+
+
+{115}
+
+APPENDIX B
+
+PREPARING THE METAL PRINTS
+
+Electricians who desire to experiment in photo-telegraphy, but who have no
+knowledge of photography, may perhaps find the following detailed
+description of preparing the metal prints of some value. The would-be
+experimenter may feel somewhat alarmed at the amount of work entailed, but
+once the various operations are thoroughly grasped, and with a little
+patience and practice, no very great difficulty should be experienced. The
+simpler photographic operations, such as developing, fixing, etc., cannot
+be described here, and the beginner is advised to study a good text-book on
+the subject.
+
+The method to be given of preparing the photographs is practically the only
+one available for wireless transmission, and although the manner given of
+preparing is perhaps not strictly professional, having been modified in
+order to meet the requirements of the ordinary amateur experimenter, the
+results obtained will be found perfectly satisfactory.
+
+As will have been gathered from Chapter II., the camera used for copying
+has to have a single line screen placed a certain distance in front of the
+photographic plate, and the object of this screen is to break the image up
+into parallel bands, each band varying in width according to the density of
+the photograph from which it has been prepared. Thus a white portion of the
+photograph would consist of very narrow lines wide apart, while a dark
+portion would be made up of wide lines close together; a black part would
+appear solid and show no lines at all. It is, of course, obvious {116} that
+the lines on the negative cannot be wider apart, centre to centre, than the
+lines of the screen. A good screen distance has been found to be 1 to 64,
+_i.e._ the diameter of the stop is 1/64th of the camera extension, and the
+distance of the screen lines from the photographic plate is 64 times the
+size of the screen opening. The following table shows what this distance is
+for the screen most likely to be used. The line screens used consist of
+glass plates upon which a number of lines are accurately ruled, the width
+of the lines and the spaces between being equal; the lines are filled in
+with an opaque substance. These ruled screens are very expensive, and are
+only made to order,[10] a screen half-plate size costing from 21s. to 27s.
+6d. An efficient substitute for a ruled screen can be made by taking a
+rather large sheet of Bristol board and ruling lines across in pure black
+drawing ink, the width of the lines and the spaces between being 1/12th of
+an inch respectively. A photograph must be taken of this card, the
+reduction in size determining the number of lines to the inch. A card 20 ×
+15 inches, with 12 lines to the inch, would, if reduced to 5 × 4 inches,
+make a screen having 48 lines to the inch. Preparing the board is rather a
+tedious operation, but the line negative produced will be found to give
+results almost as good as those obtained from a purchased screen.
+
+DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.
+
+  --------------------------------------------------------------
+  |Screen ruling  |Actual space| Distance of |In 1/32|In milli-|
+  |lines per inch.| in inches. |screen ruling| inches|  metres.|
+  |               |            |  in inches. |       |         |
+  |---------------+------------+-------------+-------+---------|
+  |       35      |    1/70    |      .91    |  28.8 |   21.8  |
+  |       50      |    1/100   |      .64    |  20.5 |   16.2  |
+  |       65      |    1/130   |      .49    |  15.7 |   12.4  |
+  --------------------------------------------------------------
+
+As it is impossible for many to have the use of professional apparatus
+designed for this particular kind of work, {117} the fixing of the screen
+into an ordinary camera must be left to the ingenuity of the worker. A
+half-plate back focussing camera will be found suitable for general
+experimental work, but if this is not available, a large box camera can be
+pressed into service.
+
+[Illustration: FIG. 57.]
+
+The writer has never seen a half-plate box camera, but one taking a 5 × 4
+inch plate can be obtained second-hand very cheaply. It is a comparatively
+simple matter to fix the line screen into a camera of this description, the
+drawings Figs. 57 and 58 showing the method adopted by the writer. The two
+clips D, made from fairly stout brass about 1/2 inch wide, are bent to the
+shape shown (an enlarged section is given at C) and soldered at the top and
+bottom of one of the metal sheaths provided for holding the plates. The
+distance between the front of the photographic plate (the film side) and
+the back of the line screen (also the film side), indicated by the arrow at
+A, is determined by the number of lines on the screen. As will be seen from
+the table given, the distance for a screen having 50 lines to the inch will
+be 41/64ths of an inch.
+
+[Illustration: FIG. 58.
+
+M, sheath; P, photographic plate; D, clips; S, line screen.]
+
+In all probability there will be enough clearance between the top of the
+sheath and the top of the camera to allow for the thickness of the clip,
+but if not, a shallow groove a little wider than the clip should be
+carefully cut in the top of the camera, so that it will slide in easily.
+The screen should be placed between the clips, the film side on the {118}
+inside, _i.e._ facing the photographic plate. As with a box camera the
+extension is a fixture, the size of stop to be used is a fixture also. The
+extension of a camera (this term really applies to a bellows camera) is
+measured from the front of the photographic plate to the diaphragm, and if
+this distance in our camera is 8 inches, then the diameter of the stop to
+give the best results would be 1/64th of this, or 1/8th inch. Although for
+all ordinary experimental work the lens fitted to the camera will be
+suitable, the best type of lens for process work of all kinds is the
+"Anastigmat."
+
+The picture or photograph from which it is desired to make a print should
+be fastened out perfectly flat upon a board with drawing pins, and if a
+copying stand is not available it must be placed upright in some convenient
+position. The diagram Fig. 59 gives the disposition of the apparatus
+required for copying. A simple and inexpensive copying stand is shown in
+Fig. 60. The blackboard A should be about 30 inches square, and must be
+fastened perfectly upright upon the base-board B. The stand C should be
+made so that it slides without any side play between the guides D, and
+should be of such a height that the lens of the camera comes exactly
+opposite the {119} [Illustration] [Illustration] centre of the board A. The
+camera, if of the box type, can be secured to the stand by means of a screw
+and wingnut, the screw being passed from the inside as shown. The beginner
+is advised to photograph only very bold and simple subjects, such as black
+and white drawings or enlargements. It is not safe to trust to the
+view-finders as to whether the whole of the picture is included on the
+plate, a piece of ground glass the same size as the plate sheaths, and used
+as a focussing screen, being much more reliable. It is a good plan to focus
+the camera for a number of different-sized pictures, marking the board A,
+and the {120} guides D, so that adjustment is afterwards a very simple
+matter.
+
+The make of plate used is also a great factor in getting a good negative,
+and Wratten Process Plates will be found excellent. As already mentioned,
+such subjects as the exposure and the development of the plate cannot be
+dealt with here, these subjects having been exhaustively treated in several
+text-books on photography. With an arc lamp the exposure is about twice as
+long as in daylight, but the exposure varies with the amount of light
+admitted to the plate, character of the source of light, and the
+sensitiveness of the plate used, etc. The writer has used acetylene gas
+lamps for this purpose with great success. The beginner is advised to use
+artificial light, as this can be kept perfectly even. With daylight,
+however, the light is constantly fluctuating, and this renders the use of
+an actinometer a necessity for correct exposure. After development, if the
+plate is required for immediate use, it can be quickly dried by soaking for
+a few minutes in methylated spirit.
+
+Having obtained a good negative, the next operation is to prepare what is
+known as a metal print. For this we shall require some stout tin-foil or
+lead-foil, about 12 or 15 square feet to the pound, and this should be cut
+into pieces of such a size that it allows a lap of 3/16 inch when wrapped
+round the drum of the transmitting machine. Obtain some good fish-glue and
+add a saturated solution of bichromate of potash in the proportion of 4
+parts of potash to 40 or 50 parts of glue. Pour a little of this glue into
+a shallow dish, lay a sheet of foil upon a flat board, and with a fairly
+stiff brush (a flat hog's-hair as wide as possible) proceed to coat the
+sheet of foil with a thin but perfectly even coating of glue. The thickness
+of the coating can only be found by trial, for if the coating is too thick
+a longer time will be required for printing; but it must not be thin enough
+to show interference colours. After the coating has been laid on, a soft
+brush, such as photographers use for dusting dry {121} plates, should be
+passed up and down, and across and across, with light, even strokes to
+remove any unevenness. A glue solution used by professional photo-engravers
+is as follows:
+
+  Fish-glue               12 oz.
+  Bichromate of Ammonia  3/4 oz.
+  Water                   18 to 24 oz.
+  Ammonia .880            30 minims.
+
+The bichromate should be dissolved in the water, and, when added to the
+glue, stir very thoroughly in order that complete mixing may take place.
+The coating may be done in a good light, not bright sunlight, but _it must
+be dried in the dark_, because, although insensitive while in a moist
+condition, it becomes sensitive immediately on desiccation. If allowed to
+dry in the light the whole coating will become insoluble, and for this
+reason the brushes used should be washed out as soon as they are finished
+with. The sheets will take about 15 minutes to dry in a perfectly dry room,
+but it is not advisable to prepare many sheets at once, as they will not
+keep for more than two or three days.
+
+The prepared negative must now be placed in an ordinary printing frame, and
+a print taken off upon one of the metal sheets in the same way as a print
+is taken off upon ordinary sensitised paper. In daylight the exposure
+varies from 5 to 20 minutes, but in artificial light various trials will
+have to be made in order to get the best results, the exposure varying with
+the amount of bichromate in the coating; the proportion of the bichromate
+to the glue should remain about 6 per cent. Light from a 25 ampere arc lamp
+for 2 to 5 minutes, at a distance of 18 inches, will generally suffice to
+"print" the impression on the metal sheets. The printing finished, the
+metal print should be laid upon a sheet of glass and held under a running
+stream of water. The washing is complete as soon as the unexposed parts of
+the glue coating have been entirely washed away leaving the bare metal, and
+this will take anything from 3 to 7 {122} minutes, depending upon the
+thickness of the film. As soon as it is dry the print is ready for use.
+
+As already mentioned, the negative from which the metal print is made
+requires that the lines be perfectly sharp and opaque, and the spaces
+between perfectly transparent. Ordinary dry plates are too rapid, a rather
+slow plate being required. Wratten Process Plates give excellent results,
+and the following is a good developer to use with them:
+
+  Glycin                15 grammes       1     oz.
+  Sulphite of Soda      40  ,,           2-1/2 ,,
+  Carbonate of Potash   80  ,,           5     ,,
+  Water               1000 c.c.         60     ,,
+
+This developer should be used for 6 minutes at a temperature of 50° F.,
+3-1/2 minutes at 65°, and 1-3/4 minutes at 80°. It is best only used once.
+If an intensifier is required, the following formula will be found to give
+satisfactory results:
+
+  Bichloride of Mercury   1 oz.    60 grammes.
+  Hot Water              16 ,,   1000 c.c.
+
+Allow to cool, completely pour off from any crystals, and add:
+
+  Hydrochloric Acid      30 minims   4 c.c.
+
+Allow negative to bleach thoroughly, wash well in water, and blacken in 10
+per cent ammonia .880, or 5 per cent sodium sulphide.
+
+In preparing the negatives and metal prints the following points should be
+observed:
+
+A good negative should have the lines perfectly sharp and opaque; there
+should be no "fluff" between the lines even when they are close together.
+
+A properly exposed and developed negative should not require any reducing
+or intensifying.
+
+If the lamps used for illuminating the copying board are placed 2 feet
+away, and the exposure required is 5 minutes, the exposure, if the lamps
+are placed 4 feet away, will be {123} 20 minutes, as the amount of light
+which falls upon an object decreases as the inverse square of the distance.
+
+Get the coating on the foil as thin as possible, and err on the side of
+over-exposure, for if the coating is thick and has been under-exposed,
+excessive washing will dissolve the whole coating; for, unless
+insolubilisation has taken place right up to the metal base, the under
+parts will remain in a more or less soluble condition.
+
+On no account must the unexposed sheets be placed near a fire, otherwise
+they will be spoilt, the whole coating becoming insoluble; heat acting in
+the same manner as light.
+
+In washing, keep the print moving so that the stream of water does not fall
+continually in one place. It is best to hold the print so that the water
+runs off in the direction of the lines.
+
+To dry the prints after washing they can be laid out flat in a moderately
+warm oven, or before a stove, the heat of course not being sufficient to
+cause the coating to peel.
+
+To render the glue image more distinct the print should be immersed for a
+few seconds in an aniline dye solution, the glue taking up the colour
+readily. These dyes are soluble in either water or alcohol. A dye known as
+"magenta" is very good.
+
+The process of coating the metal sheets must be performed as quickly as
+possible (about 10 seconds), as owing to the peculiar nature of the
+bichromated glue it soon sets, and once this has taken place it is
+impossible to smooth down any unevenness.
+
+See that the negative and metal sheet make good contact while printing.
+
+If the glue solution does not adhere to the surface of the foil in a
+perfectly even film, but assumes a streaky appearance, a little liquid
+ammonia, or a weak solution of nitric acid, rubbed over the surface of the
+foil, which is afterwards gently scoured with precipitated chalk on a tuft
+of cotton {124} wool, will remove the grease which is the cause of the
+difficulty.
+
+A photograph of a picture prepared from a line negative is given in Fig.
+61. For a great many experiments, and in order to save time, trouble, and
+expense, sketches drawn upon stout lead-foil in an insulating ink will
+answer the purpose admirably, but if any exact work is to be done a single
+line print is of course absolutely necessary. The insulating ink can be
+prepared by dissolving shellac in methylated spirit, or ordinary gum can be
+used. A very fine brush should be used in place of a pen, as the gum will
+not flow freely from an ordinary nib unless greater pressure than the foil
+will safely stand be applied. A sketch prepared in this manner is shown in
+Fig. 62. A little aniline dye should be added to the gum to render it more
+visible, or a mixture of gum and liquid indian ink will be found suitable.
+
+[Illustration: FIG. 63.]
+
+With the copying arrangement already described it is only possible to
+employ it for reducing, it being necessary to employ a bellows camera with
+a back focussing attachment for purposes of enlarging, and this constitutes
+the chief drawback to the use of a fixed focus camera. By replacing the box
+camera with a focussing camera of the same size, we shall have a piece of
+apparatus capable of reducing or enlarging, only in this case the camera
+should be a fixture and the board, A, arranged to slide backwards and
+forwards instead.
+
+[Illustration: FIG. 61.
+
+Portions of photographs (full size) of single line screen, and single line
+print. Screen 40 lines to the inch.]
+
+[Illustration: FIG. 62.]
+
+{125} An extra improvement would be to rule the surface of the copying
+board, A, in a manner similar to that shown in the diagram, Fig. 63. The
+rulings should be marked off from the centre of the board, and should
+enclose parallelograms of the various plate sizes ranging from 3-1/4 ×
+4-1/4 inches up to the full size of the board. By fastening the picture or
+photograph to be copied in the space on the board corresponding in size, we
+can ensure that it is in the correct position for the whole to be included
+on the photographic plate, providing, of course, that the centre of lens
+and board coincide.
+
+With regard to the lens required, the practice adhered to by most
+photographers is to use a lens having a focal length equal to the diagonal
+of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be
+used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 × 4 inch
+camera a 6-inch lens will be required. The following is a simple rule for
+finding the conjugate foci of a lens, and is useful in obtaining the
+distance from the lens to the photographic plate and the picture to be
+copied. Let us suppose that we wish to make a 1-1/2 times enlarged line
+negative from a 4-1/4 × 3-1/4 inch print. Add 1 to the number of times it
+is required to enlarge and multiply the result by the focal length of the
+lens in inches. In the present case this will be 1-1/2 + 1 = 2-1/2; and if
+a 6-inch lens is used, 2-1/2 × 6 = 15 inches will be the distance of the
+lens from the plate. Divide this number by the number of times it is
+desired to enlarge, and the distance of the lens from the picture to be
+copied is obtained; in this instance 15 ÷ 1-1/2 = 10 inches. The same rule
+can be followed when it is required to reduce any given number of times,
+only in this case the greater number will represent the distance between
+the lens and the picture to be copied, and the lesser number the distance
+between the lens and the plate.
+
+In reducing, a 1/4-plate lens will be found to fully cover a 5 × 4 inch
+plate, providing the reduction is not greater than three to one.
+
+       *       *       *       *       *
+
+
+{126}
+
+APPENDIX C
+
+LENSES
+
+In this small volume it is not desirable, neither is it intended, to give
+an exhaustive treatment on the subject of lenses and their action, but as
+optics plays an important part in the transmission of photographs, both by
+wireless and over ordinary conductors, the following notes relating to a
+few necessary principles have been included as likely to prove of interest.
+
+Light always travels in straight lines when in a medium of uniform density,
+such as water, air, glass, etc., but on passing from one medium to another,
+such as from air to water, or air to glass, the direction of the light rays
+is changed, or, to use the correct term, _refracted_. This refraction of
+the rays of light only takes place when the incident rays are passed
+obliquely; if the incident rays are perpendicular to the surface separating
+the two media they are not refracted, but continue their course in a
+straight line.
+
+All liquid and solid bodies that are sufficiently transparent to allow
+light rays to pass through them possess the power of bending or refracting
+the rays, the degree of refraction, as already explained, depending upon
+the nature of the body.
+
+The law relating to refraction will perhaps be better understood by means
+of the following diagram. In Fig. 64 let the line AB represent the surface
+of a vessel of water. The line CD, which is perpendicular to the surface of
+the {127} water, is termed the _normal_, and a ray of light passed in this
+direction will continue in a straight line to the point E. If, however, the
+ray is passed in an oblique direction, such as ND, it will be seen that the
+ray is bent or refracted in the direction DM; if the ray of light is passed
+in any other oblique direction, such as JD, the refracted ray will be in
+the direction DK. The angle NDC is called the _angle of incidence_ and MDE
+the _angle of refraction_. If we measure accurately the line NC, we shall
+find that it is 1-1/3, or more exactly 1.336, times greater than the line
+EM. If we repeat this measurement with the lines JH and PK we shall find
+that the line JH also bears the proportion of 1.336 to the line PK. The
+line NC is called the _sine of the angle of incidence_ NDC, and EM the
+_sine of the angle of refraction_ MDE.
+
+[Illustration: FIG. 64.]
+
+Therefore in water the sine of the angle of incidence is to the sine of the
+angle of refraction as 1.336 is to 1, and this is true whatever the
+position of the incident ray with respect to the surface of the water. From
+this we say that _the sines of the angles of incidence and refraction have
+a constant proportion or ratio to one another_.
+
+The number 1.336 is termed the _refractive index_, or _coefficient_, or the
+_refractive power_ of water. The refractive power varies, however, with
+other fluids and solids, and a complete table will be found in any good
+work on optics.
+
+Glass is the substance most commonly used for refracting the rays of light
+in optical work, the glass being worked up into different forms according
+to the purpose for which it {128} is intended. Solids formed in this way
+are termed _lenses_. A lens can be defined as a transparent medium which,
+owing to the curvature of its surfaces, is capable of converging or
+diverging the rays of light passed through it. According to its curvature
+it is either spherical, cylindrical, elliptical, or parabolic. The lenses
+used in optics are always exclusively spherical, the glass used in their
+construction being either crown glass, which is free from lead, or flint
+glass, which contains lead and is more refractive than crown glass. The
+refractive power of crown glass is from 1.534 to 1.525, and of flint glass
+from 1.625 to 1.590. Spherical surfaces in combination with each other or
+with plane surfaces give rise to six different forms of lenses, sections of
+which are given in Fig. 65.
+
+[Illustration: FIG. 65.]
+
+All lenses can be divided into two classes, convex or converging, or
+concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses,
+being thicker at the middle than at the borders, and _d_, _e_, _f_, which
+are thinner at the middle, being diverging lenses. The lenses _e_ and _g_
+are also termed meniscus lenses, and _a_ represents a prism. The line XY is
+the axis or _normal_ of these lenses to which their plane surfaces are
+perpendicular.
+
+Let us first of all notice the action of a ray of light when passed through
+a prism. The prism, Fig. 66, is represented by the triangle BBB, and the
+incident ray by the line TA. {129} Where it enters the prism at A its
+direction is changed and it is bent or refracted towards the base of the
+prism, or towards the normal, this being always the case when light passes
+from a rare medium to a dense one, and where the light leaves the opposite
+face of the prism at D it is again refracted, but away from the normal in
+an opposite direction to the incident ray, since it is passing from a dense
+to a rare medium. The line DP is called the _emergent_ or refracted ray. If
+the eye is placed at T, and a bright object at P, the object is seen not at
+P, but at the point H, since the eye cannot follow the course taken by the
+refracted rays. In other words, objects viewed through a prism always
+appear deflected towards its summit.
+
+[Illustration: FIG. 66.]
+
+In considering the action of a lens we can regard any lens as being built
+up of a number of prisms with curved faces in contact. Such a lens is shown
+in Fig. 67, the light rays being refracted towards the base of the prisms
+or towards the normal, as already explained; while the top half of the lens
+will refract all the light downwards, the bottom half will act as a series
+of inverted prisms and refract all the light upwards.
+
+[Illustration: FIG. 67.]
+
+[Illustration: FIG. 68.]
+
+If a beam of parallel light--such as light from the sun--be passed through
+a double convex lens L, Fig. 68, we shall find that the rays have been
+refracted from their parallel course and brought together at a point F.
+This point F is {130} termed the principal focus of the lens, and its
+distance from the lens is known as the focal length of that lens. In a
+double and equally convex lens of glass the focal length is equal to the
+radius of the spherical surfaces of the lens. If the lens is a plano-convex
+the focal length is twice the radius of its spherical surfaces. If the lens
+is unequally convex the focal length is found by the following rule:
+multiply the two radii of its surfaces and divide twice that product by the
+sum of the two radii, and the quotient will {131} be the focal length
+required. Conversely, by placing a source of light at the point F the rays
+will be projected in a parallel beam the same diameter as the lens. If,
+however, instead of being parallel, the rays proceed from a point farther
+from the lens than the principal focus, as at A, Fig. 69, they are termed
+divergent rays, but they also will be brought to a focus at the other side
+of the lens at the point a. If the source of light A is moved nearer to the
+principal focus of the lens to a point A^1 the rays will come to a focus at
+the point _a_^1, and similarly when the light is at A^2 the rays will come
+to a focus at the point _a_^2. It can be found by direct experiment that
+the distance _fa_ increases in the same proportion as AF diminishes, and
+diminishes in the same proportion as AF increases. The relationship which
+exists between pairs of points in this manner is termed the _conjugate
+foci_ of a lens, and though every lens has only one principal focus, yet
+its conjugate foci are innumerable.
+
+[Illustration: FIG. 69.]
+
+The formation of an image of some distant object in its principal focus is
+one of the most useful properties of a convex lens, and it is this property
+that forms the basis of several well-known optical instruments, including
+the camera, telescope, microscope, etc.
+
+If we take an oblong wooden box, AA, and substitute a sheet of ground
+glass, C, for one end, and drill a small pinhole, H, in the centre of the
+other end opposite the {132} glass plate, we shall find that a tolerably
+good image of any object placed in front of the box will be formed upon the
+glass plate. The light rays from all points of the object, BD, Fig. 70,
+will pass straight through the hole H, and illuminate the ground glass
+screen at points immediately opposite them, forming a faint inverted image
+of the object BD. The purpose of the hole H is to prevent the rays from any
+one point of the object from falling upon any other point on the glass
+screen than the point immediately opposite to it, therefore the smaller we
+make H, the more distinct will be the image obtained. Reducing the size of
+H in order to produce a more distinct image has the effect of causing the
+image to become very faint, as the smaller the hole in H, the smaller the
+number of rays that can pass through from any point of the object. By
+enlarging the hole H gradually, the image will become more and more
+indistinct until such a size is reached that it disappears altogether.
+
+[Illustration: FIG. 70.]
+
+If in this enlarged hole we place a double convex lens, LL, Fig. 71, whose
+focal length suits the length of the box, the image produced will be
+brighter and more distinct than that formed by the aperture, H, since the
+rays which proceed from any point of the object will be brought by the lens
+to a focus on the glass screen, forming a bright {133} distinct image of
+the point from which they come. The image owes its increased distinctness
+to the fact that the rays from any one point of the object cannot interfere
+with the rays from any other point, and its increased brightness to the
+great number of rays that are collected by the lens from each point of the
+object and focussed in the corresponding point of the image. It will be
+evident from a study of Fig. 71 that the image formed by a convex lens must
+necessarily be inverted, since it is impossible for the rays from the end,
+M, of the object to be carried by refraction to the upper end of the image
+at _n_. The relative positions of the object and image when placed at
+different distances from the lens are exactly the same as the conjugate
+foci of light rays as shown in Fig. 69.
+
+[Illustration: FIG. 71.]
+
+The length of the image formed by a convex lens is to the length of the
+object as the distance of the image is to the distance of the object from
+the lens. For example, if a lens having a focal length of 12 inches is
+placed at a distance of 1000 feet from some object, then the size of the
+image will be to that of the object as 12 inches to 1000 feet, or 1000
+times smaller than the object; and if the length of the object is 500
+inches, then the length of the image will be the 1/1000th part of 500
+inches, or 1/2 inch. {134}
+
+The image formed by the convex lens in Fig. 71 is known as a _real image_,
+but in addition convex lenses possess the property of forming what are
+termed _virtual images_. The distinction can be expressed by saying, _real
+images are those formed by the refracted rays themselves, and virtual
+images those formed by their prolongations_. While a real image formed by a
+convex lens is always inverted and smaller than the object, the virtual
+image is always erect and larger than the object. The power possessed by
+convex lenses of forming virtual images is made use of in that useful but
+common piece of apparatus known as a reading or magnifying glass, by which
+objects placed within its focus are made larger or magnified when viewed
+through it; but in order to properly understand how objects seem to be
+brought nearer and apparently increased in size, we must first of all
+understand what is meant by the expression, _the apparent magnitude of
+objects_.
+
+[Illustration: FIG. 72.]
+
+The apparent magnitude of an object depends upon the angle which it
+subtends to the eye of the observer. The image at A, Fig. 72, presents a
+smaller angle to the eye than the angle presented by the object when moved
+to B, and the image therefore appears smaller. When the object is moved to
+either B or C, it is viewed under a much {135} greater angle, causing the
+image to appear much larger. If we take a watch or other small circular
+object and place it at A, which we will suppose is a distance of 50 yards,
+we shall find that it will be only visible as a circular object, and its
+apparent magnitude or the angle under which it is viewed is then stated to
+be very small. If the object is now moved to the point B, which is only 5
+feet from the eye, its apparent magnitude will be found to have increased
+to such an extent that we can distinguish not only its shape, but also some
+of the marking. When moved to within a few inches from the eye as at C, we
+see it under an angle so great that all the detail can be distinctly seen.
+By having brought the object nearer the eye, thus rendering all its parts
+clearly visible, we have actually magnified it, or made it appear larger,
+although its actual size remains exactly the same. When the distance
+between the object and the observer is known, the apparent magnitude of the
+object varies inversely as the distance from the observer.
+
+Let us suppose that we wish to produce an image of a tree situated at a
+distance of 5000 feet. At this distance the light rays from the tree will
+be nearly parallel, so that if a lens having a focal length of 5 feet is
+fastened in any convenient manner in the wall of a darkened room the image
+will be formed 5 feet behind the lens at its principal focus. If a screen
+of white cardboard be placed at this point we shall find that a small but
+inverted image of the tree will be focussed upon it. As the distance of the
+object is 5000 feet, and as the size of the received image is in proportion
+to this distance divided by the focal length of the lens, the image will be
+as 5000 ÷ 5, or 1000 times smaller than the object.
+
+If now the eye is placed six inches behind the screen and the screen
+removed, so that we can view the small image distinctly in the air, we
+shall see it with an apparent magnitude as much greater than if the same
+small image were equally far off with the tree, as 6 inches is to 5000
+{136} feet, that is 10,000 times. Thus we see that although the image
+produced on the screen is 1000 times less than the tree from one cause, yet
+on account of it being brought near to the eye it is 10,000 times greater
+in apparent magnitude; therefore its apparent magnitude is increased as
+10,000 ÷ 1000, or 10 times. This means that by means of the lens it has
+actually been magnified 10 times. This magnifying power of a lens is always
+equal to the focal length divided by the distance at which we see small
+objects most distinctly, viz. 6 inches, and in the present instance is 60 ÷
+6, or 10 times.
+
+When the image is received upon a screen the apparatus is called a _camera
+obscura_, but when the eye is used and sees the inverted image in the air,
+then the apparatus is termed a _telescope_.
+
+The image formed by a convex lens can be regarded as a new object, and if a
+second lens is placed behind it a second image will be formed in the same
+manner as if the first image were a real object. A succession of images can
+thus be formed by convex lenses, the last image being always treated as a
+fresh object, and being always an inverted image of the one before. From
+this it will be evident that additional magnifying power can be given to
+our telescope with one lens by bringing the image nearer the eye, and this
+is accomplished by placing a short focus lens between the image and the
+eye. By using a lens having a focal length of 1 inch, and such a lens will
+magnify 6 times, the total magnifying power of the two lenses will be 10 ×
+6 = 60 times, or 10 times by the first lens and 6 times by the second. Such
+an instrument is known as a _compound or astronomical telescope_, and the
+first lens is called the object glass and the second lens the magnifying
+glass, or eye-piece.
+
+We are now in a position to understand how virtual images are formed, and
+the formation of a virtual image by means of a convex lens will be readily
+followed from a {137} study of Fig. 73. Let L represent a double convex
+lens, with an object, AB, placed between it and the point F, which is the
+principal focus of the lens. The rays from the object AB are refracted on
+passing through the lens, and again refracted on leaving the lens, so that
+an image of the object is formed at the eye, N. As it is impossible for the
+eye to follow the bent rays from the object, a virtual image is formed and
+is seen at A^1B^1, and is really a continuation of the emergent rays. The
+magnifying power of such a lens may be found by dividing 6 inches by the
+focal length of the lens, 6 inches being the distance at which we see small
+objects most distinctly. A lens having a focal length of 1/4 inch would
+magnify 24 times, and one with a focal length of 1/100th of an inch 600
+times, and so on. The magnifying power is greater as the lens is more
+convex and the object near to the principal focus. When a single lens is
+applied in this manner it is termed a _single microscope_, but when more
+than one lens is employed in order to increase the magnifying power, as in
+the telescope, then the apparatus is termed a _compound microscope_.
+
+[Illustration: FIG. 73.]
+
+Unlike a convex lens, which can form both real and virtual images, a
+concave lens can only produce a virtual image; and while the convex lens
+forms an image larger {138} than the object, the concave lens forms an
+image smaller than the object. Let L, Fig. 74, represent a double concave
+lens, and AB the object. The rays from AB on passing through the lens are
+refracted, and they diverge in the direction RRRR, as if they proceeded
+from the point F, which is the principal focus of the lens, and the
+prolongations of these divergent rays produce a virtual image, erect and
+smaller than the object, at A^1B^1. The principal focal distance of concave
+lenses is found by exactly the same rule as that given for convex lenses.
+
+[Illustration: FIG. 74.]
+
+Up to the present we have assumed that all the rays of light passed through
+a convex lens were brought to a focus at a point common to all the rays,
+but this is really only the case with a lens whose aperture does not exceed
+12°. By aperture is meant the angle obtained by joining the edges of a lens
+with the principal focus. With lenses having a larger aperture the amount
+of refraction is greater at the edges than at the centre, and consequently
+the rays that pass through the edges of the lens are brought to a focus
+nearer the lens than the rays that pass through the centre. Since this
+defect arises from the spherical form of the lens it is termed _spherical
+aberration_, and in lenses that {139} are used for photographic purposes
+the aberration has to be very carefully corrected.
+
+The distortion of an image formed by a convex lens is shown by the diagram,
+Fig. 75. If we receive the image upon a sheet of white cardboard placed at
+A, we shall find that while the outside edges will be clear and distinct,
+the inside will be blurred, the reverse being the case when the cardboard
+is moved to the point B.
+
+[Illustration: FIG. 75.]
+
+[Illustration: FIG. 76.]
+
+[Illustration: FIG. 77.]
+
+Aberration is to a great extent minimised by giving to the lens a meniscus
+instead of a biconvex form, but as it is desirable to reduce the aberration
+to below once the {140} thickness of the lens, and as this cannot be done
+by a single lens, we must have recourse to two lenses put together. The
+thickness of a lens is the difference between its thickness at the middle
+and at the circumference. In a double convex lens with equal convexities
+the aberration is 1-67/100ths of its thickness. In a plano-convex lens with
+the plane side turned towards parallel rays the aberration is 4-1/2 times
+its thickness, but with the convex side turned towards parallel rays the
+aberration is only 1-17/100ths of its thickness.
+
+By making use of two plano-convex lenses placed together as at Fig. 76, the
+aberration will be one-fourth of that of a single lens, but the focal
+length of the lens, L^1, must be half as much again as that of L. If their
+focal lengths are equal the aberration will only be a little more than half
+reduced. Spherical aberration, however, may be entirely destroyed by
+combining a meniscus and double convex lens, as shown in Fig. 77, the
+convex side being turned to the eye when used as a lens, and to parallel
+rays when used as a burning glass or condenser.
+
+       *       *       *       *       *
+
+
+{141}
+
+INDEX
+
+  Aberration, 139
+    spherical, 138, 140
+  Accuracy of working, 70, 72
+  Acetylene gas lamps, 120
+  Actinic power, 102
+  Actinograph, 105
+  Actinometer, 120
+  Alternating current, 82, 100
+  Ammonia, 123
+  Angle of stylus, 24, 78
+  Aniline dye, 123
+  Arcing, 27, 82
+  Arc lamps, 15, 120, 121
+  Atmospherics, 61, 85
+
+  Ballasting resistance, 100
+  Belin, 47
+  Bernochi, 7, 112
+    system of, 7, 34
+  Berzelius, 109
+  Bichromate of potash, 120
+  Blondel's oscillograph, 47
+
+  Camera obscura, 136
+    extension, 116, 118
+    choice of, 117
+  Capacity of condenser, 24, 78
+    electrostatic, 3, 5
+    of cable, 3
+    of London-Paris telephone line, 3
+  Carbon bisulphide, 53
+  Charbonelle, 48
+    receiver of, 48
+  Chemical solution, 56
+  Circuit breaker, 76
+  Clutch, details of, 88, 89, 91
+    spring, 71
+  Coating the metal sheets, 120
+  Coherer, 11, 40
+  Collecting rings, 91
+  Commercial value of photo-telegraphy, 1
+  Compensating selenium cell, 112
+  Contact breaker, 37
+  Copying arrangements, 118, 125
+  Cross screen, 21
+
+  De' Arsonval galvanometer, 47, 73
+  Decoherer, 41
+  Design of machines, 21
+  Detectors, 83
+  Developing solutions, 105, 122
+  Diaphragm, movement of, 48, 52, 84, 87
+  Dipping rods, 81, 83
+  Distance of transmission, 33
+  Duration of wave-trains, 22, 25
+
+  Early experiments, 2
+  Einthoven galvanometer, 32, 44, 45, 54, 113
+  Electric clock, 93
+  Electrolytic receiver, 4, 37, 54, 61, 64
+  Enlarging arrangements, 124, 125
+  Experimental machine, 20
+  Extraneous light, 47
+
+  Fastening electrolytic paper, 58
+  Fatigue of selenium cell, 64, 114
+  Fish glue, 120
+  Flexible couplings, 77
+  Frequency meter, 65
+  Friction brake, 88
+
+  {142}
+  High speed telegraphy, 70
+  Hughes governor, 65
+  Hughes printing telegraph, 63
+  Hurter and Driffield, 104
+  Hydrogen, 100
+
+  Incidence, angle of, 127
+  Inertia, 64, 65, 111
+    effects in photo-telegraphy, 110
+    method of counteracting, 103, 112, 113
+    effect of wave-length of light on, 114
+  Intensifying solution, 122
+  Isochroniser, 89, 91
+    details of, 91, 92, 95
+  Isochronism, 64, 69, 70, 71
+
+  Kathode rays, 53
+  Knudsen, 2
+    apparatus of, 9
+  Korn, 30, 33, 45, 65, 72
+    apparatus of, 31
+
+  Lamps, coloured, 94
+  Lenses, 85, 125, 128
+    principal focus of, 130
+    conjugate foci of, 131
+    action of, 129
+    convex, 128, 131, 136
+    concave, 128, 138
+    focal length of, 130, 138
+    aperture, 138
+    meniscus, 139
+  Light, diffusion of, 86
+    extraneous, 87
+  Limit of error in synchronising, 64
+  Line balancer, 3
+  Line screens, 9, 15, 16, 116
+    making, 116
+
+  Magnifying power, 136, 137
+  Marconi valve, 44, 54
+    coherer, 40
+  Mechanical inertia, 33
+  Mercury break, 81
+    churning of, 82
+    containers, 82
+  Mercury jet interrupter, 29
+  Metal prints, 15, 18, 32, 59, 64, 95, 120, 124
+    drying the, 121, 123
+    exposure of, 121
+    size of, 22, 24, 75, 77
+    pressing the, 22
+  Microscope, 131, 137
+  Military uses, 35
+  Mirror galvanometer, 9, 42, 73
+  Mirror, 47, 51
+  Morse code, 35
+  Motor speed, 89, 95
+    driving, 91, 93, 95
+    clockwork, 63
+    electric, 63
+
+  Nernst lamps, 43, 85, 98
+    heater of, 99
+    filament of, 99
+    principle of, 98
+    resistance of, 100
+    efficiency of, 101, 102
+    overrunning, 101
+  Nicol prism, 53
+
+  Paper for electrolytic receiver, 56
+  Parabolic reflector, 8
+  Period of galvanometer, 43, 44, 46
+  _Photographic Daily Companion_, 105
+  Photographic films, 40, 43, 45, 53, 54, 62, 85, 86, 98
+    process, 37
+    chemical inertia, 103
+    exposure of, 103, 107
+    speed of, 104, 105
+    plates, orthochromatic, 59
+    plates, 120
+  Points to be observed in preparing metal prints, 123
+  Poulsen Company, 32, 47
+    arc, 31
+  Preparing selenium, 109
+    photographs for transmitting, 15, 115
+    sketches on metal foil, 124
+  Prism, 128
+    action of, 129
+  Process plates, 122
+  Professor Nernst, 98
+
+  {143}
+  Radio-photography, requirements of, 74
+  Refraction, angle of, 127
+  Refractive power, 127
+  Relay, 25, 39, 49, 53, 55, 60, 75
+    differential, 79
+    polarised, 97
+    working speed of, 26, 75
+  Reproducing for newspapers, 60
+  Resistance of selenium, 109
+    of selenium cells, 110
+    regulating, 113
+  Retardation of current, 6
+  Retouching, 62
+  Rotary spark-gap, 28
+
+  Selenium, 99
+    cells, 8, 34, 55, 60, 64, 109, 110
+    machines, 45
+  Self-induction, 24, 78
+  Sensitiveness of selenium cells, 113
+    ratio of, 113
+  Silvered quartz threads, 44, 46
+  Spark-gap, 27
+  Speed regulator, 68
+    adjustments of, 69
+  Spring clutch, 71
+  Starting position of machines, 98
+  String galvanometer, 32
+  Stylus, 17, 18, 57, 61, 78, 95, 103
+    sparking at, 24
+  Stylus, angle of, 24, 78
+    defects of, 57
+  Submarine cable, 4
+  Synchronism, 11, 20, 36, 64, 69, 71
+
+  Telephograph, 74
+    advantages of, 76
+    method of working, 96
+  Telephone receiver, 83, 85
+    diaphragm, 48
+    improved, 51
+  Telephone relay, 48, 50, 52, 83, 85, 97
+  Telescope, 131, 136
+  Thermodetector, 32
+  Tow, 88
+  Transmission, distance of, 35, 72
+    speed of, 25, 35, 75
+
+  Vibration, natural period of, 39
+
+  Watkins, 105
+    power number, 105
+  Waves, damped, 30
+    undamped, 30, 31
+  Wheatstone bridge, 113
+  Wireless apparatus, 13
+  _Wireless World_, 31
+  Wynne, 105
+
+  Zirconia, 99
+
+
+
+THE END
+
+
+
+_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.
+
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+
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+
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+
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+
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+
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+Notes
+
+[1] These measurements only apply to a single line. Where a double line is
+employed the capacity is halved.
+
+[2] See Appendix A.
+
+[3] See Appendix B.
+
+[4] In wireless telegraphy "arcing" is principally caused by the
+continuation of the supply current in the spark-gap after the capacity has
+been charged to a potential sufficient to break down the insulation of the
+gap.
+
+[5] See Chapter V.
+
+[6] Nernst lamps are the best to use, as they produce abundantly the blue
+and violet rays which have the greatest chemical effect upon a photographic
+film. Carbon filament lamps are very poor in this respect.
+
+[7] A description of the apparatus required will be found in Ganot's
+_Physics_.
+
+[8] Great care must be exercised in using this solution, as it is
+exceedingly poisonous.
+
+[9] Two clocks would isochronise if their hands travelled at precisely the
+same rate round the dials, but would not synchronise unless they both
+registered the same time as well.
+
+[10] Line screens can be obtained from Messrs. Penrose, 109 Farringdon
+Street, London; or Messrs. Fallowfield, 146 Charing Cross Road, London.
+
+
+
+***END OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF
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+<body>
+<h1>The Project Gutenberg eBook, Wireless Transmission of Photographs, by
+Marcus J. Martin</h1>
+<pre>
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at <a href = "http://www.gutenberg.org">www.gutenberg.org</a></pre>
+<p>Title: Wireless Transmission of Photographs</p>
+<p>       Second Edition, Revised and Enlarged 1919</p>
+<p>Author: Marcus J. Martin</p>
+<p>Release Date: October 9, 2010  [eBook #34052]</p>
+<p>Language: English</p>
+<p>Character set encoding: ISO-8859-1</p>
+<p>***START OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF PHOTOGRAPHS***</p>
+<p>&nbsp;</p>
+<h4>E-text prepared by Robert Cicconetti, Keith Edkins,<br />
+    and the Online Distributed Proofreading Team<br />
+    (<a href="http://www.pgdp.net">http://www.pgdp.net</a>)<br />
+    from page images generously made available by<br />
+    Internet Archive/Canadian Libraries<br />
+    (<a href="http://www.archive.org/details/toronto">http://www.archive.org/details/toronto</a>)</h4>
+<p>&nbsp;</p>
+<table border="0" style="background-color: #ccccff;margin: 0 auto;" cellpadding="10">
+  <tr>
+    <td valign="top">
+      Note:
+    </td>
+    <td>
+      Images of the original pages are available through
+      Internet Archive/Canadian Libraries. See
+      <a href="http://www.archive.org/details/wirelesstransmis00martuoft">
+      http://www.archive.org/details/wirelesstransmis00martuoft</a>
+	</td>
+  </tr>
+</table>
+<p>&nbsp;</p>
+<hr class="pg" />
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+
+<h3>WIRELESS TRANSMISSION OF PHOTOGRAPHS</h3>
+
+  <div class="figcenter" style="width:58%;">
+      <a href="images/illo-fig10.png"><img style="width:100%" src="images/illo-fig10.png"
+      alt="Fig. 10." title="Fig. 10." /></a>
+    <span class="sc">Fig.</span> 10.
+  </div>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h2>WIRELESS TRANSMISSION</h2>
+
+<p class="cenhead">OF</p>
+
+<h2>PHOTOGRAPHS</h2>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead">BY</p>
+
+<h3>MARCUS J. MARTIN</h3>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead"><i>SECOND EDITION<br />
+REVISED AND ENLARGED 1919</i></p>
+
+  <p>&nbsp;</p>
+  <p>&nbsp;</p>
+  <p>&nbsp;</p>
+
+<h3>THE WIRELESS PRESS, LTD.</h3>
+
+<p class="cenhead">12-13 HENRIETTA STREET, STRAND</p>
+
+<p class="cenhead">LONDON, W.C. 2</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p>
+
+<h3>PREFACE TO SECOND EDITION</h3>
+
+  <p>Although during the last few years very little, in common with other
+  wireless work, has been possible in connection with the practical side of
+  the wireless transmission of photographs, yet, now that the prospect of
+  experimental work is once again occupying the minds of all wireless
+  workers, advantage has been taken of a reprint of this little volume to
+  amplify a few points that were insufficiently dealt with in the first
+  edition, and also to add some fresh matter.</p>
+
+  <p>To Chapter V. has been added a short description of the Nernst lamp,
+  and also some useful information regarding photographic films, and a few
+  notes relating to enlarging included in the Appendix B.</p>
+
+  <p>A fresh appendix dealing with the principles of optical lenses has
+  also been added. This is a subject that plays an important part in any
+  system of wireless photography, and to those experimenters whose
+  knowledge of optics is limited this section should prove useful.</p>
+
+  <p>To serious workers engaged on the problem of the wireless transmission
+  of photographs, attention <!-- Page vi --><span class="pagenum"><a
+  name="pagevi"></a>{vi}</span>is called to a series of articles which are
+  being published from time to time in the <i>Wireless World</i>, on the
+  design and construction of wireless photographic apparatus.</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>M. J. M.</p>
+    </div>
+
+    <div class="stanza">
+      <p><span class="sc">Maidstone</span>, 1919.</p>
+    </div>
+  </div>
+
+<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>{vii}</span></p>
+
+<h3>PREFACE</h3>
+
+  <p>In these progressive times it is only reasonable to expect that some
+  attempt would be made to utilise the ether-waves for other purposes than
+  that of telegraphic communication, and already many clever minds are at
+  work trying to solve the problems of the wireless control of torpedoes
+  and airships, wireless telephony, and, last but not least, the wireless
+  transmission of photographs.</p>
+
+  <p>It may seem rather premature to talk about the wireless transmission
+  of photographs at a time when the ordinary systems are not fully
+  developed; but the prospects of wireless photography are of a very
+  encouraging nature, especially for long over-water distances, as there
+  are great difficulties to be overcome in long-distance transmission over
+  ordinary land lines and cables which will be entirely eliminated by
+  wireless methods.</p>
+
+  <p>From a perusal of Chapter I. the reader will be able to understand
+  something of the difficulties that are to be encountered in working over
+  long distances, and he will also be able to appreciate something of the
+  advantages that would be derived <!-- Page viii --><span
+  class="pagenum"><a name="pageviii"></a>{viii}</span>from a reliable
+  wireless system. Apart from the value of such a system for transmitting
+  news pictures, it would also be of great advantage to transmit to ships
+  at sea photographs of criminals for identification purposes. In such a
+  small volume as this it would be impossible to deal with the working of
+  wireless apparatus and the many systems that have been devised for the
+  transmission of photographs over metallic circuits. The Author has taken
+  it for granted that other works have been studied in connection with
+  these subjects, and will therefore only describe such apparatus as is
+  likely to be of use in wireless transmission. At present the transmission
+  of photographs by wireless methods is in a purely experimental stage, and
+  this book will have served its purpose if it helps to put future
+  experimenters on the right track and prevent them from making expensive
+  and fruitless experiments, by showing them the right direction in which
+  investigations are being carried out. As there is no claim to originality
+  in respect of a good many pieces of apparatus, etc., described, I have
+  not thought it necessary to state the various sources from which the
+  information has been obtained.</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>M. J. M.</p>
+    </div>
+
+    <div class="stanza">
+      <p><span class="sc">Ashford</span>, 1916.</p>
+    </div>
+  </div>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page ix --><span class="pagenum"><a name="pageix"></a>{ix}</span></p>
+
+<h3>CONTENTS</h3>
+
+<table class="nobctr" summary="Contents." title="Contents.">
+<tr><td class="nspacsingle"> </td><td class="nspacsingle" style="text-align:right;"> PAGE</td></tr>
+<tr><td class="nspacsingle"> <span class="sc">Preface to Second Edition</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#pagev">v</a></td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Preface</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#pagevii">vii</a></td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER I</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Introductory</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page1">1</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Foreword&mdash;Early experiments&mdash;Advantages of Radio-Photography&mdash;Difficulties
+in Cable working&mdash;Bernochi's
+System&mdash;Knudsen's System.</td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER II</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Transmitting Apparatus</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page13">13</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Wireless Apparatus&mdash;Preparing the Photographs&mdash;Transmitting
+Machines&mdash;Transmitting Apparatus&mdash;Effects of
+Arcing&mdash;Spark-Gaps&mdash;Contact Breakers&mdash;Complete Station&mdash;Professor
+Korn's Apparatus&mdash;Poulsen Company's Photographic
+Recorder&mdash;Comparison of various systems&mdash;Practical
+applications.</td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER III</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Receiving Apparatus</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page37">37</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Methods of Receiving&mdash;Author's Photographic Receiver&mdash;Decohering
+Apparatus&mdash;Description of Einthoven Galvanometer&mdash;Use
+of Galvanometer in Receiving&mdash;Belin's Application
+of Blondel's Oscillograph&mdash;Description of Charbonelle's
+Receiver&mdash;Use of Telephone Relay&mdash;Description of Telephone
+Relay&mdash;Telephotographic Receiver&mdash;Polarisation Receiver&mdash;Kathode-Ray
+Receiver&mdash;Electrolytic Receiver&mdash;Atmospherics
+in Long-Distance working.</td></tr>
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2">
+<!-- Page x --><span class="pagenum"><a name="pagex"></a>{x}</span>
+CHAPTER IV</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Synchronising and Driving</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page63">63</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Driving Motors&mdash;Isochronising the Electrolytic System&mdash;Professor
+Korn's method&mdash;Description of Hughes Governor&mdash;Author's
+Speed Regulator&mdash;Problem of Synchronising&mdash;Methods
+of Synchronising&mdash;Advances made in Radio-Photography.</td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER V</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">The "Telephograph"</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page74">74</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Author's System of Radio-Photography&mdash;Requirements&mdash;Advantages&mdash;Transmitting
+machine&mdash;Description of
+Differential Relay&mdash;Wireless Receiving Apparatus&mdash;Photo-Telegraphic
+Receiving Apparatus&mdash;Circuit Breaker&mdash;Friction
+Brake&mdash;Magnetic Clutch&mdash;Description of Isochroniser&mdash;Method
+of working&mdash;Types of Nernst Lamp&mdash;Action of Nernst
+Lamp&mdash;Comparison of Actinic Value&mdash;Inertia of Photographic
+Films&mdash;Choosing Films&mdash;Speed of Films&mdash;Standard of Speed&mdash;Comparative
+Film Speeds&mdash;Effects of Minimum Exposure&mdash;Effects
+of Maximum Exposure&mdash;Considerations in working
+and choosing Films.</td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> APPENDIX A</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Selenium Cells</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page109">109</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Nature of Selenium&mdash;Preparation of Selenium&mdash;Forms of
+Selenium Cells&mdash;Action of Selenium Cells&mdash;Characteristics
+of Selenium Cells&mdash;Effects of Inertia in Photo-Telegraphy&mdash;Methods
+of counteracting Inertia&mdash;Sensitiveness of Selenium
+to Light&mdash;Effect of Heat on Selenium.</td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> APPENDIX B</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Preparing the Metal Prints</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page115">115</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Outline of Process&mdash;Line Screens&mdash;Choice of Camera&mdash;Fixing
+Line Screen in Camera&mdash;Lenses and Stops&mdash;Taking
+the Photograph&mdash;Copying Stands&mdash;Choice of Photographic
+Plates&mdash;Sources of Illumination&mdash;Metal Prints&mdash;Coating the
+<!-- Page xi --><span class="pagenum"><a name="pagexi"></a>{xi}</span>
+Metal Sheets&mdash;Sensitising Solution&mdash;Printing Operations&mdash;Developing&mdash;Intensifying&mdash;Precautions
+to be observed in
+working&mdash;Preparing Sketches on Metal&mdash;Apparatus for Reducing
+or Enlarging&mdash;Improvements to Copying Board&mdash;Lenses
+for Copying&mdash;Formula for Copying.</td></tr>
+
+<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> APPENDIX C</td></tr>
+
+<tr><td class="nspacsingle"> <span class="sc">Lenses</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page126">126</a></td></tr>
+
+<tr><td class="nspacsingle" style="padding-left:2em"> Action of Light&mdash;Law of Refraction&mdash;Lenses&mdash;Prisms&mdash;Action
+of Lenses&mdash;Focal Length of Lenses&mdash;Formation of
+Images&mdash;Apparent Magnitude of Objects&mdash;Real and Virtual
+Images&mdash;Formation of Virtual Images&mdash;Power of Magnification&mdash;Defects
+of Lenses&mdash;Aberration.</td></tr>
+</table>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page xiii --><span class="pagenum"><a name="pagexiii"></a>{xiii}</span></p>
+
+<h3>ILLUSTRATIONS</h3>
+
+<table class="nobctr" summary="Illustrations." title="Illustrations.">
+<tr><td class="nspacsingle"> FIG. </td><td class="nspacsingle" style="text-align:right;"> PAGE</td></tr>
+
+<tr><td class="nspacsingle"> 1. Diagram showing effects of capacity on an intermittent current </td><td class="nspacsingle" style="text-align:right;"> <a href="#page5">5</a></td></tr>
+
+<tr><td class="nspacsingle"> 2. Bernochi's wireless apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page7">7</a></td></tr>
+
+<tr><td class="nspacsingle"> 3. Knudsen's wireless apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page10">10</a></td></tr>
+
+<tr><td class="nspacsingle"> 4. Wireless transmitting station </td><td class="nspacsingle" style="text-align:right;"> <a href="#page13">13</a></td></tr>
+
+<tr><td class="nspacsingle"> 5. Diagram of experiment illustrating principle of line photograph </td><td class="nspacsingle" style="text-align:right;"> <a href="#page16">16</a></td></tr>
+
+<tr><td class="nspacsingle"> 6. Drawing of transmitting machine </td><td class="nspacsingle" style="text-align:right;"> <a href="#page17">17</a></td></tr>
+
+<tr><td class="nspacsingle"> 7. Drawing of transmitting machine </td><td class="nspacsingle" style="text-align:right;"> <a href="#page18">18</a></td></tr>
+
+<tr><td class="nspacsingle"> 8. Drawing of stylus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page18">18</a></td></tr>
+
+<tr><td class="nspacsingle"> 9. Electrical connections of machine </td><td class="nspacsingle" style="text-align:right;"> <a href="#page19">19</a></td></tr>
+
+<tr><td class="nspacsingle"> 10. Photograph of Author's experimental machine </td><td class="nspacsingle" style="text-align:right;"> <i>Frontispiece</i></td></tr>
+
+<tr><td class="nspacsingle">
+<table class="nob" style="margin-left: -0.25em">
+<tr><td class="nspacsingle"> 10<i>a</i>. End view of Author's experimental machine</td><td class="spacsingle" rowspan="2"> <a href="images/$rbrace.png"><img src="images/$rbrace.png" class="middle" style="height:6.5ex; width:0.75em" alt="brace" /></a> </td></tr>
+<tr><td class="nspacsingle"> 10<i>b</i>. View of image broken up by a "cross" screen</td></tr>
+</table>
+</td><td class="nspacsingle" style="text-align:right;"> <i>facing page</i> <a href="#page21">21</a></td></tr>
+
+<tr><td class="nspacsingle"> 11. Connections of complete transmitting apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page23">23</a></td></tr>
+
+<tr><td class="nspacsingle"> 12. Drawing of ordinary type of spark-gap </td><td class="nspacsingle" style="text-align:right;"> <a href="#page27">27</a></td></tr>
+
+<tr><td class="nspacsingle"> 13. Synchronous rotating spark-gap </td><td class="nspacsingle" style="text-align:right;"> <a href="#page28">28</a></td></tr>
+
+<tr><td class="nspacsingle"> 14. Non-synchronous rotating spark-gap </td><td class="nspacsingle" style="text-align:right;"> <a href="#page28">28</a></td></tr>
+
+<tr><td class="nspacsingle"> 15. Connections for complete wireless photographic station </td><td class="nspacsingle" style="text-align:right;"> <a href="#page30">30</a></td></tr>
+
+<tr><td class="nspacsingle"> 16. Connections of Professor Korn's apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page31">31</a></td></tr>
+
+<tr><td class="nspacsingle"> 17. Connections of Poulsen's photographic recorder </td><td class="nspacsingle" style="text-align:right;"> <a href="#page33">33</a></td></tr>
+
+<tr><td class="nspacsingle"> 18. Author's photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page38">38</a></td></tr>
+
+<tr><td class="nspacsingle"> 19. Enlarged drawing of cone </td><td class="nspacsingle" style="text-align:right;"> <a href="#page39">39</a></td></tr>
+
+<tr><td class="nspacsingle"> 20. End view of Author's photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page39">39</a></td></tr>
+
+<tr><td class="nspacsingle"> 21. Connections of decohering apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page41">41</a></td></tr>
+
+<tr><td class="nspacsingle"> 22. Connections for complete photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page42">42</a></td></tr>
+
+<tr><td class="nspacsingle">
+<!-- Page xiv --><span class="pagenum"><a name="pagexiv"></a>{xiv}</span>
+23. Arrangement of Einthoven galvanometer </td><td class="nspacsingle" style="text-align:right;"> <a href="#page45">45</a></td></tr>
+
+<tr><td class="nspacsingle"> 24. Einthoven galvanometer arranged for receiving </td><td class="nspacsingle" style="text-align:right;"> <a href="#page46">46</a></td></tr>
+
+<tr><td class="nspacsingle"> 25. Connection of telephone relay </td><td class="nspacsingle" style="text-align:right;"> <a href="#page49">49</a></td></tr>
+
+<tr><td class="nspacsingle"> 26. Drawing of Author's improved photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page51">51</a></td></tr>
+
+<tr><td class="nspacsingle"> 27. Diagram giving ratio of vibrating arm </td><td class="nspacsingle" style="text-align:right;"> <a href="#page51">51</a></td></tr>
+
+<tr><td class="nspacsingle"> 28. Arrangement of polarisation receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page53">53</a></td></tr>
+
+<tr><td class="nspacsingle"> 29. Arrangement of kathode-ray receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page54">54</a></td></tr>
+
+<tr><td class="nspacsingle"> 30. Connections of electrolytic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page56">56</a></td></tr>
+
+<tr><td class="nspacsingle"> 31. Drawing of improved stylus for receiving </td><td class="nspacsingle" style="text-align:right;"> <a href="#page58">58</a></td></tr>
+
+<tr><td class="nspacsingle"> 32. Drawing of Hughes telegraph governor </td><td class="nspacsingle" style="text-align:right;"> <a href="#page66">66</a></td></tr>
+
+<tr><td class="nspacsingle"> 33. Arrangement of simple speed regulator </td><td class="nspacsingle" style="text-align:right;"> <a href="#page68">68</a></td></tr>
+
+<tr><td class="nspacsingle"> 34. Diagram of connections of simple speed regulator </td><td class="nspacsingle" style="text-align:right;"> <a href="#page68">68</a></td></tr>
+
+<tr><td class="nspacsingle"> 35. Author's arrangement for complete radio-photographic station </td><td class="nspacsingle" style="text-align:right;"> <a href="#page77">77</a></td></tr>
+
+<tr><td class="nspacsingle"> 36. Drawing of transmitting machine and circuit breaker </td><td class="nspacsingle" style="text-align:right;"> <a href="#page78">78</a></td></tr>
+
+<tr><td class="nspacsingle"> 37. Drawing of special transmitting stylus showing adjusting
+ arrangements </td><td class="nspacsingle" style="text-align:right;"> <a href="#page79">79</a></td></tr>
+
+<tr><td class="nspacsingle"> 37<i>a</i>. End view of transmitting stylus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page79">79</a></td></tr>
+
+<tr><td class="nspacsingle"> 38. Connections of new type of relay designed by the Author </td><td class="nspacsingle" style="text-align:right;"> <a href="#page80">80</a></td></tr>
+
+<tr><td class="nspacsingle"> 39. Arrangement of mercury containers and dipping rods for relay </td><td class="nspacsingle" style="text-align:right;"> <a href="#page82">82</a></td></tr>
+
+<tr><td class="nspacsingle"> 40. Drawing of Author's receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr>
+
+<tr><td class="nspacsingle"> 41. Enlarged drawing of diaphragm and steel point </td><td class="nspacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr>
+
+<tr><td class="nspacsingle"> 41<i>a</i>. Drawing showing arrangement of bush and counter-weight </td><td class="nspacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr>
+
+<tr><td class="nspacsingle"> 42. Optical arrangements of receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page85">85</a></td></tr>
+
+<tr><td class="nspacsingle"> 43. Optical arrangements of receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page86">86</a></td></tr>
+
+<tr><td class="nspacsingle"> 44. Drawing of circuit breaker </td><td class="nspacsingle" style="text-align:right;"> <a href="#page88">88</a></td></tr>
+
+<tr><td class="nspacsingle"> 45. Drawing of friction brake </td><td class="nspacsingle" style="text-align:right;"> <a href="#page89">89</a></td></tr>
+
+<tr><td class="nspacsingle"> 46. Sectional drawing of magnetic clutch </td><td class="nspacsingle" style="text-align:right;"> <a href="#page90">90</a></td></tr>
+
+<tr><td class="nspacsingle"> 47. Plan of magnetic clutch </td><td class="nspacsingle" style="text-align:right;"> <a href="#page90">90</a></td></tr>
+
+<tr><td class="nspacsingle"> 48. Details of Isochroniser </td><td class="nspacsingle" style="text-align:right;"> <a href="#page92">92</a></td></tr>
+
+<tr><td class="nspacsingle"> 49. Connections of Isochroniser </td><td class="nspacsingle" style="text-align:right;"> <a href="#page94">94</a></td></tr>
+
+<tr><td class="nspacsingle"> 50. Dial of Isochroniser </td><td class="nspacsingle" style="text-align:right;"> <a href="#page94">94</a></td></tr>
+
+<tr><td class="nspacsingle"> 51. Diagram of driving mechanism </td><td class="nspacsingle" style="text-align:right;"> <a href="#page96">96</a></td></tr>
+
+<tr><td class="nspacsingle">
+<!-- Page xv --><span class="pagenum"><a name="pagexv"></a>{xv}</span>
+52. Diagram showing starting positions of machines </td><td class="nspacsingle" style="text-align:right;"> <a href="#page97">97</a></td></tr>
+
+<tr><td class="nspacsingle"> 52<i>a</i>. Arrangement of small type Nernst lamp </td><td class="nspacsingle" style="text-align:right;"> <a href="#page99">99</a></td></tr>
+
+<tr><td class="nspacsingle"> 52<i>b</i>. Ballasting resistances for Nernst lamps </td><td class="nspacsingle" style="text-align:right;"> <a href="#page100">100</a></td></tr>
+
+<tr><td class="nspacsingle"> 52<i>c</i>. Arrangement of large type Nernst lamp </td><td class="nspacsingle" style="text-align:right;"> <a href="#page101">101</a></td></tr>
+
+<tr><td class="nspacsingle"> 53. Connections of selenium cell elements </td><td class="nspacsingle" style="text-align:right;"> <a href="#page110">110</a></td></tr>
+
+<tr><td class="nspacsingle"> 53<i>a</i>. Form of selenium cell used by Bell and Tainter </td><td class="nspacsingle" style="text-align:right;"> <a href="#page110">110</a></td></tr>
+
+<tr><td class="nspacsingle"> 54. Diagram showing construction of modern cell </td><td class="nspacsingle" style="text-align:right;"> <a href="#page111">111</a></td></tr>
+
+<tr><td class="nspacsingle"> 55. Resistance curve of selenium cell </td><td class="nspacsingle" style="text-align:right;"> <a href="#page111">111</a></td></tr>
+
+<tr><td class="nspacsingle"> 55<i>a</i>. Actual curve of selenium cell </td><td class="nspacsingle" style="text-align:right;"> <a href="#page112">112</a></td></tr>
+
+<tr><td class="nspacsingle"> 56. Diagram of Professor Korn's method for counteracting inertia </td><td class="nspacsingle" style="text-align:right;"> <a href="#page113">113</a></td></tr>
+
+<tr><td class="nspacsingle"> 57. Arrangement of plate sheath and line screen </td><td class="nspacsingle" style="text-align:right;"> <a href="#page117">117</a></td></tr>
+
+<tr><td class="nspacsingle"> 58. Details of clips to hold line screen </td><td class="nspacsingle" style="text-align:right;"> <a href="#page118">118</a></td></tr>
+
+<tr><td class="nspacsingle"> 59. Arrangement of apparatus for copying </td><td class="nspacsingle" style="text-align:right;"> <a href="#page119">119</a></td></tr>
+
+<tr><td class="nspacsingle"> 60. Drawing showing method of arranging camera and copying stand for
+   adjustment </td><td class="nspacsingle" style="text-align:right;"> <a href="#page119">119</a></td></tr>
+
+<tr><td class="nspacsingle">
+<table class="nob" style="margin-left: -0.25em">
+<tr><td class="nspacsingle"> 61. Photograph of line screen and metal print </td><td class="spacsingle" rowspan="2"> <a href="images/$rbrace.png"><img src="images/$rbrace.png" class="middle" style="height:6.5ex; width:0.75em" alt="brace" /></a></td></tr>
+<tr><td class="nspacsingle"> 62. Photograph of sketch drawn upon metal foil</td></tr>
+</table>
+</td><td class="nspacsingle" style="text-align:right;"> <i>facing page</i> <a href="#page124">124</a></td></tr>
+
+<tr><td class="nspacsingle"> 63. Method of marking out copying board </td><td class="nspacsingle" style="text-align:right;"> <a href="#page124">124</a></td></tr>
+
+<tr><td class="nspacsingle"> 64. Diagram illustrating law of refraction </td><td class="nspacsingle" style="text-align:right;"> <a href="#page127">127</a></td></tr>
+
+<tr><td class="nspacsingle"> 65. Forms of lenses </td><td class="nspacsingle" style="text-align:right;"> <a href="#page128">128</a></td></tr>
+
+<tr><td class="nspacsingle"> 66. Action of light passed through a prism </td><td class="nspacsingle" style="text-align:right;"> <a href="#page129">129</a></td></tr>
+
+<tr><td class="nspacsingle"> 67. Diagram illustrating action of a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page130">130</a></td></tr>
+
+<tr><td class="nspacsingle"> 68. Formation of principal focus of a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page130">130</a></td></tr>
+
+<tr><td class="nspacsingle"> 69. Formation of conjugate foci of a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page131">131</a></td></tr>
+
+<tr><td class="nspacsingle"> 70. Apparatus illustrating principle of camera </td><td class="nspacsingle" style="text-align:right;"> <a href="#page132">132</a></td></tr>
+
+<tr><td class="nspacsingle"> 71. Formation of an image by a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page133">133</a></td></tr>
+
+<tr><td class="nspacsingle"> 72. Diagram illustrating apparent magnitude </td><td class="nspacsingle" style="text-align:right;"> <a href="#page134">134</a></td></tr>
+
+<tr><td class="nspacsingle"> 73. Formation of virtual image by a convex lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page137">137</a></td></tr>
+
+<tr><td class="nspacsingle"> 74. Formation of virtual image by a concave lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page138">138</a></td></tr>
+
+<tr><td class="nspacsingle"> 75. Diagram showing spherical aberration </td><td class="nspacsingle" style="text-align:right;"> <a href="#page139">139</a></td></tr>
+
+<tr><td class="nspacsingle"> 76. Combination of plano-convex lenses </td><td class="nspacsingle" style="text-align:right;"> <a href="#page139">139</a></td></tr>
+
+<tr><td class="nspacsingle"> 77. Combination of meniscus and convex lenses </td><td class="nspacsingle" style="text-align:right;"> <a href="#page139">139</a></td></tr>
+</table>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p>
+
+<h3>RADIO-PHOTOGRAPHY</h3>
+
+<h3>CHAPTER I</h3>
+
+<p class="cenhead">INTRODUCTORY</p>
+
+  <p>Those who desire to experiment on radio-photography, <i>i.e.</i>
+  transmitting photographs, drawings, etc., from one place to another
+  without the aid of artificial conductors, must cultivate at least an
+  elementary knowledge of optics, chemistry, mechanics, and electricity;
+  photo-telegraphy calling for a knowledge of all these sciences. There
+  are, no doubt, many wireless workers who are interested in this subject,
+  but who are deterred from experimenting owing to a lack of knowledge
+  regarding the direction developments are taking, besides which,
+  information on this subject is very difficult to obtain, the science of
+  photo-telegraphy being, at the present time, in a purely experimental
+  stage.</p>
+
+  <p>The wireless transmission of photographs has, no doubt, a great
+  commercial value, but for any system to be commercially practicable, it
+  must be simple, rapid, and reliable, besides being able to work <!-- Page
+  2 --><span class="pagenum"><a name="page2"></a>{2}</span>in conjunction
+  with the apparatus already installed for the purpose of ordinary wireless
+  telegraphy.</p>
+
+  <p>As far back as 1847 experiments were carried out with a view to
+  solving the problem of transmitting pictures and writing by electrical
+  methods over artificial conductors, but no great incentive was held forth
+  for development owing to lack of possible application; but owing to the
+  great public demand for illustrated newspapers that has recently sprung
+  into being, a large field has been opened up. During the last ten years,
+  however, development has been very rapid, and some excellent results are
+  now being obtained over a considerable length of line.</p>
+
+  <p>The wireless transmission of photographs is, on the other hand, of
+  quite recent growth, the first practicable attempt being made by Mr. Hans
+  Knudsen in 1908. It may seem rather premature to talk about the wireless
+  transmission at a time when the systems for transmitting over ordinary
+  conductors are not perfectly developed, but everything points to the fact
+  that for long-distance transmission a reliable wireless system will prove
+  to be both cheaper and quicker than transmission over ordinary land lines
+  and cables.</p>
+
+  <p>The effects of capacity and inductance&mdash;properties inherent to
+  all telegraph systems using metallic conductors&mdash;have a distinct
+  bearing upon the two questions, how far and how quickly can <!-- Page 3
+  --><span class="pagenum"><a name="page3"></a>{3}</span>photographs be
+  transmitted? Owing to the small currents received and to prevent
+  interference from earth currents it is necessary to use a complete
+  metallic circuit. If an overhead line could be employed no difficulty
+  would be experienced in working a distance of over 1000 miles, but a line
+  of this length is impossible&mdash;at least in this country&mdash;and if
+  transmission is attempted with any other country, a certain amount of
+  submarine cable is essential. It has been found that the electrostatic
+  capacity of one mile of submarine cable is equal to the capacity of 20
+  miles of overhead line, and as the effect of capacity is to retard the
+  current and reduce the speed of working, it is evident that where there
+  is any great length of cable in the circuit the distance of possible
+  transmission is enormously reduced.</p>
+
+  <p>If we take for an example the London-Paris telephone line with a
+  length of 311 miles and a capacity of 10.62 microfarads, we find that
+  about half this capacity, or 5.9 microfarads,<a name="NtA1"
+  href="#Nt1"><sup>[1]</sup></a> is contributed by the 23 miles of cable
+  connecting England with France.</p>
+
+  <p>In practice the reduction of speed due to capacity has, to a great
+  extent, been overcome by means of apparatus known as a line-balancer,
+  which hastens the slow discharge of the line and <!-- Page 4 --><span
+  class="pagenum"><a name="page4"></a>{4}</span>allows each current sent
+  out from the transmitter&mdash;the current in several systems being
+  intermittent&mdash;to be recorded separately on the receiver. Photographs
+  suitable for press work can now be sent over a line which includes only a
+  short length of cable for a distance of quite 400 miles in about ten
+  minutes, the time, of course, depending upon the size of the photograph.
+  In extending the working to other countries where there is need for a
+  great length of cable, as between England and Ireland, or America, the
+  retardation due to capacity is very great. On a cable joining this
+  country with America the current is retarded four-tenths of a second. In
+  submarine telegraphy use is made of only one cable with an earth return,
+  but special means have had to be adopted to overcome interference from
+  earth currents, as the enormous cost prohibits the laying of a second
+  cable to provide a complete metallic circuit. The current available at
+  the cable ends for receiving is very small, being only
+  <sup>1</sup>/<sub>200000</sub>th part of an ampere, and this necessitates
+  the use of apparatus of a very sensitive character. One system of
+  photo-telegraphy in use at the present time, employs what is known as an
+  electrolytic receiver (see Chapter III.) which can record signals over a
+  length of line in which the capacity effects are very slight, with the
+  marvellous speed of 12,000 a minute, but this speed rapidly decreases
+  with an increase of distance between the <!-- Page 5 --><span
+  class="pagenum"><a name="page5"></a>{5}</span><span class="figright"
+  style="width:38%;"><a href="images/illo-fig01.png"><img
+  style="width:100%" src="images/illo-fig01.png" alt="Fig. 1" title="Fig. 1"
+  /></a><span class="sc">Fig.</span> 1.</span> two stations. The effect of
+  capacity upon an intermittent current is clearly shown in Fig. 1. If we
+  were to send twenty brief currents in rapid succession over a line of
+  moderate capacity in a given time, we should find that instead of being
+  recorded separately and distinctly as at <i>a</i>, each mark would be
+  pointed at both ends and joined together as shown at <i>b</i>, while only
+  perhaps fifteen could be recorded. If the capacity be still farther
+  increased as at <i>c</i>, only perhaps half the original number of
+  currents could be recorded in the same time, owing to the fact that with
+  an increase of resistance, capacity, and inductance of the line a longer
+  time is required for it to charge up and discharge, thereby materially
+  lessening the rate at which it will allow separate signals to pass; the
+  number of signals that can therefore be recorded in a given time is
+  greatly diminished. If we were to attempt to send the same number of
+  signals over a line of great capacity, as could be sent, and recorded
+  separately and distinctly over a line of small capacity&mdash;the time
+  limit being of course the same in both instances&mdash;we should find
+  that the <!-- Page 6 --><span class="pagenum"><a
+  name="page6"></a>{6}</span>signals would be recorded practically as a
+  continuous line. The two latter cases <i>b</i>, and <i>c</i>, Fig. 1,
+  clearly shows the retardation that takes place at the commencement of a
+  current and the prolongation that takes place at the finish. If the
+  photo-telegraphic system previously mentioned could be rendered sensitive
+  enough to work on the Atlantic cables, we should find that only about
+  1200 signals a minute could be recorded, and this would mean that a
+  photograph which could be transmitted over ordinary land lines in about
+  ten minutes would take at least fifty minutes over the cable. This would
+  be both costly and impracticable, and time alone will show whether, for
+  long-distance work, transmission by wireless will be both cheaper and
+  more rapid than any other method. At present wireless telegraphy has not
+  superseded the ordinary methods of communicating over land, but there can
+  be no doubt that wireless telegraphy, if free from Government
+  restrictions, would in certain circumstances very quickly supersede
+  land-line telegraphy, while it has proved a formidable commercial
+  competitor to the cable as a means of connecting this country with
+  America. Likewise we cannot say that no system of radio-photography will
+  ever come into general use, but where there is any great distance to be
+  bridged, especially over water, wireless transmission is really the only
+  practical solution. From the <!-- Page 7 --><span class="pagenum"><a
+  name="page7"></a>{7}</span>foregoing remarks, it is evident that a
+  reliable system of radio-photography would secure a great victory in the
+  matter of time and cost alone, besides which, the photo-telegraphic
+  apparatus would be merely an accessory to the already existing wireless
+  installation.</p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/illo-fig02.png"><img style="width:100%" src="images/illo-fig02.png"
+      alt="Fig. 2." title="Fig. 2." /></a>
+    <span class="sc">Fig.</span> 2.
+  </div>
+
+  <p>There have been numerous suggestions put forward for the wireless
+  transmission of photographs, but they are all more or less impracticable.
+  One of the earliest systems was devised by de' Bernochi of Turin, but his
+  system can only be regarded interesting from an historical point of view,
+  and as in all probability it could only have been made to work over a
+  distance of a few hundred yards it is of no practical value. Fig. 2 will
+  help to explain the apparatus. A glass cylinder A' is fastened at one end
+  to a threaded steel shaft, which runs in two bearings, one bearing having
+  an internal thread corresponding with that on the <!-- Page 8 --><span
+  class="pagenum"><a name="page8"></a>{8}</span>shaft. Round the cylinder
+  is wrapped a transparent film upon which a photograph has been taken and
+  developed. Light from a powerful electric lamp L, is focussed by means of
+  the lens, N, to a point upon the photographic film. As the cylinder is
+  revolved by means of a suitable motor, it travels upwards simultaneously
+  by reason of the threaded shaft and bearing, so that the spot of light
+  traces a complete spiral over the surface of the film. The light, on
+  passing through the film (the transmission of which varies in intensity
+  according to the density of that portion of the photograph through which
+  it is passing), is refracted by the prism P on to the selenium cell S
+  which is in series with a battery B and the primary X of a form of
+  induction coil. As light of different intensities falls upon the selenium
+  cell,<a name="NtA2" href="#Nt2"><sup>[2]</sup></a> the resistance of
+  which alters in proportion, current is induced in the secondary Y of the
+  coil and influences the light of an arc lamp of whose circuit it is
+  shunted. This arc lamp T is placed at the focus of a parabolic reflector
+  R, from which the light is reflected in a parallel beam to the receiving
+  station.</p>
+
+  <p>The receiver consists of a similar reflector R' with a selenium cell E
+  placed at its focus, whose resistance is altered by the varying light
+  falling upon it from the reflector R. The selenium cell <!-- Page 9
+  --><span class="pagenum"><a name="page9"></a>{9}</span>E is in series
+  with a battery F and the mirror galvanometer H. Light falls from a lamp D
+  and is reflected by the mirror of the galvanometer on to a graduated
+  aperture J and focussed by means of the aplanatic lens U upon the
+  receiving drum A<sup>2</sup>, which carries a sensitised photographic
+  film. The two cylinders must be revolved synchronously. The above
+  apparatus is very clever, but cannot be made to work over a distance of
+  more than 200 yards.</p>
+
+  <p>A system based on more practical lines was that invented and
+  demonstrated by Mr. Hans Knudsen, but the apparatus which he employed for
+  receiving has been discarded in wireless work, as it is not suitable for
+  working with the highly-tuned systems in use at the present time.</p>
+
+  <p>Knudsen's transmitter, a diagrammatic representation of which is given
+  in Fig. 3, consists of a flat table to which a horizontal to-and-fro
+  motion is given by means of a clockwork motor. Upon this table is
+  fastened a photographic plate which has been prepared in the following
+  manner. The plate upon which the photograph is to be taken has the
+  gelatine film from three to four times thicker than that commonly used in
+  photography. In the camera, between the lens and this plate, a single
+  line screen is interposed, which has the effect of breaking the picture
+  up into parallel lines. Upon the plate being developed and before it is
+  <!-- Page 10 --><span class="pagenum"><a
+  name="page10"></a>{10}</span><span class="figright" style="width:43%;"><a
+  href="images/illo-fig03.png"><img style="width:100%"
+  src="images/illo-fig03.png" alt="Fig.3." title="Fig.3." /></a><span
+  class="sc">Fig.</span> 3.</span> completely dry, it is sprinkled over
+  with fine iron dust. With this type of plate the transparent parts dry
+  much quicker than the shaded or dark parts, and on the iron dust being
+  sprinkled over the plate it adheres to the darker portions of the film to
+  a greater extent than it does to the lighter portions; a picture partly
+  composed of iron dust is thus obtained. A steel point attached to a flat
+  spring rests upon this plate and is made to travel at right angles to the
+  motion of the table. As the picture is partly composed of iron dust, and
+  as the steel needle is fastened to a delicate spring it is evident that
+  as the plate passes to and fro under the needle, both the spring and
+  needle are set in a state of vibration. This vibrating spring makes <!--
+  Page 11 --><span class="pagenum"><a name="page11"></a>{11}</span>and
+  breaks the battery circuit of a spark coil, which in turn sets up
+  sparking in the spark-gap of the wireless apparatus.</p>
+
+  <p>The receiver consists of a similar table to that used for
+  transmitting, and carries a glass plate that has been smoked upon one
+  side. A similar spring and needle is placed over this plate, but is
+  actuated by means of a small electro-magnet in circuit with a battery and
+  a sensitive coherer. As the coherer makes and breaks the battery circuit
+  by means of the intermittent waves sent out from the transmitting aerial,
+  the needle is made to vibrate upon the smoked glass plate in unison with
+  the needle at the transmitting end. Scratches are made upon the smoked
+  plate, and these reproduce the picture on the original plate. A print can
+  be taken from this scratched plate in a similar manner to an ordinary
+  photographic negative.</p>
+
+  <p>The two tables are synchronised in the following manner. Every time
+  the transmitting table is about to start its forward stroke a powerful
+  spark is produced at the spark-gap. The waves set up by this spark
+  operate an ordinary metal filings coherer at the receiving end which
+  completes the circuit of an electro-magnet. The armature of this magnet
+  on being attracted immediately releases the motor used for driving,
+  allowing it to operate the table. The time taken to transmit a
+  photograph, quarter-plate size, is about fifteen minutes. <!-- Page 12
+  --><span class="pagenum"><a name="page12"></a>{12}</span>Although very
+  ingenious this system would not be practicable, as besides speed the
+  quality of the received pictures is a great factor, especially where they
+  are required for reproduction purposes. The results from the above
+  apparatus are said to be very crude, as with the method used to prepare
+  the photographs no very small detail could be transmitted.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 13 --><span class="pagenum"><a name="page13"></a>{13}</span></p>
+
+<h3>CHAPTER II</h3>
+
+<p class="cenhead">TRANSMITTING APPARATUS</p>
+
+  <p>Let us now consider the requirements necessary for transmitting
+  photographs by means of the wireless apparatus in use at the present
+  time.</p>
+
+  <div class="figright" style="width:24%;">
+      <a href="images/illo-fig04.png"><img style="width:100%" src="images/illo-fig04.png"
+      alt="Fig. 4." title="Fig. 4." /></a>
+    <span class="sc">Fig.</span> 4.
+  </div>
+
+  <p>The connections for an experimental syntonic wireless transmitting
+  station are shown in the diagram Fig. 4. A is the aerial; T, the
+  inductance; E, earth; L, hot-wire ammeter. The closed oscillatory circuit
+  consists of an inductance F, spark-gap G, and a block condenser C. H is a
+  spark-coil for supplying the energy, the secondary J being connected to
+  the spark-gap. A <!-- Page 14 --><span class="pagenum"><a
+  name="page14"></a>{14}</span>mercury break N and a battery B are placed
+  in the primary circuit of the coil. The Morse key K is for completing the
+  battery circuit for signalling purposes. When the key K is depressed, the
+  battery circuit is completed, and a spark passes between the balls of the
+  spark-gap G producing oscillations in the closed circuit, which are
+  transposed to the aerial circuit by induction. For signalling purposes it
+  is only necessary for the operator by means of the key K to send out a
+  long or short train of waves in some pre-arranged order, to enable the
+  operator at the receiving station to understand the message that is being
+  transmitted.</p>
+
+  <p>If a photograph could be prepared in such a manner that it would serve
+  the purpose of the key K, and could so arrange matters that a minute
+  portion of the photograph could be transmitted separately but in
+  succession, and that each portion of the photograph having the same
+  density could be given the same signal, then it would only be necessary
+  to have apparatus at the receiving station capable of arranging the
+  signals in proper sequence (each signal recorded being the same size and
+  having the same density as the transmitted portion of the photograph) in
+  order to receive a facsimile of the picture transmitted.</p>
+
+  <p>The following method of preparing the photograph<a name="NtA3"
+  href="#Nt3"><sup>[3]</sup></a> is one that has been adopted in several
+  <!-- Page 15 --><span class="pagenum"><a
+  name="page15"></a>{15}</span>systems of photo-telegraphy, and is the only
+  one at all suitable for wireless transmission. The photograph or picture
+  which is to be transmitted is fastened out perfectly flat upon a
+  copying-board. A strong light is placed on either side of this copying
+  board, and is concentrated upon the picture by means of reflectors. The
+  camera which is used for copying has a single line screen interposed
+  between the lens and sensitised plate, and the effect of this screen is
+  to break the picture up into parallel lines. Thus a white portion of the
+  photograph would consist of very narrow lines wide apart, while the dark
+  portion would be made up of wide lines close together; a black part would
+  appear solid and show no lines at all. From this line negative it will be
+  necessary to take off a print upon a specially prepared sheet of metal.
+  This consists of a sheet of thick lead- or tinfoil, coated upon one side
+  with a thin film of glue to which bichromate of potash has been added;
+  the bichromate possessing the property of rendering the glue waterproof
+  when acted upon by light. The print can be taken off by artificial light
+  (arc lamps being generally used), but the exact time to allow for
+  printing can only be found by experiment, as it varies considerably
+  according to the thickness of the film. The printing finished, the metal
+  print is washed under running water, when all those parts not acted upon
+  by light, <i>i.e.</i> the parts between the lines, are <!-- Page 16
+  --><span class="pagenum"><a name="page16"></a>{16}</span>washed away,
+  leaving the bare metal. We have now an image composed of numerous bands
+  of insulating material (each band varying in width according to the
+  density of the photograph at any point from which it is prepared)
+  attached to a metal base, so that each band of insulating material is
+  separated by a band of conducting material. It is, of course, obvious
+  that the lines on the print cannot be wider apart, centre to centre, than
+  the lines of the screen used in preparing it. A good screen to use is one
+  having 50 lines to the inch, but one is perhaps more suitable for
+  experimental work a little coarser, say 35 lines to the inch. To use a
+  screen having 50 or more lines to the inch, the transmitting apparatus,
+  as will be evident later on, will require to be very nearly perfect.</p>
+
+  <div class="figright" style="width:23%;">
+      <a href="images/illo-fig05.png"><img style="width:100%" src="images/illo-fig05.png"
+      alt="Fig. 5." title="Fig. 5." /></a>
+    <span class="sc">Fig.</span> 5.
+  </div>
+
+  <p>Before proceeding further it will perhaps be as well to make an
+  experiment. If we take one of the metal prints or, more simple, draw a
+  sketch in insulating ink upon a sheet of metal A, Fig. 5, and connect a
+  battery B and the galvanometer D as shown, we shall find on drawing the
+  free end of the wire across the metal plate that all the time the wire is
+  in contact with the lines of insulating material the needle of the
+  galvanometer will remain <!-- Page 17 --><span class="pagenum"><a
+  name="page17"></a>{17}</span>at zero, but where it is in contact with the
+  metal plate the needle is deflected.</p>
+
+  <p>From this experiment it will be seen that we have in our metal line
+  print, which consists of alternate lines of insulating and conducting
+  material, a method by which an electric circuit can be very easily made
+  and broken. It is, of course, necessary to have some arrangement whereby
+  the whole of the surface of the metal print is utilised for this purpose
+  to the best advantage. One type of transmitting machine used for this
+  purpose is represented by the diagram, Fig. 6. The cylinder A is fastened
+  to the steel shaft B, which runs in the two bearings D and D', the
+  bearing D' having an internal thread corresponding to that on the shaft.
+  The stylus in this class of machine is a fixture, the cylinder being
+  given a lateral as well as a revolving movement. As it is impossible to
+  use a rigid drive, a flexible coupling F is employed between the shaft B
+  and the motor.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/illo-fig06.png"><img style="width:100%" src="images/illo-fig06.png"
+      alt="Fig. 6." title="Fig. 6." /></a>
+    <span class="sc">Fig.</span> 6.
+  </div>
+
+  <p>Another type of machine is shown in Fig. 7. The drum in this case is
+  stationary, the table T moving laterally by reason of the screwed shaft
+  <!-- Page 18 --><span class="pagenum"><a
+  name="page18"></a>{18}</span><span class="figright" style="width:22%;"><a
+  href="images/illo-fig07.png"><img style="width:100%"
+  src="images/illo-fig07.png" alt="Fig. 7." title="Fig. 7." /></a><span
+  class="sc">Fig.</span> 7.</span> and half nut F. The table, shown
+  separate in Fig. 8, carries a stiff brass spring A, to which is attached
+  a holder B made to take a hardened steel point. The holder is provided
+  with a set screw P for securing the steel point Z. The spring and needle
+  are insulated from the rest of the machine, as shown in the drawing. In
+  working, the metal print is wrapped tightly round the cylinder of the
+  machine, the glue image being, of course, uppermost. To fasten the print
+  a little seccotine should be applied to one edge, and the joint carefully
+  smoothed down with the fingers. <span class="figleft"
+  style="width:27%;"><a href="images/illo-fig08.png"><img
+  style="width:100%" src="images/illo-fig08.png" alt="Fig. 8." title="Fig. 8."
+  /></a><span class="sc">Fig.</span> 8.</span> If there is any tendency on
+  the part of the print to slip round on the drum, a couple of small spring
+  clips placed over the ends of the drum will act as a preventive. It is
+  necessary to place the print upon the drum in such a manner that the
+  stylus draws away from the edge of the lap and not towards it, and the
+  metal prints should be of such a size that when placed round the drum of
+  the <!-- Page 19 --><span class="pagenum"><a
+  name="page19"></a>{19}</span>machine a lap of about
+  <sup>3</sup>/<sub>16</sub>ths of an inch is allowed.</p>
+
+  <div class="figright" style="width:24%;">
+      <a href="images/illo-fig09.png"><img style="width:100%" src="images/illo-fig09.png"
+      alt="Fig. 9." title="Fig. 9." /></a>
+    <span class="sc">Fig.</span> 9.
+  </div>
+
+  <p>The steel point Z (ordinary gramophone needles may be used and will be
+  found to answer the purpose admirably) is made to press lightly upon the
+  metal print, and while the pressure should be sufficient to make good
+  electrical contact, it should not be sufficient to cause the needle to
+  scratch the surface of the foil. The pressure is regulated by means of
+  the milled nut H. The electrical connections are given in Fig. 9. One
+  wire from the battery M is taken to the terminal T, and the other wires
+  from M and F lead to the relay R. The current flows from the battery M
+  through the spring Y, through the drum and metal print, the stylus Z,
+  spring A, down to the relay R, and from R back to the battery M. As the
+  drum carrying the single line half-tone print is revolved, the stylus, by
+  reason of the lateral movement given to the table or cylinder as the case
+  may be, will trace a spiral path over the entire surface of the print. As
+  the stylus traces over a conducting strip the circuit is completed, and
+  the tongue of the relay R is attracted, making contact with the stop S.
+  <!-- Page 20 --><span class="pagenum"><a name="page20"></a>{20}</span>On
+  passing over a strip of insulation the circuit is broken and the tongue
+  of the relay R returns to its normal position.</p>
+
+  <p>As already stated, the conducting and insulating bands on the print
+  vary in width according to the density of the photograph from which it is
+  prepared, so that the length of time that the tongue of the relay R is
+  held against the stop S, is in proportion to the width of the conducting
+  strip which is passing under the stylus at any instant. The function of
+  the transmitter is therefore to send to the relay R an intermittent
+  current of varying duration.</p>
+
+  <p>The two photographs Figs. 10 and 10<i>a</i> are of a machine designed
+  and used by the writer in his experiments. In this machine the drum is
+  3.5 inches long and 1.5 inches in diameter. The lead screw has 30 threads
+  to the inch, and the reduction between it and the drum is 3:1, so that
+  the table has a movement of <sup>1</sup>/<sub>90</sub>th inch per
+  revolution of the drum.</p>
+
+  <p>From the brief description of the various types of machines that have
+  been given it will be apparent that in the design of the machine proper
+  there is nothing very complicated, although the addition of the driving
+  and synchronising apparatus complicates matters rather considerably. The
+  questions of driving and synchronising the machines at the two stations
+  is fully dealt with in Chapter IV.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/illo-fig10a.png"><img style="width:100%" src="images/illo-fig10a.png"
+      alt="Fig. 10a." title="Fig. 10a." /></a>
+    <span class="sc">Fig.</span> 10a.
+  </div>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/illo-fig10b.png"><img style="width:100%" src="images/illo-fig10b.png"
+      alt="Fig. 10b." title="Fig. 10b." /></a>
+    <span class="sc">Fig.</span> 10b. Enlarged view of an image broken up
+    by a cross screen.
+  </div>
+
+<p><!-- Page 21 --><span class="pagenum"><a name="page21"></a>{21}</span></p>
+
+  <p>Although the design of the machines is rather simple great attention
+  must be paid both to accuracy of construction and accuracy of working,
+  and this applies, not only to the machines (whether for transmitting or
+  receiving) but for all the various pieces of apparatus that are used. Too
+  much care cannot be bestowed upon this point, as in the wireless
+  transmission of photographs there is a large number of instruments all
+  requiring careful adjustment, and which have to work together in perfect
+  unison at a high speed.</p>
+
+  <p>The machine shown in Figs. 10 and 10<i>a</i> was designed and used by
+  the writer solely for experimental work. It will be noticed in the
+  description given in the appendix of the method of preparing the metal
+  prints that a 5" × 4" camera is recommended, while the machine, Fig. 10,
+  is designed to take a print procured from a quarter-plate negative. This
+  size of drum was adopted for several reasons, and although it will be
+  found quite large enough for general experimental work the writer has
+  come to the conclusion that for practical commercial work a drum to take
+  a print 5" × 4" will give better results.</p>
+
+  <p>In making a negative of a picture that is required for reproduction
+  purposes, the line screen in the camera is replaced by a "cross screen,"
+  <i>i.e.</i> two single line screens placed with their lines at an angle
+  of 90° to one another, and this breaks the <!-- Page 22 --><span
+  class="pagenum"><a name="page22"></a>{22}</span>image up into small
+  squares instead of lines. By looking at any ordinary newspaper or book
+  illustration through a powerful magnifying glass the effects of a cross
+  screen will readily be seen. With a cross screen a certain amount of
+  detail is necessarily lost, but with a single line screen the amount lost
+  is much greater. If there is any very small detail in the picture most of
+  this would be lost in a coarse screen, hence the necessity of employing
+  as fine a line screen as practicable in order to get as much detail in as
+  possible. It is mainly on this account that a 5" × 4" print is
+  recommended, as, if fairly bold subjects are used for copying, the small
+  detail (this is, of course, a very vague and indefinable term) will not
+  be too fine, and the time required for transmitting reasonable. For
+  obvious reasons it is a great advantage to put the print under pressure
+  to cause the glue image to sink into the soft metal base and leave a
+  perfectly flat and smooth surface. It is essential that the bands on the
+  print lie along the axis of the cylinder, so that the stylus traces its
+  path across them, and not with them.</p>
+
+  <p>We have now an arrangement that is capable of taking the place of the
+  key K, Fig. 4, and the diagram, Fig. 11, gives the connections for the
+  complete transmitter. A is the aerial, E earth, T inductance, L ammeter.
+  The closed oscillatory circuit consists of a spark-gap G, inductance F,
+  <!-- Page 23 --><span class="pagenum"><a
+  name="page23"></a>{23}</span><span class="figright" style="width:38%;"><a
+  href="images/illo-fig11.png"><img style="width:100%"
+  src="images/illo-fig11.png" alt="Fig. 11." title="Fig. 11." /></a><span
+  class="sc">Fig.</span> 11.</span> and a condenser C. The secondary J of
+  the coil H is connected to the spark-gap, and the primary P is in circuit
+  with the mercury break N, the battery B, and the local contacts of the
+  relay R. The action is as follows. When contact is made between the
+  stylus Z and the drum V by means of the conducting bands on the line
+  print, the circuit of the relay R and the battery M is completed. The
+  closing of the local circuit of the relay R actuates the second relay R',
+  allowing the primary circuit of the coil H to be closed. As soon as the
+  primary circuit of the coil is completed sparks pass between the
+  electrodes of the spark-gap G, causing waves to radiate from the aerial.
+  The duration of the wave-trains radiated depends upon the duration of
+  contact made by the relays <!-- Page 24 --><span class="pagenum"><a
+  name="page24"></a>{24}</span>R and R', and this in turn depends upon the
+  width of the conducting strip that is passing under the stylus. The
+  battery M should be about 4 volts, and the battery D about 2 volts. The
+  two-way switch X is connected up so that the relay R' can be thrown out
+  and the key K switched in for ordinary signalling purposes. If any
+  sparking takes place at the point of the stylus, a small condenser C'
+  (about 1 microfarad capacity) should be connected as shown. In the
+  present instance the condenser should be used more as a preventive than
+  as a cure, as in all probability the voltage from M will not be
+  sufficient to cause destructive (if any) sparking; but, as most wireless
+  workers know, anything in the nature of a spark occurring in the
+  neighbourhood of a detector (this, of course, only applies when the
+  receiving apparatus is placed in close proximity to the transmitter) is
+  liable to destroy the adjustment.</p>
+
+  <p>In transmitting over ordinary conductors where the initial voltage is
+  fairly high and the self-induction of the circuit very great, the use of
+  the condenser will be found to be absolutely essential. It has also been
+  noted that the angle which the stylus presents to the drum has a marked
+  effect upon the sparking, an angle of about 60° being found to give very
+  good results.</p>
+
+  <p>If the size of the single line print used is 5 inches by 4 inches, and
+  a screen having 50 lines <!-- Page 25 --><span class="pagenum"><a
+  name="page25"></a>{25}</span>to the inch is used for preparing it, then
+  the stylus will have to make 250 contacts during one revolution of the
+  drum. Assuming the drum to make one revolution in three seconds, then the
+  time taken to transmit the complete photograph can be found from the
+  equation T = <i>w</i> × <i>t</i> × <i>s</i>, where <i>w</i> is the width
+  of the print, <i>t</i> the travel of the stylus during one revolution of
+  the drum, and <i>s</i> the time required for one revolution of the drum.
+  In the present instance this will be T = 4 × 90 × 3 = 1080 seconds = 18
+  minutes. The number of contacts made by the stylus per minute is 5000,
+  and in working at this speed the first difficulty is encountered in the
+  use of the two relays. The relay R is lightly built, and capable of
+  working at a fairly high speed, but R' is a heavier pattern, and
+  consequently works at a slightly lower rate. This relay must necessarily
+  be heavier, as more substantial contacts are needed in order to pass the
+  heavy current taken by the spark-coil.</p>
+
+  <p>Relays sensitive and accurate enough to work at this speed will in all
+  probability be beyond the reach of the majority of workers, but there are
+  several types of relays on the market very reasonable in price that will
+  answer very well for experimental work, although the speed of working
+  will no doubt be slower.</p>
+
+  <p>For the best results the duration of the wave-trains sent out should
+  be of the same duration as <!-- Page 26 --><span class="pagenum"><a
+  name="page26"></a>{26}</span>the contact made by R, and therefore equal
+  to the time taken by the stylus to trace over a conducting strip; but if
+  the duration of the contact made by R is <i>t</i>, then that made by R'
+  and consequently the duration of the groups of wave-trains would be
+  <i>t</i> - <i>v</i> where <i>v</i> equals the extra time required by R'
+  to complete its local circuit. The difference in time made by the two
+  relays, although very slight, will be found to affect very considerably
+  the quality of the received pictures. Renewing the platinum contacts is
+  also a great expense, as they are soon burnt out where a heavy current is
+  passed. If the distance experimented over is short so that the power
+  required to operate the spark-coil is not very heavy, one relay will be
+  sufficient providing the contacts are massive enough to carry the current
+  safely. It is useless to expect any of the ordinary relays in general use
+  to work satisfactorily at such a high speed, and in order to compensate
+  for this we must either increase the time of transmitting, or, as already
+  suggested, make use of a coarser line screen in preparing the
+  photographs.</p>
+
+  <p>For reasons already explained, all points of make and break should be
+  shunted by a condenser. The effective working speed of an ordinary type
+  of relay may be anything from 1000 to 2500 dots a minute, depending upon
+  accuracy of design and construction.</p>
+
+  <p>In the wireless transmission of photographs it <!-- Page 27 --><span
+  class="pagenum"><a name="page27"></a>{27}</span>is absolutely essential
+  to use some form of rotary spark-gap, as where sparks are passed in rapid
+  succession the ordinary type of gap is worse than useless. When a spark
+  passes between the electrodes of an ordinary spark-gap, Fig. 12, we find
+  that for a fraction of a second after the first spark has passed, the
+  normally high resistance of the gap has been lowered to less than one
+  ohm. If the column of hot gas which constitutes the spark is not
+  instantly dispersed, but remains between the electrodes, it will provide
+  an easy path for any further discharges, and if sparks are passed at all
+  rapidly, what was at first a disruptive and oscillatory discharge will
+  degenerate into a hot, non-oscillatory arc.<a name="NtA4"
+  href="#Nt4"><sup>[4]</sup></a></p>
+
+  <div class="figcenter" style="width:18%;">
+      <a href="images/illo-fig12.png"><img style="width:100%" src="images/illo-fig12.png"
+      alt="Fig. 12." title="Fig. 12." /></a>
+    <span class="sc">Fig.</span> 12.
+  </div>
+
+  <p>Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are
+  known as "synchronous" and "non-synchronous" gaps respectively. In the
+  synchronous gap the cog-wheel is mounted on the shaft of the alternator,
+  and a cog comes opposite the fixed electrode when the maximum of
+  potential is reached in the condenser, thus ensuring a discharge at every
+  alternation of current. With this type of gap a spark of pure tone is
+  obtained which <!-- Page 28 --><span class="pagenum"><a
+  name="page28"></a>{28}</span><span class="figleft" style="width:31%;"><a
+  href="images/illo-fig13.png"><img style="width:100%"
+  src="images/illo-fig13.png" alt="Fig. 13." title="Fig. 13." /></a><span
+  class="sc">Fig.</span> 13.</span> <span class="figright"
+  style="width:32%;"><a href="images/illo-fig14.png"><img
+  style="width:100%" src="images/illo-fig14.png" alt="Fig. 14." title="Fig. 14."
+  /></a><span class="sc">Fig.</span> 14.</span> is of great value where the
+  signals are received by means of a telephone, but where the signals are
+  to be mechanically recorded the tone of the spark is of little
+  consequence. In a non-synchronous gap a separate motor is used for
+  driving the toothed wheel, and can either be mounted on the motor shaft
+  or driven by means of a band, there being no regard given to synchronism
+  with the alternator. The fixed electrode is best made long enough to
+  cover about two of the teeth, as this ensures regular sparking and a
+  uniform sparking distance; the <!-- Page 29 --><span class="pagenum"><a
+  name="page29"></a>{29}</span>spark length is double the length of the
+  spark-gap. The toothed wheel should revolve at a high speed, anything
+  from 5000 to 8000 revolutions per minute, or even more being required.
+  The shaft of the toothed wheel is preferably mounted in
+  ball-bearings.</p>
+
+  <p>Owing to the large number of sparks that are required per minute in
+  order to transmit a photograph at even an ordinary speed, it is necessary
+  that the contact breaker be capable of working at a very high speed
+  indeed. The best break to use is what is known as a "mercury jet"
+  interrupter, the frequency of the interruptions being in some cases as
+  high as 70,000 per second. No description of these breaks will be given,
+  as the working of them is generally well understood.</p>
+
+  <p>In some cases an alternator is used in place of the battery B, Fig. 4,
+  and when this is done the break M can be dispensed with. In larger
+  stations the coil H is replaced with a special transformer.</p>
+
+  <p>The writer has designed an improved relay which will respond to
+  currents lasting only <sup>1</sup>/<sub>100</sub>th part of a second, and
+  capable of dealing with rather large currents in the local circuit.<a
+  name="NtA5" href="#Nt5"><sup>[5]</sup></a> This relay has not yet been
+  tried, but if it is successful the two relays R and R' can be dispensed
+  with, and the result will be more accurate and effective
+  transmission.</p>
+
+<p><!-- Page 30 --><span class="pagenum"><a name="page30"></a>{30}</span></p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig15.png"><img style="width:100%" src="images/illo-fig15.png"
+      alt="Fig. 15." title="Fig. 15." /></a>
+    <span class="sc">Fig.</span> 15.
+  </div>
+
+  <p>The connections for a complete experimental station, transmitting and
+  receiving apparatus combined, are given in Fig. 15. The terminals W, W
+  are for connecting to the photo-telegraphic receiving apparatus Q, being
+  a double pole two-way switch for throwing either the transmitting or
+  receiving apparatus in circuit. There is another system of transmitting
+  devised by Professor Korn, which employs an entirely different method
+  from the foregoing. By using the apparatus just described, the waves
+  generated are what are known as "damped waves," and by using these damped
+  waves, tuning, which is so essential to good commercial working, can be
+  made to reach a fairly high degree of efficiency. <!-- Page 31 --><span
+  class="pagenum"><a name="page31"></a>{31}</span></p>
+
+  <p>The question of damped <i>versus</i> undamped waves is a somewhat
+  burning one, and no attempt will be made here to deal with the merits or
+  demerits of the claims made for the respective systems. A series of
+  articles describing the production of undamped waves and their efficiency
+  in working compared with damped waves will be found in the <i>Wireless
+  World</i>, Nos. 3 and 4, 1913, and are well worth reading by any one
+  interested in the subject.</p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/illo-fig16.png"><img style="width:100%" src="images/illo-fig16.png"
+      alt="Fig. 16." title="Fig. 16." /></a>
+    <span class="sc">Fig.</span> 16.
+  </div>
+
+  <p>A diagrammatic representation of the apparatus as arranged by
+  Professor Korn is given in Fig. 16. The undamped or "continuous" waves
+  are generated by means of a high-frequency alternator or Poulsen arc. In
+  Fig. 16, X is the generator, F inductance, C condenser; the aerial
+  inductance T is connected by the aerial A and earth E. By this means the
+  waves are tuned to a certain period. <!-- Page 32 --><span
+  class="pagenum"><a name="page32"></a>{32}</span>A metal print, similar to
+  what has already been described, is wrapped round the drum D of the
+  machine, and when the stylus Z traces over an insulating strip the waves
+  generated are in tune with the receiving station, but when it traces over
+  a conducting strip, a portion of the inductance T is short-circuited, the
+  period of the oscillations is altered, and the two stations are thrown
+  out of tune.</p>
+
+  <p>The receiving station is provided with an aperiodic circuit, which
+  consists of an inductance F', condenser C', and a thermodetector N. A
+  string galvanometer H (described in Chapter III.), and the self-induction
+  coils B, B' are connected as shown, the coils B, B' preventing the
+  high-frequency currents, which change their direction, from flowing
+  through the galvanometer. The manner in which the string galvanometer is
+  arranged to reproduce a transmitted picture is shown in Fig. 24.</p>
+
+  <p>The connections adopted by the Poulsen Company for photographically
+  recording wireless messages are given in Fig. 17, a string galvanometer
+  of the Einthoven type being used. The two self-induction coils S and S'
+  are in circuit with the detector D and the galvanometer G. The condenser
+  C' prevents the continuous current produced by the detector from flowing
+  through the high frequency circuit; P is the primary of the aerial <!--
+  Page 33 --><span class="pagenum"><a
+  name="page33"></a>{33}</span>inductance and F the secondary. The method
+  of transmitting adopted by Professor Korn appears to be a simple and
+  reliable arrangement, provided that an equally reliable method of
+  producing the undamped waves can be found. Owing to the absence of
+  mechanical inertia it should be capable of working at a good speed, while
+  the absence of a number of pieces of delicate apparatus all requiring
+  careful adjustment add greatly to its reliability.</p>
+
+  <div class="figcenter" style="width:28%;">
+      <a href="images/illo-fig17.png"><img style="width:100%" src="images/illo-fig17.png"
+      alt="Fig. 17." title="Fig. 17." /></a>
+    <span class="sc">Fig.</span> 17.
+  </div>
+
+  <p>In any spark system with a properly designed aerial a coil taking ten
+  amperes is capable of transmitting signals over a distance of thirty to
+  fifty miles, but where the number of interruptions of the break required
+  per second is very high, as in radio-photography, it must be remembered
+  that a much higher voltage is needed to drive the requisite amount of
+  current through the primary winding of the coil than would be the case if
+  the interruptions were slower. It is possible to use platinum <!-- Page
+  34 --><span class="pagenum"><a name="page34"></a>{34}</span>contacts for
+  the relays, for currents up to ten amperes, but for heavier currents than
+  this some arrangement where contact is made with mercury will be found to
+  be more economical and reliable.</p>
+
+  <p>In the transmitter already described and given in Fig. 11, the best
+  results would be obtained by finding the speed at which the relay R'
+  works best, and regulating the number of contacts made by the stylus
+  accordingly.</p>
+
+  <p>The method employed by De' Bernochi (see Chapter I.) of varying the
+  intensity of a beam of light by passing it through a photographic film,
+  which in turn alters the resistance of a selenium cell, has been very
+  successfully employed in at least one system of photo-telegraphy. Its
+  application has also been suggested for wireless transmission, and
+  although with any system using continuous waves this would not be very
+  difficult, it could hardly be adapted to work with the ordinary spark
+  system. The apparatus for receiving from this type of transmitter would,
+  on the other hand, necessarily be more elaborate than the methods that
+  are described in the next chapter, and as far as the writer's experience
+  goes, experiments along these lines would not prove very profitable, as
+  simplicity is the keynote of success in any radio-photographic
+  system.</p>
+
+  <p>It has been suggested that in order to decrease the time of
+  transmission a cylinder capable of <!-- Page 35 --><span
+  class="pagenum"><a name="page35"></a>{35}</span>taking a print 7 inches
+  by 5 inches be employed, the print being prepared from rather a coarse
+  line screen&mdash;say 35 to the inch&mdash;and a traverse of about
+  <sup>1</sup>/<sub>50</sub> inch given to the stylus, thus reducing the
+  time of transmission to about twelve minutes. It is questionable,
+  however, whether the increase in speed would compensate for the loss of
+  detail, as only very bold subjects could be transmitted. As already
+  pointed out, wireless transmission would only be employed for fairly long
+  distances, and the extra time and expense required to receive a fairly
+  good detailed picture is negligible when compared with the enormous time
+  it would take to receive the original photograph by any ordinary means of
+  transit.</p>
+
+  <p>The public much prefer to have passable pictorial illustrations of
+  current events than wait several days for a more perfect
+  picture&mdash;the original, and the advantage of any newspaper being able
+  to publish photographs several days before its rivals is obvious. There
+  can also be no doubt but that a system of radio-photography, if fairly
+  reliable and capable of working over a distance of say thirty miles,
+  would be of great military use for transmitting maps and written matter
+  with a great saving of time and even life. Written matter could be
+  transmitted with even greater safety than messages which are sent in the
+  ordinary way in Morse Code, as the signals received in the receiver <!--
+  Page 36 --><span class="pagenum"><a name="page36"></a>{36}</span>of an
+  hostile installation would be but a meaningless jumble of sounds, and
+  even were they possessed of radio-photographic apparatus the received
+  message would be unintelligible, unless they knew the exact speed at
+  which the machines were running and could synchronise accurately.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 37 --><span class="pagenum"><a name="page37"></a>{37}</span></p>
+
+<h3>CHAPTER III</h3>
+
+<p class="cenhead">RECEIVING APPARATUS</p>
+
+  <p>There are only two methods available at present for receiving the
+  photographs, and both have been used in ordinary photo-telegraphic work
+  with great success. They have disadvantages when applied to wireless
+  work, however, but these will no doubt be overcome with future
+  improvements. The two methods are (1) by means of an ordinary
+  photographic process, and (2) by means of an electrolytic receiver.</p>
+
+  <p>In several photo-telegraphic systems the machine used for transmitting
+  has the cylinder twice the size of the receiving cylinder, thus making
+  the area of the received picture one-quarter the area of the picture
+  transmitted. The extra quality of the received picture does not
+  compensate for the disadvantage of having to provide two machines at each
+  station, and in the writer's opinion results, quite good enough for all
+  practical purposes, can be obtained by using a moderate size cylinder so
+  that one machine answers for both transmitting <!-- Page 38 --><span
+  class="pagenum"><a name="page38"></a>{38}</span>and receiving, and using
+  as fine a line screen as possible for preparing the photographs.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/illo-fig18.png"><img style="width:100%" src="images/illo-fig18.png"
+      alt="Fig. 18." title="Fig. 18." /></a>
+    <span class="sc">Fig.</span> 18.
+  </div>
+
+  <p>The writer, when first experimenting in photo-telegraphy, endeavoured
+  to make the receiving apparatus "self-contained," and one idea which was
+  worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and
+  is placed just within the focus of a lens which has a focal length of
+  <sup>3</sup>/<sub>4</sub> inch. When a source of light is placed at some
+  point between a lens and its principal focus, the light rays are not
+  converged, but are transmitted in a parallel beam the same size as the
+  lens. It has been found that this arrangement gives a sharper line on the
+  drum than would be the case were the light focussed direct upon the hole
+  in the cone A. An enlarged drawing of the cone is given in Fig. 19. The
+  hole in the tip of the cone A is a bare <sup>1</sup>/<sub>90</sub> inch
+  in diameter&mdash;the size of this hole depends upon the travel per
+  revolution of the drum or table of the machine used&mdash;and in working,
+  the cone is run as close as possible to the <!-- Page 39 --><span
+  class="pagenum"><a name="page39"></a>{39}</span>drum without being in
+  actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and
+  the armature is made as light as possible. The spring to which the
+  armature is attached should be of such a length that its natural period
+  of vibration is equal to the number of contacts made by the transmitting
+  stylus. The spring must be stiff enough to bring the armature back with a
+  fairly crisp movement. The spring and armature is shown separate in Fig.
+  20.</p>
+
+  <div class="figcenter" style="width:23%;">
+      <a href="images/illo-fig19.png"><img style="width:100%" src="images/illo-fig19.png"
+      alt="Fig. 19." title="Fig. 19." /></a>
+    <span class="sc">Fig.</span> 19.
+  </div>
+
+  <div class="figright" style="width:15%;">
+      <a href="images/illo-fig20.png"><img style="width:100%" src="images/illo-fig20.png"
+      alt="Fig. 20." title="Fig. 20." /></a>
+    <span class="sc">Fig.</span> 20.
+  </div>
+
+  <p>The shutter C is about <sup>1</sup>/<sub>4</sub> inch square and made
+  from thin aluminium. The hole in the centre is <sup>1</sup>/<sub>16</sub>
+  × <sup>1</sup>/<sub>8</sub> inch, and the movement of the armature is
+  limited to about <sup>3</sup>/<sub>32</sub> inch. In all arrangements of
+  this kind there is a tendency for the armature spring to vibrate, as it
+  were, sinusoidally, if the coil is magnetised and demagnetised at a
+  higher rate than the natural period of vibration of the spring. <!-- Page
+  40 --><span class="pagenum"><a name="page40"></a>{40}</span>This causes
+  an irregularity in the rate of the vibrations which affects the received
+  image very considerably. A photographic film is wrapped round the drum of
+  the machine, being fastened by means of a little celluloid cement smeared
+  along one edge.</p>
+
+  <p>This device, although it will work well over artificial conductors, is
+  not suitable for wireless work, as it is too coarse in its action; it can
+  be made sensitive enough to work at a speed of 1000 to 1500 contacts per
+  minute, with a current of .5 milliampere. It is impossible to obtain a
+  current of this magnitude from the majority of the detectors in use, so
+  that if any attempt is made to use this device for radio-photography it
+  will be necessary to employ a Marconi coherer (filings), as this is
+  practically the only coherer from which so large a current can be
+  obtained.</p>
+
+  <p>There have been many attempts made to receive with an ordinary filings
+  coherer, but as was pointed out in Chapter I. these have now been
+  discarded in serious wireless work, being only used in small amateur
+  stations or experimental sets. As the reasons for this are well known to
+  the majority of wireless workers there is no need to enumerate them
+  here.</p>
+
+  <p>A method whereby a filings coherer can be decohered, the act of
+  decohering closing a local circuit which contains the photographic <!--
+  Page 41 --><span class="pagenum"><a
+  name="page41"></a>{41}</span>receiving apparatus, is given in the diagram
+  Fig. 21.</p>
+
+  <div class="figcenter" style="width:36%;">
+      <a href="images/illo-fig21.png"><img style="width:100%" src="images/illo-fig21.png"
+      alt="Fig. 21." title="Fig. 21." /></a>
+    <span class="sc">Fig.</span> 21.
+  </div>
+
+  <p>In the figure, the coherer C is fixed in rigid supports, one support
+  being provided with a platinum pin F. To the coherer is connected the
+  sensitive electro-magnet M, which becomes magnetised as soon as the
+  incoming waves act upon the coherer. To the armature B is attached a
+  light aluminium arm S, pivoted at K, and carrying at the other end the
+  striker G, which is fitted with a platinum contact. When the armature B
+  is attracted the coherer is decohered by the force of the impact between
+  the contacts F and G. To prevent damage to the coherer the force of the
+  blow is taken off by the ability of the striker to work back through a
+  hole in the arm S, the spring <!-- Page 42 --><span class="pagenum"><a
+  name="page42"></a>{42}</span>N keeping it normally in a fixed position. T
+  and P are adjusting screws, and the terminals J are for connecting to the
+  receiving apparatus. With this arrangement a very short wave-train causes
+  only one tap of the contacts, so that only one mark is registered on the
+  receiving drum for every contact made on the transmitter.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig22.png"><img style="width:100%" src="images/illo-fig22.png"
+      alt="Fig. 22." title="Fig. 22." /></a>
+    <span class="sc">Fig.</span> 22.
+  </div>
+
+  <p>The drawing, Fig. 22, gives a diagrammatic representation of apparatus
+  arranged for another photographic method of receiving. The machine shown
+  in Fig. 6 is used in this case. A is the aerial, E earth, P primary of
+  oscillation-transformer, S secondary of transformer, C variable
+  condenser, C' block condenser, D detector, X two-way switch, T
+  telephone.</p>
+
+  <p>A De' Arsonval galvanometer H is also connected to the switch X, so
+  that either the telephone or the galvanometer can be switched in. The
+  <!-- Page 43 --><span class="pagenum"><a
+  name="page43"></a>{43}</span>galvanometer can be made sensitive enough to
+  work with a current as small as 10<sup>-7</sup> of an ampere, with a
+  period of about <sup>1</sup>/<sub>150</sub>th of a second. The screen J
+  has a small hole about <sup>1</sup>/<sub>8</sub> inch diameter drilled in
+  the centre. Under the influence of the brief currents which pass through
+  the detector every time a group of waves is received, the mirror of the
+  galvanometer swings to-and-fro in front of the screen J, and allows the
+  light reflected from the source of light M to pass through the aperture
+  in the screen, on to the lens N.</p>
+
+  <p>Round the drum V of the machine is wrapped a sensitive photographic
+  film, and this records the movements of the mirror which correspond to
+  the contacts on the half-tone print used in transmitting. Every time
+  current passes through the galvanometer, the light that is received from
+  M,<a name="NtA6" href="#Nt6"><sup>[6]</sup></a> passes through the
+  aperture in the screen J, and is focussed by the lens N to a point upon
+  the revolving film. As soon as the current ceases, the mirror swings back
+  to its original position, and the film is again in darkness. Upon being
+  developed a photograph, similar to the negative used for preparing the
+  metal print is obtained. If desired the apparatus can be so arranged that
+  the received picture is a positive instead of a negative.</p>
+
+<p><!-- Page 44 --><span class="pagenum"><a name="page44"></a>{44}</span></p>
+
+  <p>The detector used should be a Lodge wheel-coherer or a Marconi
+  valve-receiver, as these are the only detectors that can be used with a
+  recording instrument. If the swing of the galvanometer mirror is too
+  great, a small battery with a regulating resistance can be inserted in
+  order to limit the movement of the mirror to a very short range; the
+  current of course flowing in an opposite direction to the current flowing
+  through the coherer.</p>
+
+  <p>In this, as in all other methods of receiving, the results obtained
+  depend upon the fineness of the line screen used in preparing the metal
+  prints; and as already shown the fineness of the screen that can be used
+  is dependent upon the mechanical efficiency of the entire apparatus.</p>
+
+  <p>Another system, and one that has been tried as a possible means of
+  recording wireless messages, is as follows. The wireless arrangements
+  consist of apparatus similar to that shown in Fig. 22, but instead of a
+  Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is
+  substituted for the reflecting galvanometer. The Einthoven galvanometer
+  consists of a very powerful electro-magnet, the pole pieces of which
+  converge almost to points. A very fine silvered quartz thread is
+  stretched between the pole pieces, as shown in Fig. 23, the tension being
+  adjustable. The period of swing is about <sup>1</sup>/<sub>250</sub>th of
+  a second. A hole is bored through the poles, and one of them is fitted
+  <!-- Page 45 --><span class="pagenum"><a
+  name="page45"></a>{45}</span><span class="figright" style="width:30%;"><a
+  href="images/illo-fig23.png"><img style="width:100%"
+  src="images/illo-fig23.png" alt="Fig. 23." title="Fig. 23." /></a><span
+  class="sc">Fig.</span> 23.</span> with a sliding tube which carries a
+  short focus lens N. The light from M passes through the magnets, and a
+  magnified image of the quartz thread is thrown upon the ebonite screen J.
+  This screen is provided with a fine slit, and when the galvanometer is at
+  rest the shadow of the thread just covers the slit in the screen and
+  prevents any light from M reaching the photographic film. Upon signals
+  being received the shadow of the thread moves to one side for a long or
+  short period, uncovering the slit, and allowing light to pass through.
+  The lens R concentrates the collected light to a point upon the revolving
+  film. The connections for the complete receiver are given in Fig. 24.</p>
+
+  <p>The modified form of the Einthoven galvanometer, as arranged by
+  Professor Korn for use with his selenium machines for photo-telegraphy
+  over ordinary land lines, consists of two fine silver wires which are
+  displaced in a lateral direction between the pole pieces when traversed
+  by a current; the current passing through both wires in the same <!--
+  Page 46 --><span class="pagenum"><a
+  name="page46"></a>{46}</span>direction. A small shutter of aluminium foil
+  is attached to the wires at the optical centre. The silver wires used are
+  <sup>1</sup>/<sub>1000</sub> inch in diameter, with a natural period of
+  about <sup>1</sup>/<sub>120</sub>th of a second; the length of wires free
+  to swing being usually about 5 cm.</p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/illo-fig24.png"><img style="width:100%" src="images/illo-fig24.png"
+      alt="Fig. 24." title="Fig. 24." /></a>
+    <span class="sc">Fig.</span> 24.
+  </div>
+
+  <p>The period of the wires depends to a great extent upon their length
+  and diameter, and also upon their tension. By using short fine wires the
+  period can be made much smaller, but a greater current is required to
+  produce a similar displacement. Where the current available, as in
+  wireless telegraphy, is very small, and a definite displacement of the
+  wires is required, it is at once apparent that with wires of a given
+  diameter there is a limit to their length and therefore to the period.
+  Finer wires can be used, but here again there is a practical limit to
+  their fineness, although galvanometers have been constructed with a
+  single silvered quartz thread <sup>1</sup>/<sub>12000</sub>th of an inch
+  diameter, which, when placed in a powerful field, will give a good
+  displacement with a current as small as 10<sup>-8</sup> ampere. <!-- Page
+  47 --><span class="pagenum"><a name="page47"></a>{47}</span></p>
+
+  <p>With the apparatus arranged by the Poulsen Company, given in the
+  diagram, Fig. 17, for photographically recording wireless signals, the
+  current required to operate the galvanometer for signals transmitted at
+  the rate of 1500 a minute is 1 × 10<sup>-6</sup> ampere, while for
+  signals up to 2500 a minute a current about 5 × 10<sup>-6</sup> ampere is
+  necessary.</p>
+
+  <p>Another very sensitive instrument, employed by M. Belin, and known as
+  Blondel's oscillograph, consists of two fine wires stretched between the
+  poles of a powerful electro-magnet, a small and very light mirror being
+  attached to the centre of the wires. The current passes down one wire and
+  up the other, and the wires, together with the mirror, are twisted to a
+  degree depending upon the strength of the received current. In order to
+  render the instrument dead-beat the moving parts are arranged to work in
+  oil. The light reflected from the mirror is made use of in a manner
+  similar to that shown in Fig. 22.</p>
+
+  <p>In all photographic methods of receiving, the apparatus must be
+  enclosed in some way to prevent any extraneous light from reaching the
+  film, or better still placed in a room lighted only by means of a ruby
+  light.</p>
+
+  <p>The following method is given more as a suggestion than anything else,
+  as I do not think it has been tried for wireless receiving, although it
+  is stated to have given some good results over <!-- Page 48 --><span
+  class="pagenum"><a name="page48"></a>{48}</span>ordinary land lines. It
+  is the invention of Charbonelle, a French engineer, and is quite an
+  original idea. His method consists of placing a sheet of carbon paper
+  between two sheets of thin white paper, and wrapping the whole tightly
+  round the drum of the machine. A hardened steel point is fastened to the
+  diaphragm of a telephone receiver, and this receiver is placed so that
+  the steel point presses against the sheets of paper. As the diaphragm and
+  steel point vibrates under the influence of the received currents marks
+  are made by the carbon sheet on the bottom paper.</p>
+
+  <p>Over a line where a fair amount of current is available at the
+  receiver, the diaphragm would have sufficient movement to mark the paper,
+  but the movement would be very small with the current received from a
+  detector. This difficulty could no doubt be overcome to a certain extent
+  by making a special telephone receiver, with a large and very flexible
+  diaphragm, and wound for a very high resistance. The movement of an
+  ordinary telephone diaphragm for a barely audible sound is, measured at
+  the centre, about 10<sup>-6</sup> of a c.m. With a unit current the
+  movement at the centre is about <sup>1</sup>/<sub>700</sub>th of an inch.
+  Greater movement of the diaphragm could be obtained by connecting a
+  <i>Telephone relay</i> to the detector, and using the magnified current
+  from the relay to operate the telephone. <!-- Page 49 --><span
+  class="pagenum"><a name="page49"></a>{49}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/illo-fig25.png"><img style="width:100%" src="images/illo-fig25.png"
+      alt="Fig. 25." title="Fig. 25." /></a>
+    <span class="sc">Fig.</span> 25.
+  </div>
+
+  <p>The telephone relay consists of a microphone C, Fig. 25, formed of the
+  two pieces of osmium iridium alloy. The contact is separated to a minute
+  degree partly by the action of the local current from F, which flows
+  through it and also through the winding W of the two magnet coils. The
+  local current from F assists in forming the microphone by rendering the
+  space between the contacts conductive. The vibrating reed P is fastened
+  to the metal frame (not shown) which carries a micrometer screw by which
+  the distance between the contacts can be accurately regulated. It will be
+  seen from Fig. 25 that the local circuit consists of a battery F (about
+  1.5 volts), the microphone contacts C, the windings W, milliampere meter
+  B, and the terminals T, for connecting to the galvanometer or telephone,
+  all in <!-- Page 50 --><span class="pagenum"><a
+  name="page50"></a>{50}</span>series. On the top of the magnet cores N, S
+  is a smaller magnet D, wound with fine wire for a resistance of about
+  4935 ohms, the free ends of the coils being connected to the detector
+  terminals. The working is as follows. Supposing the current from the
+  detector flows through D in such a way that its magnetism is increased,
+  the reed P will be attracted, the contacts opened, and their resistance
+  increased. It will be seen that the current from F is passed through the
+  coils W, in such a way as to increase the magnetism of the permanent
+  magnet, so that any opening of the microphone contact increases their
+  resistance, causes the current to fall, and weakens the magnets to such
+  an extent that the reed P can spring back to its normal position. On the
+  other hand, if the detector current flows through D in such a direction
+  as to decrease the magnetism in the permanent magnets, the reed P will
+  rise and make better contact owing to the removal of the force opposing
+  the stiffness of the reed. Owing to the decrease in the resistance of the
+  microphone, the strength of the local current will be increased, the
+  magnets strengthened, and the reed P will be pulled back to its original
+  position. This relay gives a greatly magnified current when properly
+  adjusted, the current being easily increased from 10<sup>-4</sup> to
+  10<sup>-2</sup> amperes. It is also very sensitive, but needs careful
+  adjustment in order that the best results may <!-- Page 51 --><span
+  class="pagenum"><a name="page51"></a>{51}</span>be obtained. A greater
+  range of magnification can be obtained by placing two or more relays in
+  series.</p>
+
+  <div class="figcenter" style="width:23%;">
+      <a href="images/illo-fig26.png"><img style="width:100%" src="images/illo-fig26.png"
+      alt="Fig. 26." title="Fig. 26." /></a>
+    <span class="sc">Fig.</span> 26.
+  </div>
+
+  <p>A very sensitive receiver designed by the writer is given in the
+  figures 26 and 27. To the centre of a telephone diaphragm is fastened a
+  light steel point P, and the movement of this point is communicated to
+  the aluminium arm D, which is pivoted at C. As will be seen the telephone
+  receiver is of special construction, it containing only one coil and
+  therefore only one core; by this means the movement of the diaphragm is
+  centralised. The coil is wound for a resistance of about 200 ohms, and
+  the diaphragm should be fairly thin but very resillient.</p>
+
+  <div class="figcenter" style="width:23%;">
+      <a href="images/illo-fig27.png"><img style="width:100%" src="images/illo-fig27.png"
+      alt="Fig. 27." title="Fig. 27." /></a>
+    <span class="sc">Fig.</span> 27.
+  </div>
+
+  <p>To the free end of D is fastened the mirror T, made from thin
+  diaphragm glass about 1<sup>1</sup>/<sub>2</sub> centimetres diameter,
+  and having a focal length of 40 inches. Light from the lamp L is
+  transmitted by the lens N in a parallel beam to the mirror which <!--
+  Page 52 --><span class="pagenum"><a
+  name="page52"></a>{52}</span>concentrates it to a point upon a hole
+  <sup>1</sup>/<sub>100</sub>th of an inch in diameter in the screen J. As
+  the telephone diaphragm vibrates under the influence of the received
+  signals the arm, and consequently the mirror, vibrates also, and the hole
+  in the screen J is constantly being covered and uncovered by the spot of
+  light. It will be seen from Fig. 27 that the ratio between the centre of
+  the mirror and the pivot C, and C and the steel point P is 10:1, so that
+  if a movement of <sup>1</sup>/<sub>20000</sub>th of an inch is obtained
+  at the centre of the diaphragm the mirror will move
+  <sup>1</sup>/<sub>2000</sub>th of an inch; and as the focal length of the
+  mirror is 40 inches a movement of <sup>1</sup>/<sub>50</sub>th inch is
+  given to the spot of light.</p>
+
+  <p>This receiver is capable of working at a fairly high speed, as the
+  inertia of the moving parts is practically negligible; the weight of the
+  arm and mirror being less than 20 grains. The hole in the screen is made
+  slightly less in diameter than the traverse of the revolving cylinder,
+  the slight distance between the cylinder and the screen allowing the
+  light to disperse sufficiently to produce a line on the film of about the
+  right thickness.</p>
+
+  <p>There are two other possible means of photographically receiving the
+  picture that upon investigation may yield some results; but it is
+  doubtful whether the current available, even that obtained from a
+  telephone relay, will be sufficient to produce the desired magnetic
+  effect, and the <!-- Page 53 --><span class="pagenum"><a
+  name="page53"></a>{53}</span>insertion of a second relay would detract
+  greatly from the efficiency by decreasing the speed of working. If rays
+  of monochromatic light from a lamp L, Fig. 28, pass through a Nicol prism
+  P (polarising prism), then through a tube containing CS<sub>2</sub>
+  (carbon bisulphide), afterwards passing through the second prism P'
+  (analysing prism), and if the two Nicol prisms are set at the polarising
+  angle, no light from L would reach the photographic film wrapped round
+  the drum V of the machine. Upon the tube being subjected to a field
+  produced by a current passing through the coil C, the refractive index of
+  the liquid will be changed, and light from L will reach the photographic
+  film.<a name="NtA7" href="#Nt7"><sup>[7]</sup></a></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/illo-fig28.png"><img style="width:100%" src="images/illo-fig28.png"
+      alt="Fig. 28." title="Fig. 28." /></a>
+    <span class="sc">Fig.</span> 28.
+  </div>
+
+  <p>The second method is rather more complicated, and is based upon the
+  fact that the kathode rays in a Crookes' tube can be deflected from their
+  course by means of a magnet. In Fig. 29 the kathode K of the X-ray tube
+  sends a kathode ray discharge through an aperture in the anode A, through
+  a small aperture in the ebonite screen J <!-- Page 54 --><span
+  class="pagenum"><a name="page54"></a>{54}</span>on to the drum V of the
+  machine, round which is wrapped a photographic film; A and K being
+  connected to suitable electrical apparatus. Upon the coil M being
+  energised, the kathode-ray is deflected from its straight-line course,
+  and the drum V is left in darkness.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/illo-fig29.png"><img style="width:100%" src="images/illo-fig29.png"
+      alt="Fig. 29." title="Fig. 29." /></a>
+    <span class="sc">Fig.</span> 29.
+  </div>
+
+  <p>The method which is now going to be described is very ingenious, as it
+  makes use of what is known as an electrolytic receiver. This method of
+  receiving has proved to be the most practical and simple of all the
+  photo-telegraphic systems that have been devised.</p>
+
+  <p>The application of this system to wireless reception is as follows.
+  The aerial A, and the earth E, are joined to the primary P of a
+  transformer, the secondary S being connected to a Marconi valve receiver
+  C. The valve receiver is connected to the battery B and silvered quartz
+  thread K of an Einthoven galvanometer (already described). The thread is
+  <sup>1</sup>/<sub>12000</sub>th of an inch in diameter, and will respond
+  to currents as small as 10<sup>-8</sup> of <!-- Page 55 --><span
+  class="pagenum"><a name="page55"></a>{55}</span>an ampere. The light from
+  M throws an enlarged shadow of the thread over a slit in the screen J,
+  and as the thread moves to one side under the influence of a current, the
+  slit in J is uncovered, and the light from M is thrown upon a small
+  selenium cell R. In the dark the selenium cell has a very high
+  resistance, and therefore no current can flow from the battery D to the
+  relay F. When the string of the galvanometer moves to one side and
+  uncovers the slit in the screen J, a certain amount of light is thrown
+  upon the selenium cell lowering its resistance, allowing sufficient
+  current to pass through to operate the relay.</p>
+
+  <p>Round the drum of the machine (shown in Fig. 7) is wrapped a sheet of
+  paper that has been soaked in certain chemicals that are decomposed on
+  the passage of an electric current through them. As soon as the local
+  circuit of the relay is closed, the current from the battery Z (about 12
+  volts) flows through the paper and produces a coloured mark. The picture,
+  therefore, is composed of long or short marks which correspond to the
+  varying strips of conducting material on the single line print. In order
+  to render the marks short and crisp, a small battery Y, and regulating
+  resistance L, is placed across the drum and stylus. The diagram, Fig. 30,
+  gives the connections for the complete receiver. <!-- Page 56 --><span
+  class="pagenum"><a name="page56"></a>{56}</span></p>
+
+  <p>The paper used is soaked in a solution consisting of</p>
+
+<table class="nob" summary="Soaking solution." title="Soaking solution.">
+<tr><td class="nspacsingle"> Ferrocyanide of potassium&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </td><td class="nspacsingle" style="text-align:right;"> <sup>1</sup>/<sub>4</sub> oz.</td></tr>
+<tr><td class="nspacsingle"> Ammoniac Nitrate </td><td class="nspacsingle" style="text-align:right;"> <sup>1</sup>/<sub>2</sub> oz.</td></tr>
+<tr><td class="nspacsingle"> Distilled water<a name="NtA8" href="#Nt8"><sup>[8]</sup></a> </td><td class="nspacsingle" style="text-align:right;"> 4 oz.</td></tr>
+</table>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/illo-fig30.png"><img style="width:100%" src="images/illo-fig30.png"
+      alt="Fig. 30." title="Fig. 30." /></a>
+    <span class="sc">Fig.</span> 30.
+  </div>
+
+  <p>The paper has to be very carefully chosen, as besides being absorbent
+  enough to remain moist during the whole of the receiving, the surface
+  must also remain fairly smooth, as with a rough paper the grain shows
+  very distinctly, and if there is an excess of solution the electrolytic
+  marks are inclined to spread and so cause a blurred image. The writer
+  tried numerous specimens of paper before one could be found that gave
+  really satisfactory results. It was also found that when working in a
+  warm room the paper became nearly <!-- Page 57 --><span
+  class="pagenum"><a name="page57"></a>{57}</span>dry before the receiving
+  was finished, and the resistance of the paper being greatly increased
+  (this may be anything up to 1000 ohms), the marking became very faint. A
+  sponge moistened with the solution and applied to the undecomposed
+  portion of the paper, while still revolving, was found to help matters
+  considerably.</p>
+
+  <p>Another experience which happened during the writer's early
+  experiments, the cause of which I am still unable to explain, occurred in
+  connection with the stylus. The stylus used consisted of a sharply
+  pointed steel needle, and after working for about three minutes it was
+  noticed that the lines were becoming gradually wider, finally running
+  into each other. Upon examination it was found that the point of the
+  needle had worn away considerably, becoming in fact, almost a chisel
+  point. Almost every needle tried acted in a similar manner, and to
+  overcome this difficulty the stylus shown in Fig. 31 was devised.</p>
+
+  <p>It will be seen that it consists of a holder A, somewhat resembling a
+  drill chuck, fastened to the flat spring B in such a manner that the
+  angle the stylus makes to the drum can be altered. The needle consists of
+  a length of 36-gauge steel wire, and as this wears away slowly the jaws
+  of the holder can be loosened and a fresh length pushed through. The wire
+  should not project beyond the face of the holder more than
+  <sup>1</sup>/<sub>8</sub>th inch. The gauge <!-- Page 58 --><span
+  class="pagenum"><a name="page58"></a>{58}</span>of wire chosen would not
+  suit every machine, the best gauge to use being found by trial, but in
+  the writer's machine the pitch of the decomposition marks is much finer
+  than of those made by the commercial machines, and this gauge, with the
+  slight but unavoidable spreading of the marks, will produce a mark of
+  just the right thickness. As already mentioned, no explanation of this
+  peculiarity on the part of the stylus can be given, as there is nothing
+  very corrosive in the solution used, and the pressure of the stylus upon
+  the paper is so slight as to be almost negligible.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/illo-fig31.png"><img style="width:100%" src="images/illo-fig31.png"
+      alt="Fig. 31." title="Fig. 31." /></a>
+    <span class="sc">Fig.</span> 31.
+  </div>
+
+  <p>No special means are required for fastening the paper to the drum, the
+  moist paper adhering quite firmly. Care should be taken, however, to
+  fasten the paper&mdash;which should be long enough to allow for a lap of
+  about <sup>1</sup>/<sub>4</sub> inch&mdash;in such a manner that when
+  working the stylus draws away from the edge of the lap and not towards
+  it.</p>
+
+  <p>The current required to produce electrolysis is very small, about one
+  milliampere being sufficient. <!-- Page 59 --><span class="pagenum"><a
+  name="page59"></a>{59}</span>Providing that the voltage is sufficiently
+  high, decomposition will take place with practically "no current," it
+  being possible to decompose the solution with the discharge from a small
+  induction coil. The quantity of an element liberated is by weight the
+  product of time, current, and the electro-chemical equivalent of that
+  element, and is given by the equation W = <i>zct</i>, where</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>W = quantity of element liberated in grammes.</p>
+      <p><i>z</i> = electro-chemical equivalent,</p>
+      <p><i>c</i> = current in amperes,</p>
+      <p><i>t</i> = time in seconds.</p>
+    </div>
+  </div>
+
+  <p>The chemical action that takes place is therefore very small, as the
+  intermittent current sent out from the transmitter in some cases only
+  lasts from <sup>1</sup>/<sub>50</sub>th to <sup>1</sup>/<sub>100</sub>th
+  a second.</p>
+
+  <p>The decomposed marks on the paper are blue, and, as photographers
+  know, blue is reproduced in a photograph as a white, so that a photograph
+  taken of our electrolytic picture, which will of course be a blue image
+  upon a white ground, will be reproduced almost like a blank sheet of
+  paper. If, however, a yellow contrast filter is placed in front of the
+  camera lens, and an orthochromatic plate used, the blue will be
+  reproduced in the photograph as a dead black.</p>
+
+  <p>There is one other point that requires attention. It will be noticed
+  that the metal print used for <!-- Page 60 --><span class="pagenum"><a
+  name="page60"></a>{60}</span>transmitting is a positive, since it is
+  prepared from a negative. The received picture will therefore be a
+  negative, making the final reproduction, if it is to be used for
+  newspaper work, a negative also. Obviously this is no good. The final
+  reproduction must be a positive, therefore the received picture must be
+  also a positive. To overcome this difficulty matters must be so arranged
+  at the receiving station that in the cases of Figs. 17, 18, 22, and 24,
+  the film is kept permanently illuminated while the stylus on the
+  transmitter is tracing over an insulating strip, and in darkness when
+  tracing over a conducting strip. In Fig. 30 the relay F should allow a
+  continuous current from Z to flow through the electrolytic paper, and
+  only broken when the resistance of the selenium cell is sufficiently
+  reduced to allow the current from D to operate the relay.</p>
+
+  <p>The author has endeavoured to make direct positives on glass of the
+  picture to be transmitted, so that a negative metal print could be
+  prepared. The results obtained were not very satisfactory, but the method
+  tried is given, as it may perhaps be of interest. The plate used in the
+  camera has to be exposed three or four times longer than is required for
+  an ordinary negative. The exposed plate is then placed in a solution of
+  protoxalate of iron (ferrous oxalate) and left until the image shows
+  plainly through the back of the plate. It <!-- Page 61 --><span
+  class="pagenum"><a name="page61"></a>{61}</span>is then washed in water
+  and placed in a solution consisting of</p>
+
+<table class="nob" summary="Washing solution." title="Washing solution.">
+<tr><td class="nspacsingle"> Distilled water </td><td class="nspacsingle" style="text-align:right;"> 1000 </td><td class="nspacsingle"> cc.</td></tr>
+<tr><td class="nspacsingle"> Nitric acid </td><td class="nspacsingle" style="text-align:right;"> 2 </td><td class="nspacsingle"> cc.</td></tr>
+<tr><td class="nspacsingle"> Sulphuric acid </td><td class="nspacsingle" style="text-align:right;"> 3 </td><td class="nspacsingle"> cc.</td></tr>
+<tr><td class="nspacsingle"> Bichromate of potash&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </td><td class="nspacsingle" style="text-align:right;"> 105 </td><td class="nspacsingle"> grammes.</td></tr>
+<tr><td class="nspacsingle"> Alum </td><td class="nspacsingle" style="text-align:right;"> 80 </td><td class="nspacsingle"> &nbsp; &nbsp; &nbsp; ,,</td></tr>
+</table>
+
+  <p>After being in this bath for about fifteen minutes the plate is again
+  well washed in water, and developed in the ordinary way. The first two
+  operations should be performed in the dark room, but the remaining
+  operations can be performed in daylight, once the plate has been placed
+  in the bichromate bath. As already stated, the results obtained were not
+  very satisfactory, and such a method is not now worth following up, as it
+  is comparatively easy so to arrange matters at the receiving station that
+  a positive or negative image can be received at will.</p>
+
+  <p>It is necessary to connect the stylus of the receiving machine to the
+  positive pole of the battery Z, otherwise the marks will be made on the
+  underside of the paper. The electrolytic receiver, owing to the absence
+  of mechanical and electro-magnetic inertia, is capable of recording
+  signals at a very high speed indeed.</p>
+
+  <p>"Atmospherics," which are such a serious nuisance in long-distance
+  wireless telegraphy, will also prove a nuisance in wireless photography,
+  <!-- Page 62 --><span class="pagenum"><a name="page62"></a>{62}</span>but
+  their effects will not be so serious in a photographic method of
+  receiving as they would be in the electrolytic system. In a photographic
+  receiver where the film is, under normal conditions, constantly
+  illuminated, the received signals (both the transmitted signals and the
+  atmospheric disturbances) will be recorded, after development, as
+  transparent marks upon the film, the remainder of the film being, of
+  course, perfectly opaque. By careful retouching the marks due to the
+  disturbances can be eradicated, a print upon sensitised paper having been
+  first obtained to act as a guide during the process.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 63 --><span class="pagenum"><a name="page63"></a>{63}</span></p>
+
+<h3>CHAPTER IV</h3>
+
+<p class="cenhead">SYNCHRONISING AND DRIVING</p>
+
+  <p>Clockwork and electro-motors are the source of driving power that are
+  most suitable for photo-telegraphic work, and each has its superior
+  claims depending on the type of machine that is being used. For general
+  experimental work, however, an electro-motor is perhaps the most
+  convenient, as the speed can be regulated within very wide limits. For a
+  constant and accurate drive a falling weight has no equal, but the
+  apparatus required is very cumbersome and the work of winding both
+  tedious and heavy. This method of driving was at one time universally
+  employed with the Hughes printing telegraph, but it has now been
+  discarded in favour of electro-motors, which are more compact, besides
+  being cheaper to instal in the first instance.</p>
+
+  <p>Synchronising and isochronising the two machines are the most
+  difficult problems that require solving in connection with wireless
+  photography, and as previously mentioned, the <!-- Page 64 --><span
+  class="pagenum"><a name="page64"></a>{64}</span>synchronising of the two
+  stations must be very nearly perfect in order to obtain intelligible
+  results. The limit of error in synchronising must be about 1 in 500 in
+  order to obtain results suitable for publication.</p>
+
+  <p>The electrolytic system is perhaps the easiest to isochronise, as the
+  received picture is visible. On the metal print used for transmitting,
+  and at the commencing edge a datum line is drawn across in insulating
+  ink. The reproduction of this line is carefully observed by the operator
+  in charge of the receiving instrument, and the speed of the motor is
+  regulated until this line lies close against a line drawn across the
+  electrolytic paper. Although this may seem an ideal method there are one
+  or two considerations to be taken into account. Unless the decomposition
+  marks are made the correct length and are properly spaced, however good
+  the isochronising may be, the result will be a blurred image. Any one who
+  has worked with a selenium cell, will know that it cannot change from its
+  state of high resistance to that of low resistance with infinite
+  rapidity, and the effects of this inertia, or "fatigue" as it has been
+  called, are more pronounced when working at a high speed. In working, the
+  effects of this inertia would be to increase the time of contact of the
+  relay F (Fig. 30) as the current from D would flow for a slightly longer
+  period through R to F than the period of <!-- Page 65 --><span
+  class="pagenum"><a name="page65"></a>{65}</span>illumination allowed by
+  K. This, of course, would mean a lengthening of the marks on the paper;
+  results would also differ greatly with different selenium cells. There is
+  a method of compensation by which the inertia of a cell can almost
+  entirely be overcome, but it would add greatly to the complicacy of the
+  receiving apparatus.</p>
+
+  <p>In using an electro-motor with any optical method of receiving there
+  are two methods available. The first is an arrangement similar to that
+  used by Professor Korn in his early experiments with his selenium
+  machines. The motor used for driving has several coils in the armature
+  connected with slip rings, from which an alternating current may be
+  tapped off; the motor acting partially as a generator, besides doing good
+  work as a motor in driving the machine. This alternating current is
+  conducted to a frequency meter, which consists of a powerful
+  electro-magnet, over which are placed magnetised steel springs, having
+  different natural periods of vibration. By means of a regulating
+  resistance the motor is run until the spring which has the same period as
+  the desired armature speed vibrates freely. The speed of the motors at
+  both stations can thus be adjusted with a fair amount of accuracy.
+  Another method is to make use of a governor similar to those employed in
+  the Hughes printing telegraph system. A drawing of the governor is given
+  in Fig. 32. It consists of a <a name="page66"></a> <span class="figright"
+  style="width:33%;"><a href="images/illo-fig32.png"><img
+  style="width:100%" src="images/illo-fig32.png" alt="Fig. 32." title="Fig. 32."
+  /></a><span class="sc">Fig.</span> 32.</span> <!-- Page 67 --><span
+  class="pagenum"><a name="page67"></a>{67}</span>metal frame which
+  supports an upright steel bar S, whose ends turn on pivots. This bar is
+  rectangular in section. The gear-wheel G is fastened near the bottom of
+  this rod and gears with a similar wheel on the shaft of the driving motor
+  (not shown). Suspended from the broader sides of S are the two flexible
+  arms D, each carrying a brass ball T. These balls are not fastened to the
+  arms, but can slide up and down, being held in position by the wire
+  springs M, one end of each spring being fastened to the screws C. These
+  screws work in a slot cut in the upper part of S, and are connected to
+  the adjusting screw E. When E is turned the screws are raised or lowered
+  accordingly, and also the balls on the arms D.</p>
+
+  <p>Fastened to the arms are two brushes of tow B, and these revolve
+  inside but just clearing the inner surface of the steel ring Z. Upon the
+  motor speed increasing above the normal the arms D, and consequently the
+  balls T, swing out, making a larger circle, causing the brushes B to
+  press against the steel ring Z, setting up friction which, however, is
+  reduced as soon as the motor regains its ordinary working speed. By
+  careful adjustment the speed of the motors can be kept perfectly
+  constant. The object of having the balls T adjustable on D, is to provide
+  a means of altering the motor speed, as the lower the balls on D the
+  slower the mechanism runs, and <i>vice versa</i>. <!-- Page 68 --><span
+  class="pagenum"><a name="page68"></a>{68}</span></p>
+
+  <p><span class="figleft" style="width:27%;"><a
+  href="images/illo-fig33.png"><img style="width:100%"
+  src="images/illo-fig33.png" alt="Fig. 33." title="Fig. 33." /></a><span
+  class="sc">Fig.</span> 33.</span></p>
+
+  <p>A simple and effective speed regulator devised by the writer is given
+  in drawings 33 and 34. It comprises two parts, A and B, the part A being
+  connected to the driving motor, and the part B working independently. The
+  independent portion B consists of an ordinary clock movement M, a steel
+  spindle J being geared to one of the slower moving wheels, so that it
+  makes just one revolution in two seconds. This spindle, which runs in two
+  coned bearings, carries at its outer end a light <span class="figleft"
+  style="width:36%;"><a href="images/illo-fig34.png"><img
+  style="width:100%" src="images/illo-fig34.png" alt="Fig. 34." title="Fig. 34."
+  /></a><span class="sc">Fig.</span> 34.</span> pointer D, about two inches
+  long, to the underside of which is fastened the thin brass contact spring
+  S, which presses lightly upon the ebonite ring N. <!-- Page 69 --><span
+  class="pagenum"><a name="page69"></a>{69}</span>The portion A comprises a
+  spindle, pointer, and contact spring similar to those employed in B, the
+  spindle J' being geared to the driving motor by means of F, so that the
+  pointer D' makes a little more than one revolution in two seconds. By
+  means of a special form of brake on the driving motor, the speed is
+  reduced, so that both pointers travel at the same rate, viz. one
+  revolution in two seconds. By careful adjustment the two pointers can be
+  made to revolve in synchronism,<a name="NtA9"
+  href="#Nt9"><sup>[9]</sup></a> and when this is obtained the contact
+  springs S, S', pass over the contacts C, C', completing the circuit of
+  the battery B and lamp L. When working properly the lamp L lights up
+  regularly once every second. This regulator is an excellent one to use
+  for experimental work, although it depends a great deal upon the skill of
+  the operator, but good adjustment should be obtained in about two
+  minutes. It is a good plan to insert a clutch of some description between
+  the driving motor and the machine, so that the regulator can be adjusted
+  prior to the act of receiving or transmitting, the machine being
+  prevented from revolving by means of a catch. The motor used should be
+  powerful enough to take up the work of driving the machine without any
+  reduction in speed. The clocks M can be regulated so that they only gain
+  or lose a few seconds in <!-- Page 70 --><span class="pagenum"><a
+  name="page70"></a>{70}</span>twenty-four hours, which gives an accuracy
+  in working sufficient for all practical purposes.</p>
+
+  <p>Connection is made with the contact springs S, S', by means of the
+  springs T, T', which press against the spindles J, J'.</p>
+
+  <p>Another important point is the correct placing of the picture upon the
+  receiving drum. It is necessary that the two machines besides revolving
+  in perfect isochronism should synchronise as well, <i>i.e.</i> begin to
+  transmit and record at exactly the same position on the cylinders, viz.
+  at the edge of the lap, so that the component parts of the received image
+  shall occupy the same position on the paper or film as they do on the
+  metal print. If the receiving cylinder had, let us suppose, completed a
+  quarter of a revolution before it started to reproduce, the reproduction
+  when removed from the machine and opened out will be found to be
+  incorrectly placed; the bottom portion of the picture being joined to the
+  top portion, or <i>vice versa</i>, and this means that perhaps an
+  important piece of the picture would be rendered useless even if the
+  whole is not spoilt. It is evident, therefore, that some arrangement must
+  be employed whereby synchronism, as well as isochronism of the two
+  instruments can be maintained.</p>
+
+  <p>There are several methods of synchronising that are in constant use in
+  high-speed telegraphy, in which the limit of error is reduced to a
+  minimum, <!-- Page 71 --><span class="pagenum"><a
+  name="page71"></a>{71}</span>and some modification of these methods will
+  perhaps solve the problem, but it must be remembered that synchronism is
+  far easier to obtain where the two stations are connected by a length of
+  line than where the two stations are running independently.</p>
+
+  <p>In one system of ordinary photo-telegraphy synchronism is obtained in
+  the following manner. The receiving cylinder travels at a speed slightly
+  in excess of the transmitting cylinder, and as its revolution is finished
+  first is prevented from revolving by a check, and when in this position
+  the receiving apparatus is thrown out of circuit and an electro-magnet
+  which operates the check is switched in. When the transmitting cylinder
+  has completed its revolution (about <sup>1</sup>/<sub>100</sub>th of a
+  second later) the transmitting apparatus, by means of a special
+  arrangement, is thrown out of circuit for a period, just long enough for
+  a powerful current to be sent through the line. This current actuates the
+  electro-magnet. The check is withdrawn and the receiving cylinder
+  commences a fresh revolution in perfect synchronism with the transmitting
+  cylinder. As soon as the check is withdrawn the receiving apparatus is
+  again placed in circuit until another revolution is completed. As the
+  receiver cannot stop and start abruptly at the end of each revolution a
+  spring clutch is inserted between the driving motor and the machine. <!--
+  Page 72 --><span class="pagenum"><a name="page72"></a>{72}</span></p>
+
+  <p>Although a method of synchronising similar to this may later on be
+  devised for wireless photography, the writer, from the result of his own
+  experiments, is led to believe that results good enough for all practical
+  purposes can be obtained by fitting a synchronising device whereby the
+  two machines are started work at the same instant, and relying upon the
+  perfect regulation of the speed of the motors for correct working.</p>
+
+  <p>The method of isochronism must, however, be nearly perfect in its
+  action, as it is easy to see that with only a very slight difference in
+  the speed of either machine this error will, when multiplied by 40 or 50
+  revolutions, completely destroy the received picture for practical
+  purposes.</p>
+
+  <p>From what has been written in this and in the preceding chapters it
+  will be evident that the successful solution of transmitting photographs
+  by wireless methods will necessitate the use of a great many pieces of
+  apparatus all requiring delicate adjustment, and depending largely upon
+  each other for efficient working. As previously stated, there is at
+  present no real system of wireless photography, the whole science being
+  in a purely experimental stage, but already Professor Korn has succeeded
+  in transmitting photographs between Berlin and Paris, a distance of over
+  700 miles. If such a distance could be worked over successfully, there is
+  no reason to doubt that before long <!-- Page 73 --><span
+  class="pagenum"><a name="page73"></a>{73}</span>we shall be able to
+  receive pictures from America with as great reliability and precision as
+  we now receive messages.</p>
+
+  <p>In nearly all wireless photographic systems devised up to the present
+  the chief portion of the receiver consists of a very sensitive
+  galvanometer, and although very good results have been obtained by their
+  use they are more or less a nuisance, as the extreme delicacy of their
+  construction renders them liable to a lot of unnecessary movement caused
+  by external disturbances. A galvanometer of the De' Arsonval pattern,
+  used by the writer, was constantly being disturbed by merely walking
+  about the room, although placed upon a fairly substantial table; and for
+  the same reason it was impossible to attempt to place the driving motor
+  of the machine on the same table as the galvanometer. For ship-board work
+  it will be evident that the use of such a sensitive instrument presents a
+  great difficulty to successful working, and a good opening exists for
+  some piece of apparatus&mdash;to take the place of the
+  galvanometer&mdash;that will be as sensitive in its action but more
+  robust in its construction.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 74 --><span class="pagenum"><a name="page74"></a>{74}</span></p>
+
+<h3>CHAPTER V</h3>
+
+<p class="cenhead">THE "TELEPHOGRAPH"</p>
+
+  <p>In the present chapter it is proposed to give a brief description of a
+  system of radio-photography devised by the author, and which includes a
+  greatly improved method of transmitting and receiving, as well as an
+  ingenious arrangement for synchronising the two stations; the whole being
+  an attempt to produce a system that would be capable of working
+  commercially over fairly long distances.</p>
+
+  <p>The system about to be described, and which I have designated the
+  "telephograph," is the outcome of several years' original experimental
+  work, many difficulties that were manifest in the working of the earlier
+  systems having been overcome by apparatus that has been expressly
+  designed for the purpose.</p>
+
+  <p>In any practical system of radio-photography the following points are
+  of great importance: (1) the speed of transmission; (2) the quality of
+  the received picture; (3) the method of synchronising <!-- Page 75
+  --><span class="pagenum"><a name="page75"></a>{75}</span>the two machines
+  so that transmission and reception begin simultaneously; (4) the correct
+  regulation of the speed of the driving motors; (5) the simplicity and
+  reliability of the entire arrangement. Points 1 and 2 are dependent upon
+  several factors; the number of contacts made by the stylus per minute;
+  the size of the metal print used; the number of lines per inch on the
+  screen used in preparing the print; and the accurate and harmonious
+  working of the various pieces of apparatus employed.</p>
+
+  <p>In the system under discussion the size of the metal print used is 5
+  inches by 7 inches, and a screen having 50 lines to the inch is used for
+  preparing it. With the drum of the machine making one revolution in four
+  seconds, the stylus makes 87 contacts per second, or 5220 a minute, the
+  time for complete transmission being twenty-five minutes. By the use of
+  ordinary relays not more than 2000 contacts a minute can be obtained, and
+  in the present system it is only by means of a specially designed relay
+  that such a high rate of working has been made possible. Similarly, too,
+  with the receiving of such a large number of signals transmitted at such
+  a high speed, a special instrument has been devised that can record this
+  number of signals without any trouble, and could even record up to 8000
+  signals a minute, provided that a suitable transmitter could be designed.
+  <!-- Page 76 --><span class="pagenum"><a
+  name="page76"></a>{76}</span></p>
+
+  <p>In the present system the writer does not claim to have completely
+  solved the problem of the wireless transmission of photographs, but it is
+  a great advance on any system previously described, and the following
+  advantages are put forward for recognition: (1) a greatly improved method
+  of transmitting and receiving; (2) a simple method of regulating the
+  speed of the driving motors and maintaining isochronism with a limit of
+  error of less than 1 in 800; (3) an arrangement for synchronising the two
+  machines whereby transmitting and receiving begin simultaneously; (4) the
+  use of one machine only at each station.</p>
+
+<p class="cenhead"><span class="sc">Transmitting Apparatus</span></p>
+
+  <p>A diagrammatic representation of the apparatus required for a complete
+  station, transmitting and receiving combined, is given in Fig. 35, the
+  usual wireless equipment having been omitted from the diagram to avoid
+  confusion.</p>
+
+  <p><i>The Machine.</i>&mdash;This, as will be seen from Fig. 36, consists
+  of a base-plate M, to which are attached the two bearings B and B'. The
+  bearing B' is fitted with an internal thread to correspond with the
+  threaded portion of the shaft D. The drum V is a brass casting, being
+  fastened to the shaft by set screws. The shaft is threaded 75 to the
+  inch. The bearings are preferably of the concentric type. The circuit
+  breaker C is so arranged that when <!-- Page 77 --><span
+  class="pagenum"><a name="page77"></a>{77}</span>the drum has traversed
+  the required distance, the end of the shaft pushes back the spring M,
+  breaking the circuit of the driving gear and stopping the machine. The
+  machine is connected to the driving gear by the flexible coupling A.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig35.png"><img style="width:100%" src="images/illo-fig35.png"
+      alt="Fig. 35" title="Fig. 35" /></a>
+    <span class="sc">Fig.</span> 35.
+
+    <p class="poem">M, motor; Y, isochroniser; F, clutch; A, machine; R,
+    stylus; S, relay; X, gearing; O, circuit breaker; T, receiver; C,
+    condenser; U, telephone relay; K, polarised relay; L, contact breaker;
+    D, D<sup>1</sup>, D<sup>2</sup>, D<sup>3</sup>, batteries; P, friction
+    brake; B, B<sup>1</sup>, double-pole two-way switches; N,
+    N<sup>1</sup>, N<sup>2</sup>, single switches; W, key; E, electric
+    clock; J, telephones.</p>
+  </div>
+
+  <p>The drum measures 5 inches long by 2<sup>1</sup>/<sub>8</sub> inches
+  diameter, and this takes a metal print 5 inches by 7 inches, which allows
+  for a lap of about <sup>1</sup>/<sub>4</sub> inch. In working, the print
+  is wrapped tightly round the drum, being secured by means of a little
+  seccotine smeared along one edge. Care must be taken that the edge of the
+  lap draws away from the point of <!-- Page 78 --><span class="pagenum"><a
+  name="page78"></a>{78}</span>the stylus and not towards it. A margin of
+  bare foil, about <sup>1</sup>/<sub>8</sub> inch wide, should be left on
+  the print at the commencing edge, the purpose of which will be explained
+  later.</p>
+
+  <div class="figcenter" style="width:36%;">
+      <a href="images/illo-fig36.png"><img style="width:100%" src="images/illo-fig36.png"
+      alt="Fig. 36" title="Fig. 36" /></a>
+    <span class="sc">Fig. 36.</span>
+  </div>
+
+  <p><i>The Stylus.</i>&mdash;As the drum of the machine travels laterally,
+  by reason of the threaded shaft and bearing, the stylus must necessarily
+  be a fixture. It consists of a holder B, drilled to take a hardened steel
+  point S, attached to the spring M. The spring is arranged to work in the
+  guide F, which is provided with an adjusting screw W for regulating the
+  pressure of the stylus upon the print; the pressure being sufficient to
+  enable good contact to be made, but must not be heavy enough to scratch
+  the soft foil. The needle should present an angle of about 60° to the
+  surface of the print, as this angle has been found to give the best
+  results in working.</p>
+
+  <p>To eliminate any sparking that may take place at the point of make and
+  break, due to the self-induction of the relay coils, a condenser C, about
+  1 microfarad capacity, should be connected across <!-- Page 79 --><span
+  class="pagenum"><a name="page79"></a>{79}</span>the drum and stylus. The
+  complete stylus is given in the drawings, Figs. 37, 37<i>a</i>, and also
+  in the diagrams Figs. 8 and 9.</p>
+
+  <div class="figcenter" style="width:28%;">
+      <a href="images/illo-fig37.png"><img style="width:100%" src="images/illo-fig37.png"
+      alt="Fig. 37" title="Fig. 37" /></a>
+    <span class="sc">Fig. 37.</span>
+
+    <p class="poem">Showing the arrangement for sliding the stylus to or
+    from the machine.</p>
+  </div>
+
+  <div class="figright" style="width:25%;">
+      <a href="images/illo-fig37a.png"><img style="width:100%" src="images/illo-fig37a.png"
+      alt="Fig. 37a" title="Fig. 37a" /></a>
+    <span class="sc">Fig. 37</span><i>a</i>.
+  </div>
+
+  <p><i>The Relay.</i>&mdash;As will be seen from the diagram, Fig. 38,
+  this consists of two electro-magnets having very soft iron cores, the
+  magnet M being wound in the usual manner, while the magnet N is wound
+  differentially. The armature A is made as light as possible, and is
+  pivoted at P, and when there is no current flowing through any of the
+  coils, is held midway between the magnet cores by the two spiral springs
+  S and T, which are under slight but equal tension. The connections are as
+  follows. The wires from the winding on M are connected directly to the
+  relay terminals F and H, as are also the wires from one winding on N. The
+  other winding on N is connected in series with the battery C, ammeter B,
+  and regulating resistance R. <!-- Page 80 --><span class="pagenum"><a
+  name="page80"></a>{80}</span></p>
+
+  <div class="figcenter" style="width:36%;">
+      <a href="images/illo-fig38.png"><img style="width:100%" src="images/illo-fig38.png"
+      alt="Fig. 38" title="Fig. 38" /></a>
+    <span class="sc">Fig. 38.</span>
+  </div>
+
+  <p>When the circuit of the battery C is completed, the coil of N, to
+  which it is connected, is energised, and the armature A is attracted
+  against the stop V. When in this position the tension of the spring S is
+  released, while the tension of the spring T is increased. As soon as the
+  circuit of the battery D is completed by means of the metal line print on
+  the transmitting machine, the current divides at the terminals F and H, a
+  portion flowing through the magnet coil M, and a portion through the
+  remaining winding on N. The current which flows through the winding on N
+  produces a magnetising effect equal to that caused by the other winding
+  on N, but since the two windings are of equal length and resistance, and
+  since the current flowing through the two windings is of equal strength
+  but in opposite directions, the result is to neutralise <!-- Page 81
+  --><span class="pagenum"><a name="page81"></a>{81}</span>the magnetising
+  effects produced by each winding, and consequently no magnetism is
+  produced in the cores.</p>
+
+  <p>The other portion of the current from D flows through the coil M, and
+  it becomes magnetised at the same time that the coil N becomes
+  demagnetised. The armature A is attracted by M against the stop X, and
+  this attraction is assisted by the spring T, which was under increased
+  tension. The conditions of the springs are now reversed, the spring S
+  being under increased tension, while the tension of the spring T is
+  released.</p>
+
+  <p>As soon as the current from D is broken, the magnetism disappears from
+  M, the neutralising current in N ceases, and N once more becomes
+  magnetised, owing to the current which still flows through one winding
+  from C; the armature is therefore again attracted by N, assisted by the
+  spring S. The current flowing through the two windings of N must be
+  perfectly equal, and the regulating resistance R, and ammeters B and B',
+  are inserted for purposes of adjustment. The current from C must flow in
+  a direction opposite to that which flows from D.</p>
+
+  <div class="figright" style="width:24%;">
+      <a href="images/illo-fig39.png"><img style="width:100%" src="images/illo-fig39.png"
+      alt="Fig. 39" title="Fig. 39" /></a>
+    <span class="sc">Fig. 39.</span>
+
+    <p class="poem">H, H', containers; M, mercury; E, paraffin oil; T, T',
+    terminals; C, suspending rod; D, base; F, F', dipping rods.</p>
+  </div>
+
+  <p>The local circuit of the relay is completed by means of a copper
+  dipper in mercury, somewhat resembling an ordinary mercury break, but
+  modified to suit the present requirements. The arrangement will be seen
+  from Fig. 39. The whole of the <!-- Page 82 --><span class="pagenum"><a
+  name="page82"></a>{82}</span>moving parts are made as light as possible,
+  and for this reason the rod C and the dippers F, F' should be made as
+  short as convenient. The containers H, H' are separate, of cast iron, and
+  rectangular in shape. The dipper is of very thin copper tube&mdash;an
+  advantage where alternating current is to be used&mdash;and is made
+  adjustable for height on the suspending rod C. The leg F is of such a
+  length that permanent contact is made with the mercury in the container
+  H, while the leg F' clears the surface of the mercury by about
+  <sup>1</sup>/<sub>4</sub> inch, when the armature of the relay is in its
+  normal position. To prevent undue churning of the mercury, which would
+  necessarily take place if the dipper entered and left the mercury at each
+  movement of the armature, a pointed ebonite plug is inserted in the end
+  of the tube. This will be found to give good results at a high speed, the
+  mercury being practically undisturbed, and the production of "sludge"
+  reduced to a minimum. To prevent oxidation of the mercury, and to prevent
+  arcing, the surface is covered with paraffin oil. If this is not
+  sufficient to prevent arcing a condenser should be shunted across the
+  <!-- Page 83 --><span class="pagenum"><a
+  name="page83"></a>{83}</span>containers. The volume of mercury, and the
+  area of the dippers, should be sufficient to carry the current used for a
+  considerable period without heating up to any extent. An adjustable
+  weight J is provided in order to balance the armature and dipping
+  rod.</p>
+
+  <p>The remaining transmitting apparatus consists of the battery
+  D<sup>2</sup> and the usual wireless apparatus. The double-pole two-way
+  switch B' is to enable the photo-telegraphic set to be switched out and
+  the hand key W switched in for ordinary signalling purposes. The battery
+  D<sup>2</sup> should be about 12 volts.</p>
+
+<p class="cenhead"><span class="sc">Receiving Apparatus</span></p>
+
+  <p>The wireless portion of the receiver is similar to that given in Fig.
+  22, is of the usual syntonic type, and comprises an oscillation
+  transformer, S being the secondary, and P the primary; C' is a block
+  condenser, and C a variable condenser. The detector D is of the
+  carborundum crystal or electrolytic pattern. A two-way switch B is
+  provided so that the relay U can be switched out and the telephones J
+  switched in for ordinary receiving purposes. The relay U is a Brown's
+  telephone relay.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig40.png"><img style="width:100%" src="images/illo-fig40.png"
+      alt="Fig. 40." title="Fig. 40." /></a>
+    <span class="sc">Fig.</span> 40.
+  </div>
+
+  <p><i>The Receiver.</i>&mdash;The magnified current from the relay U is
+  taken to a special telephone receiver, the construction of which is given
+  in Fig. 40. The diaphragm F is about 2<sup>1</sup>/<sub>2</sub> inches
+  diameter, and should be fairly thin but very resilient. Only one <!--
+  Page 84 --><span class="pagenum"><a name="page84"></a>{84}</span><span
+  class="figleft" style="width:13%;"><a href="images/illo-fig41.png"><img
+  style="width:100%" src="images/illo-fig41.png" alt="Fig. 41." title="Fig. 41."
+  /></a><span class="sc">Fig.</span> 41.</span> <span class="figright"
+  style="width:26%;"><a href="images/illo-fig41a.png"><img
+  style="width:100%" src="images/illo-fig41a.png" alt="Fig. 41a."
+  title="Fig. 41a." /></a><span class="sc">Fig.</span> 41a.</span> coil is
+  provided, and this should be wound with No. 47 S.S.C. copper wire for a
+  resistance of about 2000 ohms. By using only one coil and therefore only
+  one core, the movement of the diaphragm is centralised. To the centre of
+  the diaphragm a light steel point is fastened, about
+  <sup>1</sup>/<sub>2</sub> inch long, and provided with a projecting hook
+  H. An enlarged view of this pin is given in Fig. 41. The movement of the
+  diaphragm and consequently of the steel point P is communicated to a
+  pivoted rod R, which is of special construction. A piece of aluminium
+  tube 3<sup>3</sup>/<sub>4</sub> inches long, and of the section given at
+  B, is bushed at one end with a piece of brass of the shape shown in Fig.
+  41<i>a</i>. A stiff steel wire T about 1 inch long (20 gauge) is screwed
+  into the end of Z, and carries a counterbalance weight C. A hardened <!--
+  Page 85 --><span class="pagenum"><a name="page85"></a>{85}</span>steel
+  spindle, pointed at both ends, is fastened at D, and runs between two
+  coned bearings, one of which is adjustable. The underside of Z is
+  flattened, and a small coned depression is made for the reception of the
+  pointed end of the pin. By means of the spring J the two pieces, Z and P,
+  are held firmly together, at the same time allowing perfect freedom of
+  movement. The bridge G is made from a piece of sheet aluminium placed in
+  a slot cut in the tube R, the end of the tube being pressed tight upon G,
+  and secured by means of a small rivet.</p>
+
+  <p>The optical arrangements are as follows. By means of the Nernst lamp
+  L, and the lenses B and B', Figs. 42 and 43, a magnified shadow of G is
+  thrown upon the screen J. When the shutter G is in its normal position
+  (<i>i.e.</i> at rest), its shadow is just above the small hole in J, and
+  light from L reaches the photographic film wrapped round the drum V of
+  the machine.</p>
+
+  <div class="figright" style="width:23%;">
+      <a href="images/illo-fig42.png"><img style="width:100%" src="images/illo-fig42.png"
+      alt="Fig. 42." title="Fig. 42." /></a>
+    <span class="sc">Fig.</span> 42.
+
+    <p class="poem">J, screen; L, Nernst lamp; G, shutter; B, condensing
+    lens; B<sub>1</sub>, focussing lens.</p>
+  </div>
+
+  <p>When, however, signals are sent out from the transmitting apparatus,
+  the magnified current from the relay U energises the coil of the special
+  telephone S, attracting the diaphragm F, and consequently giving movement
+  to the pivoted rod R. As by means of the optical arrangements a <!-- Page
+  86 --><span class="pagenum"><a name="page86"></a>{86}</span>magnified
+  movement as well as a magnified image of G is thrown upon the screen J,
+  the shadow of G will, when the telephone S is actuated, cover the hole in
+  the screen, and prevent any light from reaching the film on V, until
+  current from the relay U ceases to flow. Therefore, when the stylus of
+  the transmitter traces over a conducting strip on the metal print, no
+  light reaches the film on V, but when tracing over an insulating strip
+  the shadow of G on the screen J rises, and the light from L reaches the
+  film. By this means a positive picture is received, which is a great
+  advantage where the photographs are required for reproduction.
+  Atmospherics would be represented by irregular transparent marks on the
+  film after development, and these can be easily eradicated by
+  retouching.</p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/illo-fig43.png"><img style="width:100%" src="images/illo-fig43.png"
+      alt="Fig. 43." title="Fig. 43." /></a>
+    <span class="sc">Fig.</span> 43. E, ebonite screen; F, focussing lens;
+    G, shutter; O, condensing lens; L, Nernst lamp.
+  </div>
+
+  <p>The drum of the machine moves laterally <sup>1</sup>/<sub>75</sub>th
+  of an inch per revolution, and the hole in the screen is
+  <sup>1</sup>/<sub>90</sub>th of an inch in diameter. As the screen J is
+  not in direct contact with the film, the slight diffusion of the light
+  that takes place will produce <!-- Page 87 --><span class="pagenum"><a
+  name="page87"></a>{87}</span>a mark of about the right thickness. With a
+  movement of the diaphragm of only <sup>1</sup>/<sub>40000</sub>th of an
+  inch, the actual movement of G will be <sup>1</sup>/<sub>4000</sub>th of
+  an inch. If the optical arrangements have a magnifying power of 100, then
+  the movement of the shadow upon the screen will be
+  <sup>1</sup>/<sub>40</sub>th of an inch, which will be ample to cover the
+  aperture.</p>
+
+  <p>The aluminium rod R, minus the counter-weight, can be made to weigh
+  not more than 12 grains. It is necessary to enclose the optical parts in
+  a light tight box, indicated by the dotted lines in Fig. 43, in order to
+  prevent any extraneous light from reaching the film.</p>
+
+  <p><i>The Contact Breaker.</i>&mdash;The contact breaker (L, Fig. 35), as
+  will be seen from Fig. 44, consists of an electro-magnet N, the windings
+  of which are connected with the battery B and the polarised relay K. The
+  armature which is supported by the spring G carries a contact arm A,
+  which in its normal position makes permanent contact with the contact
+  screw T, and completes the circuit between the relay K and the telephone
+  relay U (Fig. 35). As soon as the transmitter sends out the first signal,
+  the magnified current from the telephone relay actuates the relay K,
+  which in turn completes the circuit of the contact breaker. Directly the
+  armature M has been attracted, the contact with T is broken, and A makes
+  fresh contact with the screw H, by means of the spring Z <!-- Page 88
+  --><span class="pagenum"><a name="page88"></a>{88}</span>fastened to the
+  underside of A. The armature, once it has been attracted, is held in
+  permanent contact with H by the catch S, independent of the magnets N. As
+  soon as contact is made with H, the clutch (F, Fig. 35) circuit is
+  completed, and the circuit of the relay K is broken. When the circuit of
+  the clutch F is broken by means of the circuit breaker C on the machine
+  (Fig. 36), the stop S is pulled back by hand, allowing the contact arm A
+  to rise, and again make fresh contact with the contact screw T.</p>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/illo-fig44.png"><img style="width:100%" src="images/illo-fig44.png"
+      alt="Fig. 44." title="Fig. 44." /></a>
+    <span class="sc">Fig.</span> 44.
+  </div>
+
+<p class="cenhead"><span class="sc">Driving Apparatus</span></p>
+
+  <p><i>The Friction Brake.</i>&mdash;This consists of a steel disc A, Fig.
+  45, about 2<sup>1</sup>/<sub>2</sub> inches diameter and
+  <sup>3</sup>/<sub>8</sub> inch or <sup>1</sup>/<sub>2</sub> inch wide on
+  the face, secured to the main shaft of the driving motor. The arm H,
+  pivoted at C, carries at one end the curved block B, which is faced with
+  a pad of tow F. The other extremity is pivoted to the steel rod P, which
+  slides <!-- Page 89 --><span class="pagenum"><a
+  name="page89"></a>{89}</span><span class="figright" style="width:23%;"><a
+  href="images/illo-fig45.png"><img style="width:100%"
+  src="images/illo-fig45.png" alt="Fig. 45." title="Fig. 45." /></a><span
+  class="sc">Fig.</span> 45.</span> in holes bored in the standards J. One
+  end of the rod P is screwed with a fine thread, about 75 to the inch, and
+  is fitted with a regulating wheel T, by means of which the block B can be
+  made to press upon the disc A with any required degree of pressure. A
+  fairly stiff steel spring R is placed upon the rod P, between one
+  standard J and the collar N. As the speed of the driving motor is
+  slightly in excess of that required by the machine, the block B, by means
+  of the wheel, is made to press upon the disc A, setting up friction which
+  reduces the motor speed until the isochroniser indicates that the correct
+  working speed has been attained.</p>
+
+  <p><i>The Clutch</i>.&mdash;The details of this will be seen from Figs.
+  46 and 47. It consists of a steel shaft coned at both ends running
+  between two countersunk bearings, one of which is adjustable. This shaft
+  carries the two portions of the clutch A and B, the portion A being a
+  fixture on the shaft, and the portion B running free upon it. The portion
+  B is a gun-metal casting bored to run accurately upon the steel shaft. A
+  soft iron annular ring is fastened to the face.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig46.png"><img style="width:100%" src="images/illo-fig46.png"
+      alt="Fig. 46." title="Fig. 46." /></a>
+    <span class="sc">Fig. 46.</span>
+
+    <p class="poem">E, spindle; R, bobbins; P, iron cores; D, copper rings;
+    T, brushes; N, back plate; V, front plate; J, gearing; S, spring; H,
+    collar; Z, iron ring; F, fixed bearing; C, insulating bush.</p>
+  </div>
+
+  <p>The portion A consists of a gun-metal casting <!-- Page 90 --><span
+  class="pagenum"><a name="page90"></a>{90}</span><span class="figright"
+  style="width:38%;"><a href="images/illo-fig47.png"><img
+  style="width:100%" src="images/illo-fig47.png" alt="Fig. 47." title="Fig. 47."
+  /></a><span class="sc">Fig.</span> 47.</span> bored a tight fit for the
+  shaft E, secured by means of a set screw. The two magnet cores P are
+  screwed into the front plate V, which is also of gun-metal, and after the
+  bobbins R have been slipped on, the shanks of the cores are passed
+  through holes drilled in the flange N of the main casting and held in
+  place with nuts. The faces of both A and B must be turned perfectly
+  square with the shaft, so that they run accurately together. The portion
+  B is <!-- Page 91 --><span class="pagenum"><a
+  name="page91"></a>{91}</span>kept in contact with A by means of a spring
+  S, the pressure being regulated by the collar H. Current is taken to the
+  magnets by means of the two insulated copper rings D mounted upon the
+  body of A. The gear-wheels on both portions have teeth of very fine
+  pitch, the number of teeth on each being regulated by the speed of the
+  driving motor and the required machine speed. Connection with the circuit
+  breaker L and the battery B<sup>2</sup> is made with the collecting rings
+  D by the brushes T. The complete connections are given in the diagram
+  Fig. 51.</p>
+
+  <p><i>The Isochroniser.</i>&mdash;This is a device for ensuring the
+  correct speed regulation of the driving motors, and is shown in detail in
+  Fig. 48. It comprises two portions, one portion being rotated at a
+  definite speed by electrical means, and the other portion rotated by the
+  driving motor.</p>
+
+  <p>The main portion consists of a metal tube N, bushed at both ends, the
+  bottom end of the tube being arranged to work on ball-bearings. An
+  ebonite bush C carries three copper rings T, T<sup>1</sup>,
+  T<sup>2</sup>, and the brushes R, R<sup>1</sup>, R<sup>2</sup> are in
+  electrical contact with them. The ebonite plate J,
+  3<sup>1</sup>/<sub>2</sub> inches diameter, is secured to the top end of
+  N, and carries a contact piece Q, shown separate at E. As will be seen
+  this is a block of ebonite with three contacts arranged on the top
+  surface. The middle contact P is <sup>1</sup>/<sub>64</sub>th of an inch
+  wide, and the contacts P<sup>1</sup> <!-- Page 92 --><span
+  class="pagenum"><a name="page92"></a>{92}</span>and P<sup>2</sup> are
+  placed on either side at a distance of <sup>1</sup>/<sub>16</sub> inch;
+  the contact strips P<sup>1</sup>, P<sup>2</sup> carry the brass pins D,
+  which are about <sup>1</sup>/<sub>16</sub> inch diameter, and spaced
+  <sup>3</sup>/<sub>8</sub> inch apart. A connecting wire is carried from
+  the contact P to the copper ring T, another from P<sup>1</sup> to
+  T<sup>1</sup>, and one from P<sup>2</sup> to T<sup>2</sup>.</p>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/illo-fig48.png"><img style="width:100%" src="images/illo-fig48.png"
+      alt="Fig. 48." title="Fig. 48." /></a>
+    <span class="sc">Fig. 48.</span>
+
+    <p class="poem">N, brass tube; S, bushes; G, ball-bearing; H,
+    gear-wheel; T, T<sup>1</sup>, T<sup>2</sup>, copper rings; C,
+    insulating block; R, R<sup>1</sup>, R<sup>2</sup>, brushes; J, ebonite
+    disc; Q, contact block; D, metal pins; O, pulley, P, P<sup>1</sup>,
+    P<sup>2</sup>, contact plates; K, needle; Z, spring; W, steel rod; E,
+    countersunk bearing.</p>
+  </div>
+
+  <p>The bushes S are bored a running fit for the steel rod W (shown
+  separate at A), which is coned at both ends, and runs between two
+  countersunk bearings, the bottom bearing E being fixed while <!-- Page 93
+  --><span class="pagenum"><a name="page93"></a>{93}</span>the top bearing
+  (not shown) is adjustable. A needle K is fastened near the end of the rod
+  W, and attached to this needle is the spring Z, which presses lightly but
+  firmly upon the contact block Q. To provide a level surface for Z to work
+  over, the spaces between the contact pieces are filled in with an
+  insulating material, and the whole surface finished off perfectly smooth.
+  The spring Z is <sup>1</sup>/<sub>8</sub> inch wide for portion of its
+  length, but at the point where it presses upon Q it is reduced in width
+  to <sup>1</sup>/<sub>64</sub>th of an inch (see Fig. 48). The driving
+  arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a
+  grooved pulley, is run in bearings parallel with the shaft W, and is
+  connected by suitable gearing to the shaft of the driving motor, so that
+  the needle K makes one revolution in about 2<sup>1</sup>/<sub>2</sub>
+  seconds. A belt passing over the pulleys connects the two shafts, and the
+  tension of the belt is regulated by means of an adjustable jockey
+  pulley.</p>
+
+  <p>The tube N, carrying the disc J, must be rotated at a fixed speed, and
+  this is accomplished in the following manner. An ordinary electric clock
+  impulse dial, actuated from a master clock, is connected by suitable
+  gearing H, so that the tube N makes exactly one revolution in 2 seconds;
+  it being possible to adjust an electric clock of the "Synchronome" type,
+  so that it only gains or loses about 1 second in 24 hours, and this
+  provides <!-- Page 94 --><span class="pagenum"><a
+  name="page94"></a>{94}</span>an accuracy sufficient for all practical
+  purposes. The connections are given in Fig. 49, and the face of the
+  instrument in Fig. 50. It will be seen that a connecting wire is run from
+  the steel spindle W to one terminal each of the lamps L, L<sup>1</sup>,
+  L<sup>2</sup>, and from the other terminal of the lamps to one terminal
+  of the batteries J, the battery comprising a set of three 4-volt
+  accumulators. The other terminals of the batteries are joined one to each
+  of the brushes R, R<sup>1</sup>, R<sup>2</sup>.</p>
+
+  <div class="figleft" style="width:17%;">
+      <a href="images/illo-fig49.png"><img style="width:100%" src="images/illo-fig49.png"
+      alt="Fig. 49." title="Fig. 49." /></a>
+    <span class="sc">Fig. 49.</span>
+  </div>
+
+  <div class="figright" style="width:20%;">
+      <a href="images/illo-fig50.png"><img style="width:100%" src="images/illo-fig50.png"
+      alt="Fig. 50." title="Fig. 50." /></a>
+    <span class="sc">Fig. 50.</span>
+
+    <p class="poem">M, terminals for connecting to electric clock; L, white
+    lamp; L<sup>1</sup>, blue lamp; L<sup>2</sup>, red lamp.</p>
+  </div>
+
+  <p>The lamps are coloured, the lamp L being white, and the lamps
+  L<sup>1</sup> and L<sup>2</sup> blue and red respectively, and care must
+  be taken in connecting up that when the needle K makes contact with the
+  stud P the white lamp L is in circuit. When the machines are working, the
+  operator, by means of the brake (already described), reduces the speed of
+  the driving motor until the needle K travels in unison with the disc J,
+  making permanent contact with P on the contact <!-- Page 95 --><span
+  class="pagenum"><a name="page95"></a>{95}</span>block Q, which is
+  evidenced by the lamp L remaining alight. If, however, the needle travels
+  faster than the disc J, contact with P is broken and fresh contact is
+  made with P<sup>2</sup>, the lamp L is extinguished and the red lamp
+  L<sup>2</sup> lights up, and remains alight until the operator reduces
+  the speed. Similarly, too, if the needle travels slower than J, contact
+  is made with P<sup>1</sup>, and the circuit of the blue lamp
+  L<sup>1</sup> is completed. When the speed is either above or below the
+  normal, the needle K engages with one or the other of the pins D, and as
+  the tension of the driving belt is only such as is required to drive the
+  needle, the belt slips on the pulleys until the normal speed is
+  regained.</p>
+
+<p class="cenhead"><span class="sc">Method of Working</span></p>
+
+  <p>The clockwork motor M, Fig. 51, should be capable of running for
+  several hours with one winding, and powerful enough to take up the work
+  of driving the machine without any appreciable effort. The main spindle
+  of the motor is so arranged that it makes one revolution in two minutes,
+  and the reduction in speed between the motor shaft and the shaft to which
+  the coupling A is attached is 30:1. The metal line print having been
+  wrapped round the drum of the machine, the stylus is put into position,
+  at the edge of the lap, and with the needle resting about half-way on
+  <!-- Page 96 --><span class="pagenum"><a name="page96"></a>{96}</span>the
+  margin of the bare foil left at the commencing edge of the print. Now,
+  when the two stations are in perfect readiness for work, the motors are
+  started and the speed adjusted; the speed of the machine being just under
+  one revolution in four seconds.</p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/illo-fig51.png"><img style="width:100%" src="images/illo-fig51.png"
+      alt="Fig. 51." title="Fig. 51." /></a>
+    <span class="sc">Fig. 51.</span>
+
+    <p class="poem">M, clockwork motor; S, isochroniser; E, friction break;
+    T, brushes; F, electric clutch; X, gearing; D, D<sup>1</sup>, switches;
+    A, flexible coupling; K, polarised relay; L, circuit breaker;
+    B<sub>1</sub>, B<sub>2</sub>, B<sub>3</sub>, batteries; P, electric
+    clock; W, terminals for connection to telephone relay; H, terminals for
+    connection to terminals J, on transmitting machine.</p>
+  </div>
+
+  <p>The switch D is then closed, and the arm of the switch D<sup>1</sup>
+  placed on the contact stud (1), at the transmitting station only. As soon
+  as the switches are closed the clutch F comes into action, and the
+  transmitting machine begins to revolve. When the whole of the line print
+  wrapped round the drum of the machine has passed under the stylus, the
+  end of the shaft D, Fig. 36, engages <!-- Page 97 --><span
+  class="pagenum"><a name="page97"></a>{97}</span>with the spring <i>m</i>,
+  breaking the clutch circuit and allowing the motor to run free. As soon
+  as the machine stops, the switch D is opened and the machine run back to
+  its starting position by hand.</p>
+
+  <p>At the receiving station the switch D is also closed, and the arm of
+  the switch D<sup>1</sup> placed on the contact stud (2). The closing of
+  these switches does not bring the clutch F into operation until current
+  from the telephone relay U connected to the wireless receiving apparatus
+  works the sensitive polarised relay K, which in turn completes the
+  circuit of the circuit-breaker L. When the armature of L is attracted,
+  the circuit of the relay K is broken, the circuit of the clutch F is
+  completed, and the machine starts revolving.</p>
+
+  <div class="figright" style="width:13%;">
+      <a href="images/illo-fig52.png"><img style="width:100%" src="images/illo-fig52.png"
+      alt="Fig. 52." title="Fig. 52." /></a>
+    <span class="sc">Fig. 52.</span>
+  </div>
+
+  <p>The current from the relay U, due to the transmitting stylus passing
+  over <i>one</i> contact strip on the metal print, is too brief to actuate
+  the heavier mechanism of the relay K, hence the need of the margin of
+  bare foil at the commencing edge of the metal print, so that a
+  practically continuous current will flow to the relay K until the
+  armature is attracted. As, however, the relay is not actuated at the
+  receipt of the first signal, and as it is necessary for the machine to
+  start recording at a certain point on the film, viz. <!-- Page 98
+  --><span class="pagenum"><a name="page98"></a>{98}</span>at the edge of
+  the lap&mdash;the reason for this was given in Chapter IV.&mdash;the
+  starting position of the receiving drum will be similar to that given in
+  the diagram Fig. 52, where X indicates the lap of the photographic film,
+  and the arrow the direction of rotation.</p>
+
+  <p>It is, of course, obvious that a somewhat similar adjustment must be
+  made with regard to the position of the stylus on the metal print at the
+  transmitting machine.</p>
+
+  <p>In the present system, as in almost every photographic method of
+  receiving that has been described, the Nernst lamp is invariably
+  mentioned as the source of illumination. Since the advent of the
+  high-voltage metal-filament lamps the Nernst lamp has fallen somewhat
+  into disuse for commercial purposes, but it possesses certain
+  characteristics that render it eminently suitable for the purpose under
+  discussion.</p>
+
+  <p>The main principle of this type of lamp depends upon the discovery
+  made by Professor Nernst in 1898, after whom the lamp is named, that
+  filaments of certain earthy bodies when raised to a red heat became
+  conductive sufficiently well to pass a current which raised it to a white
+  heat, and furthermore that the glowing filament emitted a brighter light
+  for a given amount of current than carbon filaments.</p>
+
+  <div class="figright" style="width:19%;">
+      <a href="images/illo-fig52a.png"><img style="width:100%" src="images/illo-fig52a.png"
+      alt="Fig. 52a." title="Fig. 52a." /></a>
+    <span class="sc">Fig.</span> 52<i>a</i>.
+  </div>
+
+  <p>Nernst lamps are made in two sizes, the larger <!-- Page 99 --><span
+  class="pagenum"><a name="page99"></a>{99}</span>being intended for the
+  same work as usually done by arc lamps, and the smaller to replace
+  incandescent lamps; the smaller type being made to fit into the ordinary
+  bayonet lampholders. The principal parts of a Nernst lamp consist of the
+  filament, the heater, the automatic cut-out, and the resistance, and
+  their arrangement in the smaller type of lamp is given in the diagram,
+  Fig. 52<i>a</i>. The current enters at the positive terminal, passes
+  through the heater M, and out through the negative terminal. The filament
+  B, which consists of a short length of an infusible earth made of the
+  oxides of several rare minerals, of which zirconia is one, is a
+  non-conductor at first, but becomes a conductor upon being raised to a
+  high temperature by means of the heater M. As soon as the filament
+  becomes conductive the current then passes through the automatic cut-out
+  H, and the armature D is attracted, thus breaking the heater circuit. The
+  current then flows from the positive terminal <!-- Page 100 --><span
+  class="pagenum"><a name="page100"></a>{100}</span><span class="figleft"
+  style="width:34%;"><a href="images/illo-fig52b.png"><img
+  style="width:100%" src="images/illo-fig52b.png" alt="Fig. 52b."
+  title="Fig. 52b." /></a><span class="sc">Fig.</span> 52<i>b</i>.</span>
+  through the cut-out H, resistance J, and filament B, and from thence out
+  of the lamp. Since the resistance of the filament decreases the hotter it
+  gets, it is necessary to insert a ballasting resistance in series with it
+  which has the opposite property of increasing its resistance as it gets
+  hotter, to prevent the filament taking too much current and destroying
+  itself. Such a resistance, J, consists of a filament of fine iron wire,
+  which, to prevent oxidation from exposure to the air, is enclosed in a
+  glass bulb filled with hydrogen gas. Fig. 52<i>b</i> shows the form of
+  ballast resistance used in the small and large type of lamp
+  respectively.</p>
+
+  <p>Either direct or alternating current can be used with these lamps, and
+  with direct current the polarity must be strictly observed, and that the
+  positive wire is connected to the positive and the <!-- Page 101 --><span
+  class="pagenum"><a name="page101"></a>{101}</span>negative wire to the
+  negative terminal. With the smaller type of lamp once it has been
+  correctly placed in its holder it is essential that it should not be
+  turned, as a change in the direction of the current will rapidly destroy
+  the filament.</p>
+
+  <div class="figright" style="width:19%;">
+      <a href="images/illo-fig52c.png"><img style="width:100%" src="images/illo-fig52c.png"
+      alt="Fig. 52c." title="Fig. 52c." /></a>
+    <span class="sc">Fig.</span> 52<i>c</i>.
+  </div>
+
+  <p>The arrangement of the larger type of Nernst lamp can be readily seen
+  from the drawing, Fig. 52<i>c</i>.</p>
+
+  <p>Care must be taken to see that the voltage required by the burner and
+  resistance equals the voltage of the supply circuit, and that only parts
+  of the same amperage are used together on the same lamp. No advantage is
+  obtained by over-running a Nernst lamp, this only shortening its life
+  without increasing the light. Under normal conditions the average life of
+  the burner is about 700 hours.</p>
+
+  <p>The efficiency of the Nernst lamp is fairly high, being only 1.45 to
+  1.75 watts per c.p. The light given is remarkably steady, and the lamps
+  are adaptable for all voltages from 100 to 300. In one of the large type
+  of lamps for use on a 235-volt <!-- Page 102 --><span class="pagenum"><a
+  name="page102"></a>{102}</span>circuit the burner takes 0.5 ampere at 215
+  volts, and the resistance 0.5 ampere at 20 volts, while one of the
+  smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts
+  and 0.25 ampere at 20 volts for the burner and resistance respectively.
+  The burner and heater are very fragile, and should never be handled
+  except by the porcelain plate to which they are attached. The lamps burn
+  in air and emit a brilliant white light of high actinic power, the
+  intrinsic brilliancy (c.p./square inch) varying from 1000 to 2500, as
+  compared with 1000 to 1200 for ordinary metal filament lamps, and 300 to
+  500 for carbon filament lamps.</p>
+
+  <p>The chief advantage of the Nernst lamp from a photographic point of
+  view lies in the fact that it produces abundantly the blue and violet
+  rays which have the greatest chemical effect upon a photographic plate or
+  film. These rays are known as chemical or actinic rays, and are only
+  slightly produced in some types of incandescent electric lamps.
+  Carbon-filament lamps are very poor in this respect.</p>
+
+  <p>Because a light is visually brilliant it must by no means be assumed
+  that it is the best to use for purposes of photography, and this is a
+  point over which many photographers stumble when using artificial light.
+  Many sources of light, while excellent for illumination, have very low
+  actinic powers, while others may have low illuminating but high <!-- Page
+  103 --><span class="pagenum"><a name="page103"></a>{103}</span>actinic
+  powers. A lamp giving a light yellowish in colour has usually low actinic
+  power, while all those lamps giving a soft white light are generally
+  found to be highly actinic.</p>
+
+  <p>In addition to the actinic value of the source of illumination, the
+  photographic film used must be very carefully chosen, as the chemical
+  inertia of the sensitised film plays an important part in the successful
+  reproduction of the picture, and also, to a certain extent, affects the
+  speed of transmission. The length of exposure, the amount of light
+  admitted to the film, and the characteristics of the film itself, are all
+  factors which have a decided bearing upon the quality of the results
+  obtained, and the film found to be most suitable in one case will perhaps
+  give very unsatisfactory results in another.</p>
+
+  <p>In photo-telegraphy the length of exposure is determined by the time
+  taken by the transmitting stylus to trace over a conducting strip on the
+  metal print, and this time, of course, varies with the density of the
+  image and also with the speed of transmission.</p>
+
+  <p>The film in ordinary photography is chosen with regard to the subject
+  and the existing light conditions, and the amount of light admitted to
+  the film and the length of exposure are regulated accordingly. No such
+  latitude is, however, possible in photo-telegraphy. With each set of
+  apparatus <!-- Page 104 --><span class="pagenum"><a
+  name="page104"></a>{104}</span>the various factors, such as the light
+  value, the amount of light admitted to the film, and the length of
+  exposure, will be practically fixed quantities, and the film that will
+  give the most satisfactory results under these fixed conditions can only
+  be found by the rough-and-ready method of "trial and error."</p>
+
+  <p>The films in common use are manufactured in four qualities, namely,
+  ordinary, studio, rapid, and extra rapid. These terms should really
+  relate to the light sensitiveness of the film (or, as it is technically
+  termed, the speed), but at the best they are a rough and very
+  unsatisfactory guide, for the reason that some unscrupulous makers,
+  purely for business purposes, do not hesitate to label their films and
+  plates as slow, rapid, etc., without troubling to make any tests for
+  correct classification.</p>
+
+  <p>The speed of photographic films and plates is generally indicated by a
+  number, and the system of standardisation adopted by the majority of
+  makers in this country is that originated by Messrs. Hurter &amp;
+  Driffield, abbreviated H. &amp; D. In their system the speed of the film
+  and the exposure varies in geometrical proportion, a film marked H. &amp;
+  D. 50 requiring double the exposure of one marked H. &amp; D. 100. The
+  highest number always denotes the highest speed, and the exposure varies
+  inversely with the speed.</p>
+
+  <p>Besides the Hurter &amp; Driffield method of <!-- Page 105 --><span
+  class="pagenum"><a name="page105"></a>{105}</span>obtaining the speed
+  numbers of plates and films adopted by a large number of makers in this
+  country, there are also two standard English systems known as the W.P.
+  No. (Watkin's power number) and Wynne F. No., both of which are used to a
+  fair extent.</p>
+
+  <p>The "Actinograph" number or speed number of a plate in the H. &amp; D.
+  system is found by dividing 34 by a number known as the Inertia, the
+  Inertia, which is a measure of the insensitiveness of the plate, being
+  determined according to the directions laid down by Hurter &amp;
+  Driffield&mdash;that is, by using pyro-soda developer and the straight
+  portion only of the density curve. If, for instance, the Inertia was
+  found to be one-fifth, then the speed number would be 34 ÷
+  <sup>1</sup>/<sub>5</sub> = 170, and the plate is H. &amp; D. 170. The
+  W.P. No. is found by dividing 50 by the Inertia. Thus 50 ÷
+  <sup>1</sup>/<sub>5</sub> = 250, and the plate is W.P. 250, but for all
+  practical purposes the W.P. No. can be taken as one and a half times H.
+  &amp; D. The Wynne F. numbers may be found by multiplying the square root
+  of the Watkins number by 6.4. Thus</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>&radic;250 = 15.81, and 15.81 × 6.4 = W.F. 101.</p>
+    </div>
+  </div>
+
+  <p>For those photographers who are in the habit of using an actinometer
+  giving the plate speeds in H. &amp; D. numbers, the following table,
+  taken from the <i>Photographer's Daily Companion</i>, is given, <!-- Page
+  106 --><span class="pagenum"><a name="page106"></a>{106}</span>which
+  shows at a glance the relative speed numbers for the various systems. The
+  Watkins and Wynne numbers only hold good, however, when the inertia has
+  been found by the H. &amp; D. method.</p>
+
+<p class="cenhead"><span class="sc">Table of Comparative Speed Numbers for Plates and Films</span></p>
+
+<table class="allbctr" summary="Speed Numbers." title="Speed Numbers.">
+<tr><td class="allb" style="text-align:center"> H. &amp; D.</td><td class="allb" style="text-align:center"> W.P. No.</td><td class="allb" style="text-align:center"> W.F. No.</td><td class="muspac"> </td><td class="allb" style="text-align:center"> H. &amp; D.</td><td class="allb" style="text-align:center"> W.P. No.</td><td class="allb" style="text-align:center"> W.F. No.</td></tr>
+<tr><td class="vertb" style="text-align:center"> &nbsp; 10 </td><td class="vertb" style="text-align:center"> &nbsp; 15 </td><td class="vertb" style="text-align:center"> &nbsp; 24 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 220 </td><td class="vertb" style="text-align:center"> 323 </td><td class="vertb" style="text-align:center"> 114 </td></tr>
+<tr><td class="vertb" style="text-align:center"> &nbsp; 20 </td><td class="vertb" style="text-align:center"> &nbsp; 30 </td><td class="vertb" style="text-align:center"> &nbsp; 28 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 240 </td><td class="vertb" style="text-align:center"> 352 </td><td class="vertb" style="text-align:center"> 120 </td></tr>
+<tr><td class="vertb" style="text-align:center"> &nbsp; 40 </td><td class="vertb" style="text-align:center"> &nbsp; 60 </td><td class="vertb" style="text-align:center"> &nbsp; 49 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 260 </td><td class="vertb" style="text-align:center"> 382 </td><td class="vertb" style="text-align:center"> 124 </td></tr>
+<tr><td class="vertb" style="text-align:center"> &nbsp; 80 </td><td class="vertb" style="text-align:center"> 120 </td><td class="vertb" style="text-align:center"> &nbsp; 69 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 280 </td><td class="vertb" style="text-align:center"> 412 </td><td class="vertb" style="text-align:center"> 129 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 100 </td><td class="vertb" style="text-align:center"> 147 </td><td class="vertb" style="text-align:center"> &nbsp; 77 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 300 </td><td class="vertb" style="text-align:center"> 441 </td><td class="vertb" style="text-align:center"> 134 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 120 </td><td class="vertb" style="text-align:center"> 176 </td><td class="vertb" style="text-align:center"> &nbsp; 84 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 320 </td><td class="vertb" style="text-align:center"> 470 </td><td class="vertb" style="text-align:center"> 138 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 140 </td><td class="vertb" style="text-align:center"> 206 </td><td class="vertb" style="text-align:center"> &nbsp; 91 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 340 </td><td class="vertb" style="text-align:center"> 500 </td><td class="vertb" style="text-align:center"> 142 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 160 </td><td class="vertb" style="text-align:center"> 235 </td><td class="vertb" style="text-align:center"> 103 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 380 </td><td class="vertb" style="text-align:center"> 558 </td><td class="vertb" style="text-align:center"> 150 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 200 </td><td class="vertb" style="text-align:center"> 294 </td><td class="vertb" style="text-align:center"> 109 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 400 </td><td class="vertb" style="text-align:center"> 588 </td><td class="vertb" style="text-align:center"> 154 </td></tr>
+</table>
+
+  <p>Although theoretically the higher the speed of the film the less the
+  duration of exposure required, there is a practical limit, as besides the
+  intensity and actinic value of the light admitted to the film a definite
+  time is necessary for it to overcome the chemical inertia of the
+  sensitised coating and produce a useful effect. With every make of film
+  it is possible to give so short an exposure that although light does fall
+  upon the film it does no work at all&mdash;in other words, we can say
+  that for every film there is a minimum amount of light action, and
+  anything below this is of no use. The exposure that enables the smallest
+  amount of light action to take place is termed the limit of the smallest
+  useful exposure. <!-- Page 107 --><span class="pagenum"><a
+  name="page107"></a>{107}</span></p>
+
+  <p>There is also a maximum exposure in which the light affects
+  practically all the silver in the film, and any increased light action
+  has no increased effect. This is the limit of the greatest useful
+  exposure.</p>
+
+  <p>In photo-telegraphy the duration of exposure, as already pointed out,
+  is determined by certain conditions connected with the transmitting
+  apparatus, and with conditions similar to those mentioned on page 75 the
+  length of exposure will vary roughly from 1-50th to 1-150th of a
+  second.</p>
+
+  <p>The most suitable film to use for purposes of photo-telegraphy is one
+  having a fairly slow speed in which the range of exposure required comes
+  well within the limits of the film. There is no advantage in using a film
+  having a speed of, say, H. &amp; D. 300 if good results can be obtained
+  from one with a speed of, say, H. &amp; D. 200, as the use of the higher
+  speed increases the risk of overexposure. With the high-speeded films the
+  difficulties of development are also greatly increased, there being more
+  latitude in both exposure and development with the slower speeds, and
+  consequently a better chance of obtaining a good negative.</p>
+
+  <p>Another point, often puzzling to the beginner, and which increases the
+  difficulty of choosing a suitable make of film, is that, although one
+  make of film marked H. &amp; D. 100 will give good results, another make,
+  also marked H. &amp; D. 100, will give <!-- Page 108 --><span
+  class="pagenum"><a name="page108"></a>{108}</span>very poor results. This
+  is owing, not to a poor quality film, as many suppose, but to the almost
+  insurmountable difficulty of makers being able to employ exactly the same
+  standard of light for testing purposes, so that although various makes
+  may all be standardised by the H. &amp; D. method, films bearing the same
+  speed numbers may vary in their actual speed by as much as 30 to 50 per
+  cent.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 109 --><span class="pagenum"><a name="page109"></a>{109}</span></p>
+
+<h3>APPENDIX A</h3>
+
+<p class="cenhead">SELENIUM CELLS</p>
+
+  <p>Selenium is a non-metallic element, and was first discovered by
+  Berzelius in 1817, in the deposit from sulphuric acid chambers, which
+  still continues the source from which it is obtained for commercial
+  purposes, although it is found to a small extent in native sulphur. Its
+  at. wt. is 79.2, and its sp. gr. 4.8. Symbol, Se.</p>
+
+  <p>In its natural state selenium is practically a non-conductor of
+  electricity, its resistance being forty thousand million times greater
+  than copper. Its practical value lies in the property which it possesses,
+  that when in a prepared condition it is capable of varying its electrical
+  resistance according to the amount of light to which it is exposed, the
+  resistance decreasing as the light increases.</p>
+
+  <p>Selenium is prepared by heating it to a temperature of 120° C.,
+  keeping it there for some hours, and allowing it to cool slowly, when it
+  assumes a crystalline form and changes from a bluish grey to a dull slate
+  colour. A selenium cell in its simplest form consists merely of some
+  prepared selenium placed between two or more metal electrodes, the
+  selenium acting as a high resistance conductor between them. The form
+  given by Bell and Tainter to the cells used in their experiments is given
+  in Figs. 53 and 53<i>a</i>. It consists of a number of rectangular brass
+  plates P, P', separated by very thin sheets of mica M, the mica sheets
+  being slightly narrower than the brass plates, the whole being clamped
+  together in the frame F by the two bolts B. <!-- Page 110 --><span
+  class="pagenum"><a name="page110"></a>{110}</span>By means of a sand-bath
+  the cell is raised to the desired temperature, and selenium is rubbed
+  over the surface, which melts and fills the small spaces between the
+  brass plates. All the plates P are connected together to form one
+  terminal, and the plates P' to form the other. By using very thin mica
+  sheets, and a large number of elements, a very narrow transverse section
+  of selenium, together with a large active surface, can be obtained.</p>
+
+  <p>The cell used for commercial purposes is usually constructed as
+  follows. A small rectangular piece of porcelain, slate, mica, or other
+  insulator, is wound with many turns of fine platinum wire. The wire is
+  wound double, as shown in Fig. 54, the spaces between the turns being
+  filled with prepared selenium. A thin glass cover is sometimes placed
+  over the cell to protect the surface from injury.</p>
+
+  <div class="figcenter" style="width:17%;">
+      <a href="images/illo-fig53.png"><img style="width:100%" src="images/illo-fig53.png"
+      alt="Fig. 53." title="Fig. 53." /></a>
+    <span class="sc">Fig.</span> 53.
+
+    <p class="poem">P, P', plates; M, mica; S, selenium.</p>
+  </div>
+
+  <div class="figright" style="width:18%;">
+      <a href="images/illo-fig53a.png"><img style="width:100%" src="images/illo-fig53a.png"
+      alt="Fig. 53a." title="Fig. 53a." /></a>
+    <span class="sc">Fig.</span> 53<i>a</i>.
+  </div>
+
+  <p>A strong light falling upon a cell lowers its resistance, and <i>vice
+  versa</i>, the resistance of a cell being at its highest when unexposed
+  to light; the light is apparently absorbed and made to do work by varying
+  the electrical resistance of the selenium. Selenium cells vary very
+  considerably as regards their quality as well as in their electrical
+  resistance, it being possible to obtain cells of the same size for any
+  resistance between 10 and 1,000,000 ohms, and also, a cell may remain in
+  good working condition for several months, while another will become
+  useless in as many weeks.</p>
+
+  <p>The ability of a cell to respond to very rapid changes in the
+  illumination to which it is exposed is determined largely upon its
+  inertia, it being taken as a general rule <!-- Page 111 --><span
+  class="pagenum"><a name="page111"></a>{111}</span>that the higher the
+  resistance of a cell the less the inertia, and <i>vice versa</i>, and
+  also, that the higher the resistance the greater the ratio of
+  sensitiveness. Inertia plays an important part in the working of a cell,
+  slightly opposing the drop in resistance when illuminated, and opposing
+  to a <span class="figleft" style="width:22%;"><a
+  href="images/illo-fig54.png"><img style="width:100%"
+  src="images/illo-fig54.png" alt="Fig. 54." title="Fig. 54." /></a><span
+  class="sc">Fig.</span> 54</span> much greater degree the return to normal
+  for no-illumination. The effects of inertia or "lag," as it is termed,
+  can readily be seen by reference to Fig. 55. It will be noticed that the
+  current value rapidly increases when the cell is first illuminated, but
+  if after a short time <i>t</i> the light is cut off, the current value,
+  instead of returning at once to normal for no-illumination, only
+  partially rises owing to the interference of the inertia, and some time
+  elapses before the cell returns to its normal condition; the time varying
+  from a few seconds to several minutes, depending upon the characteristics
+  of the cell and the amount of light to which it is exposed. An actual
+  curve is given in Fig. 55<i>a</i>. The inertia or "lag" of a cell
+  produces upon an intermittent current an effect similar to that produced
+  by the capacity <span class="figright" style="width:17%;"><a
+  href="images/illo-fig55.png"><img style="width:100%"
+  src="images/illo-fig55.png" alt="Fig. 55." title="Fig. 55." /></a><span
+  class="sc">Fig.</span> 55</span> of a line, as was noted in Chapter I.,
+  preventing the incoming signals from being recorded separately, and
+  distinctly. To obtain the best results in photo-telegraphy, the
+  resistance of a cell should only be decreased to an extent sufficient to
+  pass the current required to operate the recording apparatus, and the
+  illumination should be regulated so that this condition of the cell takes
+  place.</p>
+
+  <p>The comparative slowness of selenium in responding to <!-- Page 112
+  --><span class="pagenum"><a name="page112"></a>{112}</span>any great
+  changes in the illumination offers a serious difficulty to its use in
+  photo-telegraphy, but various methods have been devised whereby the
+  effects of inertia can be counteracted. In the system of De' Bernochi
+  (see Chapter I.) the changes in the illumination are neither very rapid
+  nor very great, and the inertia effects would therefore be very slight;
+  but in any photo-telegraphic system in which a metal line print is used
+  for transmitting, where the source of illumination is constant and the
+  resistance of the cell is required to drop to a definite value and return
+  to normal instantly, many times in succession, the inertia effects are
+  very pronounced. The most successful method of counteracting the inertia
+  is that adopted by Professor Korn of always keeping the cell sufficiently
+  illuminated to overcome it, so that any additional light acts very
+  rapidly. Another method worked out and patented by Professor Korn, and
+  known as the "compensating cell" method, gives a practically dead beat
+  action, the resistance returning to its normal condition as soon as the
+  illumination ceases. The arrangement is given in the diagram Fig. 56.</p>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/illo-fig55a.png"><img style="width:100%" src="images/illo-fig55a.png"
+      alt="Fig. 55a." title="Fig. 55a." /></a>
+    <span class="sc">Fig.</span> 55<i>a</i>.
+  </div>
+
+  <p>Light from the transmitting or receiving apparatus, as the case may
+  be, falls upon the selenium cell S<sup>1</sup>, which is <!-- Page 113
+  --><span class="pagenum"><a name="page113"></a>{113}</span>placed on one
+  arm of a Wheatstone bridge, a second cell S<sup>2</sup> being placed on
+  the opposite arm. The selenium cell S<sup>1</sup> should have great
+  sensitiveness and small inertia, the compensating cell S<sup>2</sup>
+  having proportionally small sensitiveness and large inertia. Two
+  batteries B, B', of about 100 volts, are connected as shown, B being
+  provided with a compensating variable resistance W; W' is also a
+  regulating resistance. When no light is falling upon the cell
+  S<sup>1</sup>, light from L is prevented from reaching the second cell
+  S<sup>2</sup> by a small shutter which is fastened to the strings of the
+  Einthoven galvanometer (described in Chapter III.), and the piece of
+  apparatus C&mdash;relay or galvanometer as the case may be&mdash;remains
+  in a normal condition. When, however, light falls upon the cell
+  S<sup>1</sup>, the balance of the bridge is upset, and light from L falls
+  a fraction of a second later upon the second cell S<sup>2</sup>, and the
+  current flowing through C completes the circuit. Needless to say it is
+  necessary that the two cells be well matched, as it is very easy to have
+  over-compensation, in which case the current is brought below zero.</p>
+
+  <div class="figcenter" style="width:20%;">
+      <a href="images/illo-fig56.png"><img style="width:100%" src="images/illo-fig56.png"
+      alt="Fig. 56." title="Fig. 56." /></a>
+    <span class="sc">Fig.</span> 56.
+  </div>
+
+  <p>It is also stated that by enclosing the cells in exhausted glass
+  tubes, their inertia can be greatly reduced and their life considerably
+  prolonged. The sensitiveness of a cell is the ratio between its
+  resistance in the dark and its resistance when illuminated. The majority
+  of cells have a ratio between 2:1 and 3:1, but Professor Korn has shown
+  mathematically that by conforming to certain conditions regarding the
+  construction the ratio of sensitiveness may be between 4:1 and 5:1. Thus
+  a cell of R = 250,000 ohms can be reduced to 60,000 ohms from the light
+  of a 16 c.p. lamp placed only a short distance away; the resistance may
+  be still <!-- Page 114 --><span class="pagenum"><a
+  name="page114"></a>{114}</span>further decreased by continuing the
+  illumination, but this produces a permanent defect in the cells termed
+  "fatigue," the cells becoming very sluggish in their action and their
+  sensitiveness gradually becoming less, the ratio between their resistance
+  in the dark and their resistance when illuminated being reduced by as
+  much as 30 per cent.</p>
+
+  <p>Excessive illumination will also produce similar results. The inertia
+  of a cell is practically unaffected by the wavelength of the light used,
+  but the maximum sensitiveness of a cell is towards the yellow-orange
+  portion of the spectrum.</p>
+
+  <p>In addition to light, heat has also been found to vary the electrical
+  resistance of selenium in a very remarkable manner. At 80° C. selenium is
+  a non-conductor, but up to 210° C. the conductivity gradually increases,
+  after which it again diminishes.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 115 --><span class="pagenum"><a name="page115"></a>{115}</span></p>
+
+<h3>APPENDIX B</h3>
+
+<p class="cenhead">PREPARING THE METAL PRINTS</p>
+
+  <p>Electricians who desire to experiment in photo-telegraphy, but who
+  have no knowledge of photography, may perhaps find the following detailed
+  description of preparing the metal prints of some value. The would-be
+  experimenter may feel somewhat alarmed at the amount of work entailed,
+  but once the various operations are thoroughly grasped, and with a little
+  patience and practice, no very great difficulty should be experienced.
+  The simpler photographic operations, such as developing, fixing, etc.,
+  cannot be described here, and the beginner is advised to study a good
+  text-book on the subject.</p>
+
+  <p>The method to be given of preparing the photographs is practically the
+  only one available for wireless transmission, and although the manner
+  given of preparing is perhaps not strictly professional, having been
+  modified in order to meet the requirements of the ordinary amateur
+  experimenter, the results obtained will be found perfectly
+  satisfactory.</p>
+
+  <p>As will have been gathered from Chapter II., the camera used for
+  copying has to have a single line screen placed a certain distance in
+  front of the photographic plate, and the object of this screen is to
+  break the image up into parallel bands, each band varying in width
+  according to the density of the photograph from which it has been
+  prepared. Thus a white portion of the photograph would consist of very
+  narrow lines wide apart, while a dark portion would be made up of wide
+  lines close together; a black part would appear solid and show no lines
+  at all. It is, of course, obvious <!-- Page 116 --><span
+  class="pagenum"><a name="page116"></a>{116}</span>that the lines on the
+  negative cannot be wider apart, centre to centre, than the lines of the
+  screen. A good screen distance has been found to be 1 to 64, <i>i.e.</i>
+  the diameter of the stop is <sup>1</sup>/<sub>64</sub>th of the camera
+  extension, and the distance of the screen lines from the photographic
+  plate is 64 times the size of the screen opening. The following table
+  shows what this distance is for the screen most likely to be used. The
+  line screens used consist of glass plates upon which a number of lines
+  are accurately ruled, the width of the lines and the spaces between being
+  equal; the lines are filled in with an opaque substance. These ruled
+  screens are very expensive, and are only made to order,<a name="NtA10"
+  href="#Nt10"><sup>[10]</sup></a> a screen half-plate size costing from
+  21s. to 27s. 6d. An efficient substitute for a ruled screen can be made
+  by taking a rather large sheet of Bristol board and ruling lines across
+  in pure black drawing ink, the width of the lines and the spaces between
+  being <sup>1</sup>/<sub>12</sub>th of an inch respectively. A photograph
+  must be taken of this card, the reduction in size determining the number
+  of lines to the inch. A card 20 × 15 inches, with 12 lines to the inch,
+  would, if reduced to 5 × 4 inches, make a screen having 48 lines to the
+  inch. Preparing the board is rather a tedious operation, but the line
+  negative produced will be found to give results almost as good as those
+  obtained from a purchased screen.</p>
+
+<p class="cenhead"><span class="sc">Diameter of Stop used <sup>1</sup>/<sub>64</sub>th of Camera Extension.</span></p>
+
+<table class="allbctr" summary="Screen line spacing." title="Screen line spacing.">
+<tr><td class="allb" style="text-align:center; vertical-align:top;"> Screen ruling<br />
+lines per inch.
+
+</td><td class="allb" style="text-align:center; vertical-align:top;"> Actual space<br />
+in inches.
+
+</td><td class="allb" style="text-align:center; vertical-align:top;"> Distance of<br />
+screen ruling<br />
+in inches.
+
+</td><td class="allb" style="text-align:center; vertical-align:top;"> In <sup>1</sup>/<sub>32</sub><br />
+inches
+
+</td><td class="allb" style="text-align:center; vertical-align:top;"> In milli-<br />
+metres</td></tr>
+<tr><td class="vertb" style="text-align:center"> 35 </td><td class="vertb" style="text-align:center"> <sup>1</sup>/<sub>70</sub> </td><td class="vertb" style="text-align:center"> .91 </td><td class="vertb" style="text-align:center"> 28.8 </td><td class="vertb" style="text-align:center"> 21.8 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 50 </td><td class="vertb" style="text-align:center"> <sup>1</sup>/<sub>100</sub> </td><td class="vertb" style="text-align:center"> .64 </td><td class="vertb" style="text-align:center"> 20.5 </td><td class="vertb" style="text-align:center"> 16.2 </td></tr>
+<tr><td class="vertb" style="text-align:center"> 65 </td><td class="vertb" style="text-align:center"> <sup>1</sup>/<sub>130</sub> </td><td class="vertb" style="text-align:center"> .49 </td><td class="vertb" style="text-align:center"> 15.7 </td><td class="vertb" style="text-align:center"> 12.4 </td></tr>
+</table>
+
+  <p>As it is impossible for many to have the use of professional apparatus
+  designed for this particular kind of work, <!-- Page 117 --><span
+  class="pagenum"><a name="page117"></a>{117}</span>the fixing of the
+  screen into an ordinary camera must be left to the ingenuity of the
+  worker. A half-plate back focussing camera will be found suitable for
+  general experimental work, but if this is not available, a large box
+  camera can be pressed into service.</p>
+
+  <div class="figleft" style="width:24%;">
+      <a href="images/illo-fig57.png"><img style="width:100%" src="images/illo-fig57.png"
+      alt="Fig. 57." title="Fig. 57." /></a>
+    <span class="sc">Fig.</span> 57.
+  </div>
+
+  <p>The writer has never seen a half-plate box camera, but one taking a 5
+  × 4 inch plate can be obtained second-hand very cheaply. It is a
+  comparatively simple matter to fix the line screen into a camera of this
+  description, the drawings Figs. 57 and 58 showing the method adopted by
+  the writer. The two clips D, made from fairly stout brass about
+  <sup>1</sup>/<sub>2</sub> inch wide, are bent to the shape shown (an
+  enlarged section is given at C) and soldered at the top and bottom of one
+  of the metal sheaths provided for holding the plates. The distance
+  between the front of the photographic plate (the film side) and the back
+  of the line screen (also the film side), indicated by the arrow at A, is
+  determined by the number of lines on the screen. As will be seen from the
+  table given, the distance for a screen having 50 lines to the inch will
+  be <sup>41</sup>/<sub>64</sub>ths of an inch.</p>
+
+  <div class="figright" style="width:24%;">
+      <a href="images/illo-fig58.png"><img style="width:100%" src="images/illo-fig58.png"
+      alt="Fig. 58." title="Fig. 58." /></a>
+    <span class="sc">Fig.</span> 58.
+
+    <p class="poem">M, sheath; P, photographic plate; D, clips; S, line
+    screen.</p>
+  </div>
+
+  <p>In all probability there will be enough clearance between the top of
+  the sheath and the top of the camera to allow for the thickness of the
+  clip, but if not, a shallow groove a little wider than the clip should be
+  carefully cut in the top of the camera, so that it will slide in easily.
+  The screen should be placed between the clips, the film side on the <!--
+  Page 118 --><span class="pagenum"><a
+  name="page118"></a>{118}</span>inside, <i>i.e.</i> facing the
+  photographic plate. As with a box camera the extension is a fixture, the
+  size of stop to be used is a fixture also. The extension of a camera
+  (this term really applies to a bellows camera) is measured from the front
+  of the photographic plate to the diaphragm, and if this distance in our
+  camera is 8 inches, then the diameter of the stop to give the best
+  results would be <sup>1</sup>/<sub>64</sub>th of this, or
+  <sup>1</sup>/<sub>8</sub>th inch. Although for all ordinary experimental
+  work the lens fitted to the camera will be suitable, the best type of
+  lens for process work of all kinds is the "Anastigmat."</p>
+
+  <p>The picture or photograph from which it is desired to make a print
+  should be fastened out perfectly flat upon a board with drawing pins, and
+  if a copying stand is not available it must be placed upright in some
+  convenient position. The diagram Fig. 59 gives the disposition of the
+  apparatus required for copying. A simple and inexpensive copying stand is
+  shown in Fig. 60. The blackboard A should be about 30 inches square, and
+  must be fastened perfectly upright upon the base-board B. The stand C
+  should be made so that it slides without any side play between the guides
+  D, and should be of such a height that the lens of the camera comes
+  exactly opposite the <!-- Page 119 --><span class="pagenum"><a
+  name="page119"></a>{119}</span><span class="figleft"
+  style="width:32%;"><a href="images/illo-fig59.png"><img
+  style="width:100%" src="images/illo-fig59.png" alt="Fig. 59." title="Fig. 59."
+  /></a><span class="sc">Fig.</span> 59. L, L, lamps; A, board with
+  picture; S, line screen; P, photographic plate.</span> <span
+  class="figright" style="width:41%;"><a href="images/illo-fig60.png"><img
+  style="width:100%" src="images/illo-fig60.png" alt="Fig. 60." title="Fig. 60."
+  /></a><span class="sc">Fig.</span> 60.</span> centre of the board A. The
+  camera, if of the box type, can be secured to the stand by means of a
+  screw and wingnut, the screw being passed from the inside as shown. The
+  beginner is advised to photograph only very bold and simple subjects,
+  such as black and white drawings or enlargements. It is not safe to trust
+  to the view-finders as to whether the whole of the picture is included on
+  the plate, a piece of ground glass the same size as the plate sheaths,
+  and used as a focussing screen, being much more reliable. It is a good
+  plan to focus the camera for a number of different-sized pictures,
+  marking the board A, and the <!-- Page 120 --><span class="pagenum"><a
+  name="page120"></a>{120}</span>guides D, so that adjustment is afterwards
+  a very simple matter.</p>
+
+  <p>The make of plate used is also a great factor in getting a good
+  negative, and Wratten Process Plates will be found excellent. As already
+  mentioned, such subjects as the exposure and the development of the plate
+  cannot be dealt with here, these subjects having been exhaustively
+  treated in several text-books on photography. With an arc lamp the
+  exposure is about twice as long as in daylight, but the exposure varies
+  with the amount of light admitted to the plate, character of the source
+  of light, and the sensitiveness of the plate used, etc. The writer has
+  used acetylene gas lamps for this purpose with great success. The
+  beginner is advised to use artificial light, as this can be kept
+  perfectly even. With daylight, however, the light is constantly
+  fluctuating, and this renders the use of an actinometer a necessity for
+  correct exposure. After development, if the plate is required for
+  immediate use, it can be quickly dried by soaking for a few minutes in
+  methylated spirit.</p>
+
+  <p>Having obtained a good negative, the next operation is to prepare what
+  is known as a metal print. For this we shall require some stout tin-foil
+  or lead-foil, about 12 or 15 square feet to the pound, and this should be
+  cut into pieces of such a size that it allows a lap of
+  <sup>3</sup>/<sub>16</sub> inch when wrapped round the drum of the
+  transmitting machine. Obtain some good fish-glue and add a saturated
+  solution of bichromate of potash in the proportion of 4 parts of potash
+  to 40 or 50 parts of glue. Pour a little of this glue into a shallow
+  dish, lay a sheet of foil upon a flat board, and with a fairly stiff
+  brush (a flat hog's-hair as wide as possible) proceed to coat the sheet
+  of foil with a thin but perfectly even coating of glue. The thickness of
+  the coating can only be found by trial, for if the coating is too thick a
+  longer time will be required for printing; but it must not be thin enough
+  to show interference colours. After the coating has been laid on, a soft
+  brush, such as photographers use for dusting dry <!-- Page 121 --><span
+  class="pagenum"><a name="page121"></a>{121}</span>plates, should be
+  passed up and down, and across and across, with light, even strokes to
+  remove any unevenness. A glue solution used by professional
+  photo-engravers is as follows:</p>
+
+<table class="nob" summary="Glue solution." title="Glue solution.">
+<tr><td class="spacsingle"> Fish-glue </td><td class="spacsingle"> 12 oz.</td></tr>
+<tr><td class="spacsingle"> Bichromate of Ammonia </td><td class="spacsingle"> <sup>3</sup>/<sub>4</sub> oz.</td></tr>
+<tr><td class="spacsingle"> Water </td><td class="spacsingle"> 18 to 24 oz.</td></tr>
+<tr><td class="spacsingle"> Ammonia .880 </td><td class="spacsingle"> 30 minims.</td></tr>
+</table>
+
+  <p>The bichromate should be dissolved in the water, and, when added to
+  the glue, stir very thoroughly in order that complete mixing may take
+  place. The coating may be done in a good light, not bright sunlight, but
+  <i>it must be dried in the dark</i>, because, although insensitive while
+  in a moist condition, it becomes sensitive immediately on desiccation. If
+  allowed to dry in the light the whole coating will become insoluble, and
+  for this reason the brushes used should be washed out as soon as they are
+  finished with. The sheets will take about 15 minutes to dry in a
+  perfectly dry room, but it is not advisable to prepare many sheets at
+  once, as they will not keep for more than two or three days.</p>
+
+  <p>The prepared negative must now be placed in an ordinary printing
+  frame, and a print taken off upon one of the metal sheets in the same way
+  as a print is taken off upon ordinary sensitised paper. In daylight the
+  exposure varies from 5 to 20 minutes, but in artificial light various
+  trials will have to be made in order to get the best results, the
+  exposure varying with the amount of bichromate in the coating; the
+  proportion of the bichromate to the glue should remain about 6 per cent.
+  Light from a 25 ampere arc lamp for 2 to 5 minutes, at a distance of 18
+  inches, will generally suffice to "print" the impression on the metal
+  sheets. The printing finished, the metal print should be laid upon a
+  sheet of glass and held under a running stream of water. The washing is
+  complete as soon as the unexposed parts of the glue coating have been
+  entirely washed away leaving the bare metal, and this will take anything
+  from 3 to 7 <!-- Page 122 --><span class="pagenum"><a
+  name="page122"></a>{122}</span>minutes, depending upon the thickness of
+  the film. As soon as it is dry the print is ready for use.</p>
+
+  <p>As already mentioned, the negative from which the metal print is made
+  requires that the lines be perfectly sharp and opaque, and the spaces
+  between perfectly transparent. Ordinary dry plates are too rapid, a
+  rather slow plate being required. Wratten Process Plates give excellent
+  results, and the following is a good developer to use with them:</p>
+
+<table class="nob" summary="Developer." title="Developer.">
+<tr><td class="spacsingle"> Glycin </td><td class="spacsingle"> &nbsp; &nbsp; 15 grammes </td><td class="spacsingle"> &nbsp; 1 oz.</td></tr>
+<tr><td class="spacsingle"> Sulphite of Soda </td><td class="spacsingle"> &nbsp; &nbsp; 40 &nbsp; &nbsp; &nbsp; ,, </td><td class="spacsingle"> 2½ &nbsp;,,</td></tr>
+<tr><td class="spacsingle"> Carbonate of Potash </td><td class="spacsingle"> &nbsp; &nbsp; 80 &nbsp; &nbsp; &nbsp; ,, </td><td class="spacsingle"> &nbsp; 5 &nbsp; ,,</td></tr>
+<tr><td class="spacsingle"> Water </td><td class="spacsingle"> 1000 c.c. </td><td class="spacsingle"> 60 &nbsp; ,,</td></tr>
+</table>
+
+  <p>This developer should be used for 6 minutes at a temperature of 50°
+  F., 3<sup>1</sup>/<sub>2</sub> minutes at 65°, and
+  1<sup>3</sup>/<sub>4</sub> minutes at 80°. It is best only used once. If
+  an intensifier is required, the following formula will be found to give
+  satisfactory results:</p>
+
+<table class="nob" summary="Intensifier." title="Intensifier.">
+<tr><td class="spacsingle"> Bichloride of Mercury </td><td class="spacsingle"> &nbsp; 1 oz. </td><td class="spacsingle"> &nbsp; &nbsp; 60 grammes.</td></tr>
+<tr><td class="spacsingle"> Hot Water </td><td class="spacsingle"> 16 &nbsp;,, </td><td class="spacsingle"> 1000 c.c.</td></tr>
+</table>
+
+  <p>Allow to cool, completely pour off from any crystals, and add:</p>
+
+<table class="nob" summary="Acid." title="Acid.">
+<tr><td class="spacsingle"> Hydrochloric Acid </td><td class="spacsingle"> 30 minims </td><td class="spacsingle"> &nbsp; &nbsp; 4 c.c.</td></tr>
+</table>
+
+  <p>Allow negative to bleach thoroughly, wash well in water, and blacken
+  in 10 per cent ammonia .880, or 5 per cent sodium sulphide.</p>
+
+  <p>In preparing the negatives and metal prints the following points
+  should be observed:</p>
+
+  <p>A good negative should have the lines perfectly sharp and opaque;
+  there should be no "fluff" between the lines even when they are close
+  together.</p>
+
+  <p>A properly exposed and developed negative should not require any
+  reducing or intensifying.</p>
+
+  <p>If the lamps used for illuminating the copying board are placed 2 feet
+  away, and the exposure required is 5 minutes, the exposure, if the lamps
+  are placed 4 feet away, will be <!-- Page 123 --><span class="pagenum"><a
+  name="page123"></a>{123}</span>20 minutes, as the amount of light which
+  falls upon an object decreases as the inverse square of the distance.</p>
+
+  <p>Get the coating on the foil as thin as possible, and err on the side
+  of over-exposure, for if the coating is thick and has been under-exposed,
+  excessive washing will dissolve the whole coating; for, unless
+  insolubilisation has taken place right up to the metal base, the under
+  parts will remain in a more or less soluble condition.</p>
+
+  <p>On no account must the unexposed sheets be placed near a fire,
+  otherwise they will be spoilt, the whole coating becoming insoluble; heat
+  acting in the same manner as light.</p>
+
+  <p>In washing, keep the print moving so that the stream of water does not
+  fall continually in one place. It is best to hold the print so that the
+  water runs off in the direction of the lines.</p>
+
+  <p>To dry the prints after washing they can be laid out flat in a
+  moderately warm oven, or before a stove, the heat of course not being
+  sufficient to cause the coating to peel.</p>
+
+  <p>To render the glue image more distinct the print should be immersed
+  for a few seconds in an aniline dye solution, the glue taking up the
+  colour readily. These dyes are soluble in either water or alcohol. A dye
+  known as "magenta" is very good.</p>
+
+  <p>The process of coating the metal sheets must be performed as quickly
+  as possible (about 10 seconds), as owing to the peculiar nature of the
+  bichromated glue it soon sets, and once this has taken place it is
+  impossible to smooth down any unevenness.</p>
+
+  <p>See that the negative and metal sheet make good contact while
+  printing.</p>
+
+  <p>If the glue solution does not adhere to the surface of the foil in a
+  perfectly even film, but assumes a streaky appearance, a little liquid
+  ammonia, or a weak solution of nitric acid, rubbed over the surface of
+  the foil, which is afterwards gently scoured with precipitated chalk on a
+  tuft of cotton <!-- Page 124 --><span class="pagenum"><a
+  name="page124"></a>{124}</span>wool, will remove the grease which is the
+  cause of the difficulty.</p>
+
+  <p>A photograph of a picture prepared from a line negative is given in
+  Fig. 61. For a great many experiments, and in order to save time,
+  trouble, and expense, sketches drawn upon stout lead-foil in an
+  insulating ink will answer the purpose admirably, but if any exact work
+  is to be done a single line print is of course absolutely necessary. The
+  insulating ink can be prepared by dissolving shellac in methylated
+  spirit, or ordinary gum can be used. A very fine brush should be used in
+  place of a pen, as the gum will not flow freely from an ordinary nib
+  unless greater pressure than the foil will safely stand be applied. A
+  sketch prepared in this manner is shown in Fig. 62. A little aniline dye
+  should be added to the gum to render it more visible, or a mixture of gum
+  and liquid indian ink will be found suitable.</p>
+
+  <div class="figcenter" style="width:22%;">
+      <a href="images/illo-fig63.png"><img style="width:100%" src="images/illo-fig63.png"
+      alt="Fig. 63." title="Fig. 63." /></a>
+    <span class="sc">Fig.</span> 63.
+  </div>
+
+  <p>With the copying arrangement already described it is only possible to
+  employ it for reducing, it being necessary to employ a bellows camera
+  with a back focussing attachment for purposes of enlarging, and this
+  constitutes the chief drawback to the use of a fixed focus camera. By
+  replacing the box camera with a focussing camera of the same size, we
+  shall have a piece of apparatus capable of reducing or enlarging, only in
+  this case the camera should be a fixture and the board, A, arranged to
+  slide backwards and forwards instead.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/illo-fig61.png"><img style="width:100%" src="images/illo-fig61.png"
+      alt="Fig. 61." title="Fig. 61." /></a>
+    <span class="sc">Fig.</span> 61.
+
+    <p class="poem">Portions of photographs (full size) of single line
+    screen, and single line print. Screen 40 lines to the inch.</p>
+  </div>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/illo-fig62.png"><img style="width:100%" src="images/illo-fig62.png"
+      alt="Fig. 62." title="Fig. 62." /></a>
+    <span class="sc">Fig.</span> 62.
+  </div>
+
+<p><!-- Page 125 --><span class="pagenum"><a name="page125"></a>{125}</span></p>
+
+  <p>An extra improvement would be to rule the surface of the copying
+  board, A, in a manner similar to that shown in the diagram, Fig. 63. The
+  rulings should be marked off from the centre of the board, and should
+  enclose parallelograms of the various plate sizes ranging from
+  3<sup>1</sup>/<sub>4</sub> × 4<sup>1</sup>/<sub>4</sub> inches up to the
+  full size of the board. By fastening the picture or photograph to be
+  copied in the space on the board corresponding in size, we can ensure
+  that it is in the correct position for the whole to be included on the
+  photographic plate, providing, of course, that the centre of lens and
+  board coincide.</p>
+
+  <p>With regard to the lens required, the practice adhered to by most
+  photographers is to use a lens having a focal length equal to the
+  diagonal of the plate used. Thus for a <sup>1</sup>/<sub>4</sub>-plate
+  camera a 5-inch lens should be used, and for a
+  <sup>1</sup>/<sub>2</sub>-plate an 8-inch lens, and so on. For a 5 × 4
+  inch camera a 6-inch lens will be required. The following is a simple
+  rule for finding the conjugate foci of a lens, and is useful in obtaining
+  the distance from the lens to the photographic plate and the picture to
+  be copied. Let us suppose that we wish to make a
+  1<sup>1</sup>/<sub>2</sub> times enlarged line negative from a
+  4<sup>1</sup>/<sub>4</sub> × 3<sup>1</sup>/<sub>4</sub> inch print. Add 1
+  to the number of times it is required to enlarge and multiply the result
+  by the focal length of the lens in inches. In the present case this will
+  be 1<sup>1</sup>/<sub>2</sub> + 1 = 2<sup>1</sup>/<sub>2</sub>; and if a
+  6-inch lens is used, 2<sup>1</sup>/<sub>2</sub> × 6 = 15 inches will be
+  the distance of the lens from the plate. Divide this number by the number
+  of times it is desired to enlarge, and the distance of the lens from the
+  picture to be copied is obtained; in this instance 15 ÷
+  1<sup>1</sup>/<sub>2</sub> = 10 inches. The same rule can be followed
+  when it is required to reduce any given number of times, only in this
+  case the greater number will represent the distance between the lens and
+  the picture to be copied, and the lesser number the distance between the
+  lens and the plate.</p>
+
+  <p>In reducing, a <sup>1</sup>/<sub>4</sub>-plate lens will be found to
+  fully cover a 5 × 4 inch plate, providing the reduction is not greater
+  than three to one.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 126 --><span class="pagenum"><a name="page126"></a>{126}</span></p>
+
+<h3>APPENDIX C</h3>
+
+<p class="cenhead">LENSES</p>
+
+  <p>In this small volume it is not desirable, neither is it intended, to
+  give an exhaustive treatment on the subject of lenses and their action,
+  but as optics plays an important part in the transmission of photographs,
+  both by wireless and over ordinary conductors, the following notes
+  relating to a few necessary principles have been included as likely to
+  prove of interest.</p>
+
+  <p>Light always travels in straight lines when in a medium of uniform
+  density, such as water, air, glass, etc., but on passing from one medium
+  to another, such as from air to water, or air to glass, the direction of
+  the light rays is changed, or, to use the correct term, <i>refracted</i>.
+  This refraction of the rays of light only takes place when the incident
+  rays are passed obliquely; if the incident rays are perpendicular to the
+  surface separating the two media they are not refracted, but continue
+  their course in a straight line.</p>
+
+  <p>All liquid and solid bodies that are sufficiently transparent to allow
+  light rays to pass through them possess the power of bending or
+  refracting the rays, the degree of refraction, as already explained,
+  depending upon the nature of the body.</p>
+
+  <p>The law relating to refraction will perhaps be better understood by
+  means of the following diagram. In Fig. 64 let the line AB represent the
+  surface of a vessel of water. The line CD, which is perpendicular to the
+  surface of the <!-- Page 127 --><span class="pagenum"><a
+  name="page127"></a>{127}</span>water, is termed the <i>normal</i>, and a
+  ray of light passed in this direction will continue in a straight line to
+  the point E. If, however, the ray is passed in an oblique direction, such
+  as ND, it will be seen that the ray is bent or refracted in the direction
+  DM; if the ray of light is passed in any other oblique direction, such as
+  JD, the refracted ray will be in the direction DK. The angle NDC is
+  called the <i>angle of incidence</i> and MDE the <i>angle of
+  refraction</i>. If we measure accurately the line NC, we shall find that
+  it is 1<sup>1</sup>/<sub>3</sub>, or more exactly 1.336, times greater
+  than the line EM. If we repeat this measurement with the lines JH and PK
+  we shall find that the line JH also bears the proportion of 1.336 to the
+  line PK. The line NC is called the <i>sine of the angle of incidence</i>
+  NDC, and EM the <i>sine of the angle of refraction</i> MDE.</p>
+
+  <div class="figcenter" style="width:20%;">
+      <a href="images/illo-fig64.png"><img style="width:100%" src="images/illo-fig64.png"
+      alt="Fig. 64." title="Fig. 64." /></a>
+    <span class="sc">Fig.</span> 64.
+  </div>
+
+  <p>Therefore in water the sine of the angle of incidence is to the sine
+  of the angle of refraction as 1.336 is to 1, and this is true whatever
+  the position of the incident ray with respect to the surface of the
+  water. From this we say that <i>the sines of the angles of incidence and
+  refraction have a constant proportion or ratio to one another</i>.</p>
+
+  <p>The number 1.336 is termed the <i>refractive index</i>, or
+  <i>coefficient</i>, or the <i>refractive power</i> of water. The
+  refractive power varies, however, with other fluids and solids, and a
+  complete table will be found in any good work on optics.</p>
+
+  <p>Glass is the substance most commonly used for refracting the rays of
+  light in optical work, the glass being worked up into different forms
+  according to the purpose for which it <!-- Page 128 --><span
+  class="pagenum"><a name="page128"></a>{128}</span>is intended. Solids
+  formed in this way are termed <i>lenses</i>. A lens can be defined as a
+  transparent medium which, owing to the curvature of its surfaces, is
+  capable of converging or diverging the rays of light passed through it.
+  According to its curvature it is either spherical, cylindrical,
+  elliptical, or parabolic. The lenses used in optics are always
+  exclusively spherical, the glass used in their construction being either
+  crown glass, which is free from lead, or flint glass, which contains lead
+  and is more refractive than crown glass. The refractive power of crown
+  glass is from 1.534 to 1.525, and of flint glass from 1.625 to 1.590.
+  Spherical surfaces in combination with each other or with plane surfaces
+  give rise to six different forms of lenses, sections of which are given
+  in Fig. 65.</p>
+
+  <div class="figcenter" style="width:41%;">
+      <a href="images/illo-fig65.png"><img style="width:100%" src="images/illo-fig65.png"
+      alt="Fig. 65." title="Fig. 65." /></a>
+    <span class="sc">Fig.</span> 65.
+  </div>
+
+  <p>All lenses can be divided into two classes, convex or converging, or
+  concave or diverging. In the figure, <i>b</i>, <i>c</i>, <i>g</i> are
+  converging lenses, being thicker at the middle than at the borders, and
+  <i>d</i>, <i>e</i>, <i>f</i>, which are thinner at the middle, being
+  diverging lenses. The lenses <i>e</i> and <i>g</i> are also termed
+  meniscus lenses, and <i>a</i> represents a prism. The line XY is the axis
+  or <i>normal</i> of these lenses to which their plane surfaces are
+  perpendicular.</p>
+
+  <p>Let us first of all notice the action of a ray of light when passed
+  through a prism. The prism, Fig. 66, is represented by the triangle BBB,
+  and the incident ray by the line TA. <!-- Page 129 --><span
+  class="pagenum"><a name="page129"></a>{129}</span>Where it enters the
+  prism at A its direction is changed and it is bent or refracted towards
+  the base of the prism, or towards the normal, this being always the case
+  when light passes from a rare medium to a dense one, and where the light
+  leaves the opposite face of the prism at D it is again refracted, but
+  away from the normal in an opposite direction to the incident ray, since
+  it is passing from a dense to a rare medium. The line DP is called the
+  <i>emergent</i> or refracted ray. If the eye is placed at T, and a bright
+  object at P, the object is seen not at P, but at the point H, since the
+  eye cannot follow the course taken by the refracted rays. In other words,
+  objects viewed through a prism always appear deflected towards its
+  summit.</p>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/illo-fig66.png"><img style="width:100%" src="images/illo-fig66.png"
+      alt="Fig. 66." title="Fig. 66." /></a>
+    <span class="sc">Fig.</span> 66.
+  </div>
+
+  <p>In considering the action of a lens we can regard any lens as being
+  built up of a number of prisms with curved faces in contact. Such a lens
+  is shown in Fig. 67, the light rays being refracted towards the base of
+  the prisms or towards the normal, as already explained; while the top
+  half of the lens will refract all the light downwards, the bottom half
+  will act as a series of inverted prisms and refract all the light
+  upwards.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/illo-fig67.png"><img style="width:100%" src="images/illo-fig67.png"
+      alt="Fig. 67." title="Fig. 67." /></a>
+    <span class="sc">Fig.</span> 67.
+  </div>
+
+  <div class="figright" style="width:27%;">
+      <a href="images/illo-fig68.png"><img style="width:100%" src="images/illo-fig68.png"
+      alt="Fig. 68." title="Fig. 68." /></a>
+    <span class="sc">Fig.</span> 68.
+  </div>
+
+  <p>If a beam of parallel light&mdash;such as light from the sun&mdash;be
+  passed through a double convex lens L, Fig. 68, we shall find that the
+  rays have been refracted from their parallel course and brought together
+  at a point F. This point F is <!-- Page 130 --><span class="pagenum"><a
+  name="page130"></a>{130}</span>termed the principal focus of the lens,
+  and its distance from the lens is known as the focal length of that lens.
+  In a double and equally convex lens of glass the focal length is equal to
+  the radius of the spherical surfaces of the lens. If the lens is a
+  plano-convex the focal length is twice the radius of its spherical
+  surfaces. If the lens is unequally convex the focal length is found by
+  the following rule: multiply the two radii of its surfaces and divide
+  twice that product by the sum of the two radii, and the quotient will
+  <!-- Page 131 --><span class="pagenum"><a
+  name="page131"></a>{131}</span>be the focal length required. Conversely,
+  by placing a source of light at the point F the rays will be projected in
+  a parallel beam the same diameter as the lens. If, however, instead of
+  being parallel, the rays proceed from a point farther from the lens than
+  the principal focus, as at A, Fig. 69, they are termed divergent rays,
+  but they also will be brought to a focus at the other side of the lens at
+  the point <i>a</i>. If the source of light A is moved nearer to the
+  principal focus of the lens to a point A<sup>1</sup> the rays will come
+  to a focus at the point <i>a</i><sup>1</sup>, and similarly when the
+  light is at A<sup>2</sup> the rays will come to a focus at the point
+  <i>a</i><sup>2</sup>. It can be found by direct experiment that the
+  distance <i>fa</i> increases in the same proportion as AF diminishes, and
+  diminishes in the same proportion as AF increases. The relationship which
+  exists between pairs of points in this manner is termed the <i>conjugate
+  foci</i> of a lens, and though every lens has only one principal focus,
+  yet its conjugate foci are innumerable.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig69.png"><img style="width:100%" src="images/illo-fig69.png"
+      alt="Fig. 69." title="Fig. 69." /></a>
+    <span class="sc">Fig.</span> 69.
+  </div>
+
+  <p>The formation of an image of some distant object in its principal
+  focus is one of the most useful properties of a convex lens, and it is
+  this property that forms the basis of several well-known optical
+  instruments, including the camera, telescope, microscope, etc.</p>
+
+  <p>If we take an oblong wooden box, AA, and substitute a sheet of ground
+  glass, C, for one end, and drill a small pinhole, H, in the centre of the
+  other end opposite the <!-- Page 132 --><span class="pagenum"><a
+  name="page132"></a>{132}</span>glass plate, we shall find that a
+  tolerably good image of any object placed in front of the box will be
+  formed upon the glass plate. The light rays from all points of the
+  object, BD, Fig. 70, will pass straight through the hole H, and
+  illuminate the ground glass screen at points immediately opposite them,
+  forming a faint inverted image of the object BD. The purpose of the hole
+  H is to prevent the rays from any one point of the object from falling
+  upon any other point on the glass screen than the point immediately
+  opposite to it, therefore the smaller we make H, the more distinct will
+  be the image obtained. Reducing the size of H in order to produce a more
+  distinct image has the effect of causing the image to become very faint,
+  as the smaller the hole in H, the smaller the number of rays that can
+  pass through from any point of the object. By enlarging the hole H
+  gradually, the image will become more and more indistinct until such a
+  size is reached that it disappears altogether.</p>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/illo-fig70.png"><img style="width:100%" src="images/illo-fig70.png"
+      alt="Fig. 70." title="Fig. 70." /></a>
+    <span class="sc">Fig.</span> 70.
+  </div>
+
+  <p>If in this enlarged hole we place a double convex lens, LL, Fig. 71,
+  whose focal length suits the length of the box, the image produced will
+  be brighter and more distinct than that formed by the aperture, H, since
+  the rays which proceed from any point of the object will be brought by
+  the lens to a focus on the glass screen, forming a bright <!-- Page 133
+  --><span class="pagenum"><a name="page133"></a>{133}</span>distinct image
+  of the point from which they come. The image owes its increased
+  distinctness to the fact that the rays from any one point of the object
+  cannot interfere with the rays from any other point, and its increased
+  brightness to the great number of rays that are collected by the lens
+  from each point of the object and focussed in the corresponding point of
+  the image. It will be evident from a study of Fig. 71 that the image
+  formed by a convex lens must necessarily be inverted, since it is
+  impossible for the rays from the end, M, of the object to be carried by
+  refraction to the upper end of the image at <i>n</i>. The relative
+  positions of the object and image when placed at different distances from
+  the lens are exactly the same as the conjugate foci of light rays as
+  shown in Fig. 69.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/illo-fig71.png"><img style="width:100%" src="images/illo-fig71.png"
+      alt="Fig. 71." title="Fig. 71." /></a>
+    <span class="sc">Fig.</span> 71.
+  </div>
+
+  <p>The length of the image formed by a convex lens is to the length of
+  the object as the distance of the image is to the distance of the object
+  from the lens. For example, if a lens having a focal length of 12 inches
+  is placed at a distance of 1000 feet from some object, then the size of
+  the image will be to that of the object as 12 inches to 1000 feet, or
+  1000 times smaller than the object; and if the length of the object is
+  500 inches, then the length of the image will be the
+  <sup>1</sup>/<sub>1000</sub>th part of 500 inches, or
+  <sup>1</sup>/<sub>2</sub> inch. <!-- Page 134 --><span class="pagenum"><a
+  name="page134"></a>{134}</span></p>
+
+  <p>The image formed by the convex lens in Fig. 71 is known as a <i>real
+  image</i>, but in addition convex lenses possess the property of forming
+  what are termed <i>virtual images</i>. The distinction can be expressed
+  by saying, <i>real images are those formed by the refracted rays
+  themselves, and virtual images those formed by their prolongations</i>.
+  While a real image formed by a convex lens is always inverted and smaller
+  than the object, the virtual image is always erect and larger than the
+  object. The power possessed by convex lenses of forming virtual images is
+  made use of in that useful but common piece of apparatus known as a
+  reading or magnifying glass, by which objects placed within its focus are
+  made larger or magnified when viewed through it; but in order to properly
+  understand how objects seem to be brought nearer and apparently increased
+  in size, we must first of all understand what is meant by the expression,
+  <i>the apparent magnitude of objects</i>.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/illo-fig72.png"><img style="width:100%" src="images/illo-fig72.png"
+      alt="Fig. 72." title="Fig. 72." /></a>
+    <span class="sc">Fig.</span> 72.
+  </div>
+
+  <p>The apparent magnitude of an object depends upon the angle which it
+  subtends to the eye of the observer. The image at A, Fig. 72, presents a
+  smaller angle to the eye than the angle presented by the object when
+  moved to B, and the image therefore appears smaller. When the object is
+  moved to either B or C, it is viewed under a much <!-- Page 135 --><span
+  class="pagenum"><a name="page135"></a>{135}</span>greater angle, causing
+  the image to appear much larger. If we take a watch or other small
+  circular object and place it at A, which we will suppose is a distance of
+  50 yards, we shall find that it will be only visible as a circular
+  object, and its apparent magnitude or the angle under which it is viewed
+  is then stated to be very small. If the object is now moved to the point
+  B, which is only 5 feet from the eye, its apparent magnitude will be
+  found to have increased to such an extent that we can distinguish not
+  only its shape, but also some of the marking. When moved to within a few
+  inches from the eye as at C, we see it under an angle so great that all
+  the detail can be distinctly seen. By having brought the object nearer
+  the eye, thus rendering all its parts clearly visible, we have actually
+  magnified it, or made it appear larger, although its actual size remains
+  exactly the same. When the distance between the object and the observer
+  is known, the apparent magnitude of the object varies inversely as the
+  distance from the observer.</p>
+
+  <p>Let us suppose that we wish to produce an image of a tree situated at
+  a distance of 5000 feet. At this distance the light rays from the tree
+  will be nearly parallel, so that if a lens having a focal length of 5
+  feet is fastened in any convenient manner in the wall of a darkened room
+  the image will be formed 5 feet behind the lens at its principal focus.
+  If a screen of white cardboard be placed at this point we shall find that
+  a small but inverted image of the tree will be focussed upon it. As the
+  distance of the object is 5000 feet, and as the size of the received
+  image is in proportion to this distance divided by the focal length of
+  the lens, the image will be as 5000 ÷ 5, or 1000 times smaller than the
+  object.</p>
+
+  <p>If now the eye is placed six inches behind the screen and the screen
+  removed, so that we can view the small image distinctly in the air, we
+  shall see it with an apparent magnitude as much greater than if the same
+  small image were equally far off with the tree, as 6 inches is to 5000
+  <!-- Page 136 --><span class="pagenum"><a
+  name="page136"></a>{136}</span>feet, that is 10,000 times. Thus we see
+  that although the image produced on the screen is 1000 times less than
+  the tree from one cause, yet on account of it being brought near to the
+  eye it is 10,000 times greater in apparent magnitude; therefore its
+  apparent magnitude is increased as 10,000 ÷ 1000, or 10 times. This means
+  that by means of the lens it has actually been magnified 10 times. This
+  magnifying power of a lens is always equal to the focal length divided by
+  the distance at which we see small objects most distinctly, viz. 6
+  inches, and in the present instance is 60 ÷ 6, or 10 times.</p>
+
+  <p>When the image is received upon a screen the apparatus is called a
+  <i>camera obscura</i>, but when the eye is used and sees the inverted
+  image in the air, then the apparatus is termed a <i>telescope</i>.</p>
+
+  <p>The image formed by a convex lens can be regarded as a new object, and
+  if a second lens is placed behind it a second image will be formed in the
+  same manner as if the first image were a real object. A succession of
+  images can thus be formed by convex lenses, the last image being always
+  treated as a fresh object, and being always an inverted image of the one
+  before. From this it will be evident that additional magnifying power can
+  be given to our telescope with one lens by bringing the image nearer the
+  eye, and this is accomplished by placing a short focus lens between the
+  image and the eye. By using a lens having a focal length of 1 inch, and
+  such a lens will magnify 6 times, the total magnifying power of the two
+  lenses will be 10 × 6 = 60 times, or 10 times by the first lens and 6
+  times by the second. Such an instrument is known as a <i>compound or
+  astronomical telescope</i>, and the first lens is called the object glass
+  and the second lens the magnifying glass, or eye-piece.</p>
+
+  <p>We are now in a position to understand how virtual images are formed,
+  and the formation of a virtual image by means of a convex lens will be
+  readily followed from a <!-- Page 137 --><span class="pagenum"><a
+  name="page137"></a>{137}</span>study of Fig. 73. Let L represent a double
+  convex lens, with an object, AB, placed between it and the point F, which
+  is the principal focus of the lens. The rays from the object AB are
+  refracted on passing through the lens, and again refracted on leaving the
+  lens, so that an image of the object is formed at the eye, N. As it is
+  impossible for the eye to follow the bent rays from the object, a virtual
+  image is formed and is seen at A<sup>1</sup>B<sup>1</sup>, and is really
+  a continuation of the emergent rays. The magnifying power of such a lens
+  may be found by dividing 6 inches by the focal length of the lens, 6
+  inches being the distance at which we see small objects most distinctly.
+  A lens having a focal length of <sup>1</sup>/<sub>4</sub> inch would
+  magnify 24 times, and one with a focal length of
+  <sup>1</sup>/<sub>100</sub>th of an inch 600 times, and so on. The
+  magnifying power is greater as the lens is more convex and the object
+  near to the principal focus. When a single lens is applied in this manner
+  it is termed a <i>single microscope</i>, but when more than one lens is
+  employed in order to increase the magnifying power, as in the telescope,
+  then the apparatus is termed a <i>compound microscope</i>.</p>
+
+  <div class="figcenter" style="width:35%;">
+      <a href="images/illo-fig73.png"><img style="width:100%" src="images/illo-fig73.png"
+      alt="Fig. 73." title="Fig. 73." /></a>
+    <span class="sc">Fig.</span> 73.
+  </div>
+
+  <p>Unlike a convex lens, which can form both real and virtual images, a
+  concave lens can only produce a virtual image; and while the convex lens
+  forms an image larger <!-- Page 138 --><span class="pagenum"><a
+  name="page138"></a>{138}</span>than the object, the concave lens forms an
+  image smaller than the object. Let L, Fig. 74, represent a double concave
+  lens, and AB the object. The rays from AB on passing through the lens are
+  refracted, and they diverge in the direction RRRR, as if they proceeded
+  from the point F, which is the principal focus of the lens, and the
+  prolongations of these divergent rays produce a virtual image, erect and
+  smaller than the object, at A<sup>1</sup>B<sup>1</sup>. The principal
+  focal distance of concave lenses is found by exactly the same rule as
+  that given for convex lenses.</p>
+
+  <div class="figcenter" style="width:37%;">
+      <a href="images/illo-fig74.png"><img style="width:100%" src="images/illo-fig74.png"
+      alt="Fig. 74." title="Fig. 74." /></a>
+    <span class="sc">Fig.</span> 74.
+  </div>
+
+  <p>Up to the present we have assumed that all the rays of light passed
+  through a convex lens were brought to a focus at a point common to all
+  the rays, but this is really only the case with a lens whose aperture
+  does not exceed 12°. By aperture is meant the angle obtained by joining
+  the edges of a lens with the principal focus. With lenses having a larger
+  aperture the amount of refraction is greater at the edges than at the
+  centre, and consequently the rays that pass through the edges of the lens
+  are brought to a focus nearer the lens than the rays that pass through
+  the centre. Since this defect arises from the spherical form of the lens
+  it is termed <i>spherical aberration</i>, and in lenses that <!-- Page
+  139 --><span class="pagenum"><a name="page139"></a>{139}</span>are used
+  for photographic purposes the aberration has to be very carefully
+  corrected.</p>
+
+  <p>The distortion of an image formed by a convex lens is shown by the
+  diagram, Fig. 75. If we receive the image upon a sheet of white cardboard
+  placed at A, we shall find that while the outside edges will be clear and
+  distinct, the inside will be blurred, the reverse being the case when the
+  cardboard is moved to the point B.</p>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/illo-fig75.png"><img style="width:100%" src="images/illo-fig75.png"
+      alt="Fig. 75." title="Fig. 75." /></a>
+    <span class="sc">Fig.</span> 75.
+  </div>
+
+  <div class="figleft" style="width:8%;">
+      <a href="images/illo-fig76.png"><img style="width:100%" src="images/illo-fig76.png"
+      alt="Fig. 76." title="Fig. 76." /></a>
+    <span class="sc">Fig.</span> 76.
+  </div>
+
+  <div class="figright" style="width:10%;">
+      <a href="images/illo-fig77.png"><img style="width:100%" src="images/illo-fig77.png"
+      alt="Fig. 70." title="Fig. 70." /></a>
+    <span class="sc">Fig.</span> 77.
+  </div>
+
+  <p>Aberration is to a great extent minimised by giving to the lens a
+  meniscus instead of a biconvex form, but as it is desirable to reduce the
+  aberration to below once the <!-- Page 140 --><span class="pagenum"><a
+  name="page140"></a>{140}</span>thickness of the lens, and as this cannot
+  be done by a single lens, we must have recourse to two lenses put
+  together. The thickness of a lens is the difference between its thickness
+  at the middle and at the circumference. In a double convex lens with
+  equal convexities the aberration is 1<sup>67</sup>/<sub>100</sub>ths of
+  its thickness. In a plano-convex lens with the plane side turned towards
+  parallel rays the aberration is 4<sup>1</sup>/<sub>2</sub> times its
+  thickness, but with the convex side turned towards parallel rays the
+  aberration is only 1<sup>17</sup>/<sub>100</sub>ths of its thickness.</p>
+
+  <p>By making use of two plano-convex lenses placed together as at Fig.
+  76, the aberration will be one-fourth of that of a single lens, but the
+  focal length of the lens, L<sup>1</sup>, must be half as much again as
+  that of L. If their focal lengths are equal the aberration will only be a
+  little more than half reduced. Spherical aberration, however, may be
+  entirely destroyed by combining a meniscus and double convex lens, as
+  shown in Fig. 77, the convex side being turned to the eye when used as a
+  lens, and to parallel rays when used as a burning glass or condenser.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 141 --><span class="pagenum"><a name="page141"></a>{141}</span></p>
+
+<h3>INDEX</h3>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>Aberration, <a href="#page139">139</a></p>
+      <p class="i2">spherical, <a href="#page138">138</a>, <a href="#page140">140</a></p>
+      <p>Accuracy of working, <a href="#page70">70</a>, <a href="#page72">72</a></p>
+      <p>Acetylene gas lamps, <a href="#page120">120</a></p>
+      <p>Actinic power, <a href="#page102">102</a></p>
+      <p>Actinograph, <a href="#page105">105</a></p>
+      <p>Actinometer, <a href="#page120">120</a></p>
+      <p>Alternating current, <a href="#page82">82</a>, <a href="#page100">100</a></p>
+      <p>Ammonia, <a href="#page123">123</a></p>
+      <p>Angle of stylus, <a href="#page24">24</a>, <a href="#page78">78</a></p>
+      <p>Aniline dye, <a href="#page123">123</a></p>
+      <p>Arcing, <a href="#page27">27</a>, <a href="#page82">82</a></p>
+      <p>Arc lamps, <a href="#page15">15</a>, <a href="#page120">120</a>, <a href="#page121">121</a></p>
+      <p>Atmospherics, <a href="#page61">61</a>, <a href="#page85">85</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Ballasting resistance, <a href="#page100">100</a></p>
+      <p>Belin, <a href="#page47">47</a></p>
+      <p>Bernochi, <a href="#page7">7</a>, <a href="#page112">112</a></p>
+      <p class="i2">system of, <a href="#page7">7</a>, <a href="#page34">34</a></p>
+      <p>Berzelius, <a href="#page109">109</a></p>
+      <p>Bichromate of potash, <a href="#page120">120</a></p>
+      <p>Blondel's oscillograph, <a href="#page47">47</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Camera obscura, <a href="#page136">136</a></p>
+      <p class="i2">extension, <a href="#page116">116</a>, <a href="#page118">118</a></p>
+      <p class="i2">choice of, <a href="#page117">117</a></p>
+      <p>Capacity of condenser, <a href="#page24">24</a>, <a href="#page78">78</a></p>
+      <p class="i2">electrostatic, <a href="#page3">3</a>, <a href="#page5">5</a></p>
+      <p class="i2">of cable, <a href="#page3">3</a></p>
+      <p class="i2">of London-Paris telephone line, <a href="#page3">3</a></p>
+      <p>Carbon bisulphide, <a href="#page53">53</a></p>
+      <p>Charbonelle, <a href="#page48">48</a></p>
+      <p class="i2">receiver of, <a href="#page48">48</a></p>
+      <p>Chemical solution, <a href="#page56">56</a></p>
+      <p>Circuit breaker, <a href="#page76">76</a></p>
+      <p>Clutch, details of, <a href="#page88">88</a>, <a href="#page89">89</a>, <a href="#page91">91</a></p>
+      <p class="i2">spring, <a href="#page71">71</a></p>
+      <p>Coating the metal sheets, <a href="#page120">120</a></p>
+      <p>Coherer, <a href="#page11">11</a>, <a href="#page40">40</a></p>
+      <p>Collecting rings, <a href="#page91">91</a></p>
+      <p>Commercial value of photo-telegraphy, <a href="#page1">1</a></p>
+      <p>Compensating selenium cell, <a href="#page112">112</a></p>
+      <p>Contact breaker, <a href="#page37">37</a></p>
+      <p>Copying arrangements, <a href="#page118">118</a>, <a href="#page125">125</a></p>
+      <p>Cross screen, <a href="#page21">21</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>De' Arsonval galvanometer, <a href="#page47">47</a>, <a href="#page73">73</a></p>
+      <p>Decoherer, <a href="#page41">41</a></p>
+      <p>Design of machines, <a href="#page21">21</a></p>
+      <p>Detectors, <a href="#page83">83</a></p>
+      <p>Developing solutions, <a href="#page105">105</a>, <a href="#page122">122</a></p>
+      <p>Diaphragm, movement of, <a href="#page48">48</a>, <a href="#page52">52</a>, <a href="#page84">84</a>, <a href="#page87">87</a></p>
+      <p>Dipping rods, <a href="#page81">81</a>, <a href="#page83">83</a></p>
+      <p>Distance of transmission, <a href="#page33">33</a></p>
+      <p>Duration of wave-trains, <a href="#page22">22</a>, <a href="#page25">25</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Early experiments, <a href="#page2">2</a></p>
+      <p>Einthoven galvanometer, <a href="#page32">32</a>, <a href="#page44">44</a>, <a href="#page45">45</a>, <a href="#page54">54</a>, <a href="#page113">113</a></p>
+      <p>Electric clock, <a href="#page93">93</a></p>
+      <p>Electrolytic receiver, <a href="#page4">4</a>, <a href="#page37">37</a>, <a href="#page54">54</a>, <a href="#page61">61</a>, <a href="#page64">64</a></p>
+      <p>Enlarging arrangements, <a href="#page124">124</a>, <a href="#page125">125</a></p>
+      <p>Experimental machine, <a href="#page20">20</a></p>
+      <p>Extraneous light, <a href="#page47">47</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Fastening electrolytic paper, <a href="#page58">58</a></p>
+      <p>Fatigue of selenium cell, <a href="#page64">64</a>, <a href="#page114">114</a></p>
+      <p>Fish glue, <a href="#page120">120</a></p>
+      <p>Flexible couplings, <a href="#page77">77</a></p>
+      <p>Frequency meter, <a href="#page65">65</a></p>
+      <p>Friction brake, <a href="#page88">88</a></p>
+    </div>
+
+    <div class="stanza">
+<!-- Page 142 --><span class="pagenum"><a name="page142"></a>{142}</span>
+      <p>High speed telegraphy, <a href="#page70">70</a></p>
+      <p>Hughes governor, <a href="#page65">65</a></p>
+      <p>Hughes printing telegraph, <a href="#page63">63</a></p>
+      <p>Hurter and Driffield, <a href="#page104">104</a></p>
+      <p>Hydrogen, <a href="#page100">100</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Incidence, angle of, <a href="#page127">127</a></p>
+      <p>Inertia, <a href="#page64">64</a>, <a href="#page65">65</a>, <a href="#page111">111</a></p>
+      <p class="i2">effects in photo-telegraphy, <a href="#page110">110</a></p>
+      <p class="i2">method of counteracting, <a href="#page103">103</a>, <a href="#page112">112</a>, <a href="#page113">113</a></p>
+      <p class="i2">effect of wave-length of light on, <a href="#page114">114</a></p>
+      <p>Intensifying solution, <a href="#page122">122</a></p>
+      <p>Isochroniser, <a href="#page89">89</a>, <a href="#page91">91</a></p>
+      <p class="i2">details of, <a href="#page91">91</a>, <a href="#page92">92</a>, <a href="#page95">95</a></p>
+      <p>Isochronism, <a href="#page64">64</a>, <a href="#page69">69</a>, <a href="#page70">70</a>, <a href="#page71">71</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Kathode rays, <a href="#page53">53</a></p>
+      <p>Knudsen, <a href="#page2">2</a></p>
+      <p class="i2">apparatus of, <a href="#page9">9</a></p>
+      <p>Korn, <a href="#page30">30</a>, <a href="#page33">33</a>, <a href="#page45">45</a>, <a href="#page65">65</a>, <a href="#page72">72</a></p>
+      <p class="i2">apparatus of, <a href="#page31">31</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Lamps, coloured, <a href="#page94">94</a></p>
+      <p>Lenses, <a href="#page85">85</a>, <a href="#page125">125</a>, <a href="#page128">128</a></p>
+      <p class="i2">principal focus of, <a href="#page130">130</a></p>
+      <p class="i2">conjugate foci of, <a href="#page131">131</a></p>
+      <p class="i2">action of, <a href="#page129">129</a></p>
+      <p class="i2">convex, <a href="#page128">128</a>, <a href="#page131">131</a>, <a href="#page136">136</a></p>
+      <p class="i2">concave, <a href="#page128">128</a>, <a href="#page138">138</a></p>
+      <p class="i2">focal length of, <a href="#page130">130</a>, <a href="#page138">138</a></p>
+      <p class="i2">aperture, <a href="#page138">138</a></p>
+      <p class="i2">meniscus, <a href="#page139">139</a></p>
+      <p>Light, diffusion of, <a href="#page86">86</a></p>
+      <p class="i2">extraneous, <a href="#page87">87</a></p>
+      <p>Limit of error in synchronising, <a href="#page64">64</a></p>
+      <p>Line balancer, <a href="#page3">3</a></p>
+      <p>Line screens, <a href="#page9">9</a>, <a href="#page15">15</a>, <a href="#page16">16</a>, <a href="#page116">116</a></p>
+      <p class="i2">making, <a href="#page116">116</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Magnifying power, <a href="#page136">136</a>, <a href="#page137">137</a></p>
+      <p>Marconi valve, <a href="#page44">44</a>, <a href="#page54">54</a></p>
+      <p class="i2">coherer, <a href="#page40">40</a></p>
+      <p>Mechanical inertia, <a href="#page33">33</a></p>
+      <p>Mercury break, <a href="#page81">81</a></p>
+      <p class="i2">churning of, <a href="#page82">82</a></p>
+      <p class="i2">containers, <a href="#page82">82</a></p>
+      <p>Mercury jet interrupter, <a href="#page29">29</a></p>
+      <p>Metal prints, <a href="#page15">15</a>, <a href="#page18">18</a>, <a href="#page32">32</a>, <a href="#page59">59</a>, <a href="#page64">64</a>, <a href="#page95">95</a>, <a href="#page120">120</a>, <a href="#page124">124</a></p>
+      <p class="i2">drying the, <a href="#page121">121</a>, <a href="#page123">123</a></p>
+      <p class="i2">exposure of, <a href="#page121">121</a></p>
+      <p class="i2">size of, <a href="#page22">22</a>, <a href="#page24">24</a>, <a href="#page75">75</a>, <a href="#page77">77</a></p>
+      <p class="i2">pressing the, <a href="#page22">22</a></p>
+      <p>Microscope, <a href="#page131">131</a>, <a href="#page137">137</a></p>
+      <p>Military uses, <a href="#page35">35</a></p>
+      <p>Mirror galvanometer, <a href="#page9">9</a>, <a href="#page42">42</a>, <a href="#page73">73</a></p>
+      <p>Mirror, <a href="#page47">47</a>, <a href="#page51">51</a></p>
+      <p>Morse code, <a href="#page35">35</a></p>
+      <p>Motor speed, <a href="#page89">89</a>, <a href="#page95">95</a></p>
+      <p class="i2">driving, <a href="#page91">91</a>, <a href="#page93">93</a>, <a href="#page95">95</a></p>
+      <p class="i2">clockwork, <a href="#page63">63</a></p>
+      <p class="i2">electric, <a href="#page63">63</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Nernst lamps, <a href="#page43">43</a>, <a href="#page85">85</a>, <a href="#page98">98</a></p>
+      <p class="i2">heater of, <a href="#page99">99</a></p>
+      <p class="i2">filament of, <a href="#page99">99</a></p>
+      <p class="i2">principle of, <a href="#page98">98</a></p>
+      <p class="i2">resistance of, <a href="#page100">100</a></p>
+      <p class="i2">efficiency of, <a href="#page101">101</a>, <a href="#page102">102</a></p>
+      <p class="i2">overrunning, <a href="#page101">101</a></p>
+      <p>Nicol prism, <a href="#page53">53</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Paper for electrolytic receiver, <a href="#page56">56</a></p>
+      <p>Parabolic reflector, <a href="#page8">8</a></p>
+      <p>Period of galvanometer, <a href="#page43">43</a>, <a href="#page44">44</a>, <a href="#page46">46</a></p>
+      <p><i>Photographic Daily Companion</i>, <a href="#page105">105</a></p>
+      <p>Photographic films, <a href="#page40">40</a>, <a href="#page43">43</a>, <a href="#page45">45</a>, <a href="#page53">53</a>, <a href="#page54">54</a>, <a href="#page62">62</a>, <a href="#page85">85</a>, <a href="#page86">86</a>, <a href="#page98">98</a></p>
+      <p class="i2">process, <a href="#page37">37</a></p>
+      <p class="i2">chemical inertia, <a href="#page103">103</a></p>
+      <p class="i2">exposure of, <a href="#page103">103</a>, <a href="#page107">107</a></p>
+      <p class="i2">speed of, <a href="#page104">104</a>, <a href="#page105">105</a></p>
+      <p class="i2">plates, orthochromatic, <a href="#page59">59</a></p>
+      <p class="i2">plates, <a href="#page120">120</a></p>
+      <p>Points to be observed in preparing metal prints, <a href="#page123">123</a></p>
+      <p>Poulsen Company, <a href="#page32">32</a>, <a href="#page47">47</a></p>
+      <p class="i2">arc, <a href="#page31">31</a></p>
+      <p>Preparing selenium, <a href="#page109">109</a></p>
+      <p class="i2">photographs for transmitting, <a href="#page15">15</a>, <a href="#page115">115</a></p>
+      <p class="i2">sketches on metal foil, <a href="#page124">124</a></p>
+      <p>Prism, <a href="#page128">128</a></p>
+      <p class="i2">action of, <a href="#page129">129</a></p>
+      <p>Process plates, <a href="#page122">122</a></p>
+      <p>Professor Nernst, <a href="#page98">98</a></p>
+    </div>
+
+    <div class="stanza">
+<!-- Page 143 --><span class="pagenum"><a name="page143"></a>{143}</span>
+      <p>Radio-photography, requirements of, <a href="#page74">74</a></p>
+      <p>Refraction, angle of, <a href="#page127">127</a></p>
+      <p>Refractive power, <a href="#page127">127</a></p>
+      <p>Relay, <a href="#page25">25</a>, <a href="#page39">39</a>, <a href="#page49">49</a>, <a href="#page53">53</a>, <a href="#page55">55</a>, <a href="#page60">60</a>, <a href="#page75">75</a></p>
+      <p class="i2">differential, <a href="#page79">79</a></p>
+      <p class="i2">polarised, <a href="#page97">97</a></p>
+      <p class="i2">working speed of, <a href="#page26">26</a>, <a href="#page75">75</a></p>
+      <p>Reproducing for newspapers, <a href="#page60">60</a></p>
+      <p>Resistance of selenium, <a href="#page109">109</a></p>
+      <p class="i2">of selenium cells, <a href="#page110">110</a></p>
+      <p class="i2">regulating, <a href="#page113">113</a></p>
+      <p>Retardation of current, <a href="#page6">6</a></p>
+      <p>Retouching, <a href="#page62">62</a></p>
+      <p>Rotary spark-gap, <a href="#page28">28</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Selenium, <a href="#page99">99</a></p>
+      <p class="i2">cells, <a href="#page8">8</a>, <a href="#page34">34</a>, <a href="#page55">55</a>, <a href="#page60">60</a>, <a href="#page64">64</a>, <a href="#page109">109</a>, <a href="#page110">110</a></p>
+      <p class="i2">machines, <a href="#page45">45</a></p>
+      <p>Self-induction, <a href="#page24">24</a>, <a href="#page78">78</a></p>
+      <p>Sensitiveness of selenium cells, <a href="#page113">113</a></p>
+      <p class="i2">ratio of, <a href="#page113">113</a></p>
+      <p>Silvered quartz threads, <a href="#page44">44</a>, <a href="#page46">46</a></p>
+      <p>Spark-gap, <a href="#page27">27</a></p>
+      <p>Speed regulator, <a href="#page68">68</a></p>
+      <p class="i2">adjustments of, <a href="#page69">69</a></p>
+      <p>Spring clutch, <a href="#page71">71</a></p>
+      <p>Starting position of machines, <a href="#page98">98</a></p>
+      <p>String galvanometer, <a href="#page32">32</a></p>
+      <p>Stylus, <a href="#page17">17</a>, <a href="#page18">18</a>, <a href="#page57">57</a>, <a href="#page61">61</a>, <a href="#page78">78</a>, <a href="#page95">95</a>, <a href="#page103">103</a></p>
+      <p class="i2">sparking at, <a href="#page24">24</a></p>
+      <p>Stylus, angle of, <a href="#page24">24</a>, <a href="#page78">78</a></p>
+      <p class="i2">defects of, <a href="#page57">57</a></p>
+      <p>Submarine cable, <a href="#page4">4</a></p>
+      <p>Synchronism, <a href="#page11">11</a>, <a href="#page20">20</a>, <a href="#page36">36</a>, <a href="#page64">64</a>, <a href="#page69">69</a>, <a href="#page71">71</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Telephograph, <a href="#page74">74</a></p>
+      <p class="i2">advantages of, <a href="#page76">76</a></p>
+      <p class="i2">method of working, <a href="#page96">96</a></p>
+      <p>Telephone receiver, <a href="#page83">83</a>, <a href="#page85">85</a></p>
+      <p class="i2">diaphragm, <a href="#page48">48</a></p>
+      <p class="i2">improved, <a href="#page51">51</a></p>
+      <p>Telephone relay, <a href="#page48">48</a>, <a href="#page50">50</a>, <a href="#page52">52</a>, <a href="#page83">83</a>, <a href="#page85">85</a>, <a href="#page97">97</a></p>
+      <p>Telescope, <a href="#page131">131</a>, <a href="#page136">136</a></p>
+      <p>Thermodetector, <a href="#page32">32</a></p>
+      <p>Tow, <a href="#page88">88</a></p>
+      <p>Transmission, distance of, <a href="#page35">35</a>, <a href="#page72">72</a></p>
+      <p class="i2">speed of, <a href="#page25">25</a>, <a href="#page35">35</a>, <a href="#page75">75</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Vibration, natural period of, <a href="#page39">39</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Watkins, <a href="#page105">105</a></p>
+      <p class="i2">power number, <a href="#page105">105</a></p>
+      <p>Waves, damped, <a href="#page30">30</a></p>
+      <p class="i2">undamped, <a href="#page30">30</a>, <a href="#page31">31</a></p>
+      <p>Wheatstone bridge, <a href="#page113">113</a></p>
+      <p>Wireless apparatus, <a href="#page13">13</a></p>
+      <p><i>Wireless World</i>, <a href="#page31">31</a></p>
+      <p>Wynne, <a href="#page105">105</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Zirconia, <a href="#page99">99</a></p>
+    </div>
+  </div>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead">THE END</p>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead"><i>Printed by</i> <span class="sc">R. &amp; R. Clark, Limited</span>, <i>Edinburgh</i>.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>PUBLICATIONS OF</h3>
+
+<h1>THE WIRELESS PRESS, <span class="sc">Ltd.</span></h1>
+
+<h3>12 AND 13 HENRIETTA STREET,<br />
+STRAND, LONDON, W.C.2.</h3>
+
+  <p><b>The Year Book of Wireless Telegraphy and Telephony.</b></p>
+
+  <p>With Map of the World, showing Wireless Stations; British, Colonial
+  and foreign "Wireless" Laws and Regulations. Price <b>10s. 6d.</b> net.
+  (<b>Post free, 11s. Inland; 11s. 4d. Abroad.</b>)</p>
+
+  <p><b>The Wireless Telegraphists' Pocket Book of Notes, Formulæ and
+  Calculations.</b></p>
+
+  <p>By Dr. <span class="sc">J. A. Fleming</span>, M.A., D.Sc., F.R.S.,
+  M.Inst.E.E., etc. A valuable compendium for Wireless Engineers and
+  Operators. Price <b>9s.</b> net. (<b>Postage 5d.</b>)</p>
+
+  <p><b>The Handbook of Technical Instruction for Wireless
+  Telegraphists.</b></p>
+
+  <p>By <span class="sc">J. C. Hawkhead</span> and <span class="sc">H. M.
+  Dowsett</span>, M.I.E.E. Provides a complete theoretical course for the
+  Postmaster-General's certificate of proficiency. 310 pages. 240 Diagrams
+  and Illustrations. Price <b>7s.</b> net. (<b>Postage 6d.</b>)</p>
+
+  <p><b>Manual de Instrucción Técnica para Operadores de Telegrafia sin
+  Hilos.</b></p>
+
+  <p>Por <span class="sc">J. C. Hawkhead</span> y <span class="sc">H. M.
+  Dowsett</span>, M.I.E.E. Precio: España, <b>10</b> pesetas; Franqueo, 1
+  peseta extra. América Latina, <b>$2.25</b>, oro, neto; Franqueo, 25 cents
+  extra. (Great Britain, <b>9s.</b>; <b>Postage 6d.</b>)</p>
+
+  <p><b>The Elementary Principles of Wireless Telegraphy.</b></p>
+
+  <p>By <span class="sc">R. D. Bangay</span>. In two Parts. Price
+  <b>3s.</b> each. (<b>Postage 4d.</b>) Or in one Volume, price <b>7s.</b>
+  net. (<b>Postage 6d.</b>) Used by H.M. Government for instructional
+  purposes.</p>
+
+  <p><b>Principios Element ales de Telegrafia sin Hilos.</b></p>
+
+  <p>Por <span class="sc">R. D. Bangay</span>. (Partes 1a y 2a en un
+  Volumen.) <b>Precio</b>: España, <b>10</b> pesetas; Franqueo, 1 peseta
+  extra. América Latina, <b>$2.25</b>, oro, neto; Franqueo, 25 cents extra.
+  (Great Britain, <b>9s.</b>; <b>Postage 6d.</b>)</p>
+
+  <p><b>Principes Élémentaires de Télégraphie sans Fil.</b></p>
+
+  <p>Par <span class="sc">R. D. Bangay</span>. (Great Britain, <b>9s.</b>;
+  <b>Postage 6d.</b>)</p>
+
+  <p><b>Magnetism and Electricity for Home Study.</b></p>
+
+  <p>By <span class="sc">H. E. Penrose</span>. Crown 8vo. Over 500 pages.
+  Price <b>5s</b>. net, (<b>Postage 6d.</b>) Contains fifty complete
+  lessons.</p>
+
+  <p><b>The Calculation and Measurement of Inductance and Capacity.</b></p>
+
+  <p>By <span class="sc">W. H. Nottage</span>, B.Sc. Invaluable to all
+  engaged in Telegraph Engineering. Indispensable to the Wireless Engineer,
+  Student and Experimenter. Price <b>3s. 6d.</b> net. (<b>Postage
+  5d.</b>)</p>
+
+  <p><b>A Short Course in Elementary Mathematics and their application to
+  Wireless Telegraphy.</b></p>
+
+  <p>By <span class="sc">S. J. Willis</span>. To Students in Wireless
+  Telegraphy, as well as those engaged in the practical application of this
+  Science, this book should prove of real value. Price <b>3s. 6d.</b> net.
+  (<b>Postage 6d.</b>)</p>
+
+  <p><b>The Marconi Official Gramophone Records.</b></p>
+
+  <p>For self-tuition in receiving Morse Signals. Price <b>4s.</b> each,
+  double-sided. (<b>Postage 9d.</b>) Set of Six Records, <b>24s.</b> post
+  <b>free</b>.</p>
+
+  <p><b>The Maintenance of Wireless Telegraph Apparatus.</b></p>
+
+  <p>By <span class="sc">P. W. Harris</span>. An up-to-date Manual, full of
+  practical hints and explanations. Diagrams of all ship installations,
+  from ¼ kw. to 5 kw. Price <b>2s. 6d.</b> net. (<b>Postage 4d.</b>)</p>
+
+  <p><b>Dictionary of Technical Terms used in Wireless Telegraphy.</b></p>
+
+  <p>By <span class="sc">Harold Ward</span>. Vest Pocket Edition. 2nd
+  Edition, revised and enlarged. Contains over 1500 definitions. Price
+  <b>2s. 6d.</b> net. (<b>Postage 2d.</b>)</p>
+
+  <p><b>Armature Model for 1½ kw. Rotary Converter.</b></p>
+
+  <p>Shows every Winding of the Converter Armature from start to finish.
+  Price <b>1s.</b> net. (<b>Postage 3d.</b>)</p>
+
+  <p><b>Morse Made Easy.</b></p>
+
+  <p>By <span class="sc">A. L. Rye</span>. Linen backed, for rapidly
+  learning the Morse Code. Price <b>3d.</b> net, or post free
+  <b>3½d.</b></p>
+
+  <p><b>Morse Code Card.</b></p>
+
+  <p>Contains full alphabet, with punctuation marks, figures, abbreviations
+  and contractions. Price <b>2d.</b>, post free.</p>
+
+  <p><b>Practical Wireless Telegraphy.</b></p>
+
+  <p>By <span class="sc">E. E. Bucher</span>. 352 pages. 340 Illustrations.
+  Price <b>12s. 6d.</b> (<b>Postage 6d.</b>)</p>
+
+  <p><b>Radio-Telephony.</b></p>
+
+  <p>By <b>Alfred N. Goldsmith</b>, Ph.D. 256 pages. 226 Illustrations.
+  Price <b>15s.</b> net. (<b>Postage 6d.</b>)</p>
+
+  <p><b>Standard Tables and Equations in Radio-Telegraphy.</b></p>
+
+  <p>By <span class="sc">Bertram Hoyle</span>, M.Sc.Tech., A.M.I.E.E. 159
+  pages. Price <b>9s.</b> net. (<b>Postage 6d.</b>)</p>
+
+  <p><b>Vacuum Tubes in Wireless Communication.</b></p>
+
+  <p>By <span class="sc">E. E. Bucher</span>. Deals with the Oscillation
+  Valve. 178 pages. 130 Illustrations. Price <b>12s. 6d.</b> net.
+  (<b>Postage 6d.</b>)</p>
+
+  <p><b>Useful Notes on Wireless Telegraphy.</b> (Students' Library.)</p>
+
+  <p>By <span class="sc">Harold E. Penrose</span>. Price <b>1s. 4d.</b> net
+  each. (<b>Postage 2d.</b>)</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>Book  I. DIRECT CURRENT.</p>
+      <p>Book II. ALTERNATING CURRENT.</p>
+      <p>Book III. HIGH-FREQUENCY CURRENT AND WAVE PRODUCTION.</p>
+      <p>Book IV. THE 1½ KW. SHIP SET.</p>
+      <p>Book V. THE OSCILLATION VALVE.</p>
+    </div>
+  </div>
+
+  <p><b>The Oscillation Valve: The Elementary Principles of its Application
+  to Wireless Telegraphy.</b></p>
+
+  <p>By <span class="sc">R. D. Bangay</span>. 215 pages. Price <b>5s.</b>
+  (<b>Postage 3d.</b>)</p>
+
+  <p><b>The Thermionic Valve and its Developments in Radio-Telegraphy and
+  Telephony.</b></p>
+
+  <p>By Dr. <span class="sc">J. A. Fleming</span>, M.A., D.Sc., F.R.S.,
+  M.Inst.E.E., etc. 279 pages. Price <b>15s.</b> (<b>Postage 6d.</b>)</p>
+
+  <p><b>Alternating Current Work: An Outline for Students of Wireless
+  Telegraphy.</b></p>
+
+  <p>By <span class="sc">A. Shore</span>. 163 pages. Price <b>3s. 6d.</b>
+  (<b>Postage 4d.</b>)</p>
+
+  <p><b>Telephony without Wires.</b></p>
+
+  <p>By <span class="sc">Philip R. Coursey</span>, B.Sc., A.M.I.E.E.,
+  F.P.S.L. 414 pages. Price <b>15s.</b> (<b>Postage 6d.</b>)</p>
+
+  <p><b>The Wireless World.</b></p>
+
+  <p>A Monthly Magazine devoted to Wireless Telegraphy and Telephony. Price
+  <b>9d.</b> (<b>Postage 3d.</b>) Annual Subscription, <b>11s.</b> post
+  free.</p>
+
+  <p><b>The Radio Review.</b></p>
+
+  <p>A Monthly Record of Scientific Progress in Radio-telegraphy and
+  Telephony. Price <b>2s. 6d.</b> (<b>Postage 3d.</b>) Annual Subscription,
+  <b>30s.</b> post free.</p>
+
+  <p><b>Conquest.</b></p>
+
+  <p>A Popular Illustrated Monthly Magazine dealing with Science, Industry
+  and Invention. Price <b>1s.</b> (<b>Postage 3d.</b>) Annual Subscription,
+  <b>15s.</b> post free.</p>
+
+  <p><b>Continuous Wave Wireless Telegraphy.</b> Part I.</p>
+
+  <p>By Dr. <span class="sc">W. H. Eccles</span>, D.Sc., A.R.C.S., M.I.E.E.
+  [<i>In the Press.</i></p>
+
+  <p><br style="clear:both" /></p>
+<hr class="short" />
+
+<h3>COMPLETE CATALOGUE POST FREE.</h3>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>Notes</h3>
+
+<div class="note">
+  <p><a name="Nt1" href="#NtA1">[1]</a> These measurements only apply to a
+  single line. Where a double line is employed the capacity is halved.</p>
+
+  <p><a name="Nt2" href="#NtA2">[2]</a> See Appendix A.</p>
+
+  <p><a name="Nt3" href="#NtA3">[3]</a> See Appendix B.</p>
+
+  <p><a name="Nt4" href="#NtA4">[4]</a> In wireless telegraphy "arcing" is
+  principally caused by the continuation of the supply current in the
+  spark-gap after the capacity has been charged to a potential sufficient
+  to break down the insulation of the gap.</p>
+
+  <p><a name="Nt5" href="#NtA5">[5]</a> See Chapter V.</p>
+
+  <p><a name="Nt6" href="#NtA6">[6]</a> Nernst lamps are the best to use,
+  as they produce abundantly the blue and violet rays which have the
+  greatest chemical effect upon a photographic film. Carbon filament lamps
+  are very poor in this respect.</p>
+
+  <p><a name="Nt7" href="#NtA7">[7]</a> A description of the apparatus
+  required will be found in Ganot's <i>Physics</i>.</p>
+
+  <p><a name="Nt8" href="#NtA8">[8]</a> Great care must be exercised in
+  using this solution, as it is exceedingly poisonous.</p>
+
+  <p><a name="Nt9" href="#NtA9">[9]</a> Two clocks would isochronise if
+  their hands travelled at precisely the same rate round the dials, but
+  would not synchronise unless they both registered the same time as
+  well.</p>
+
+  <p><a name="Nt10" href="#NtA10">[10]</a> Line screens can be obtained
+  from Messrs. Penrose, 109 Farringdon Street, London; or Messrs.
+  Fallowfield, 146 Charing Cross Road, London.</p>
+
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<hr class="pg" />
+</div>
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@@ -0,0 +1,4170 @@
+The Project Gutenberg eBook, Wireless Transmission of Photographs, by
+Marcus J. Martin
+
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+
+
+
+Title: Wireless Transmission of Photographs
+       Second Edition, Revised and Enlarged 1919
+
+
+Author: Marcus J. Martin
+
+
+
+Release Date: October 9, 2010  [eBook #34052]
+
+Language: English
+
+Character set encoding: ISO-646-US (US-ASCII)
+
+
+***START OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF
+PHOTOGRAPHS***
+
+
+E-text prepared by Robert Cicconetti, Keith Edkins, and the Online
+Distributed Proofreading Team (http://www.pgdp.net) from page images
+generously made available by Internet Archive/Canadian Libraries
+(http://www.archive.org/details/toronto)
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 34052-h.htm or 34052-h.zip:
+      (http://www.gutenberg.org/files/34052/34052-h/34052-h.htm)
+      or
+      (http://www.gutenberg.org/files/34052/34052-h.zip)
+
+
+      Images of the original pages are available through
+      Internet Archive/Canadian Libraries. See
+      http://www.archive.org/details/wirelesstransmis00martuoft
+
+
+Transcriber's note:
+
+      A carat character indicates that the following character
+      (or characters in curly braces) is a superscript. Examples:
+      A^2 (A raised to the second power), 10^{-7} (10 raised to
+      the -7 power).
+
+      Text enclosed between underscores was italicized in the
+      original (_italics_).
+
+      A single underscore indicates that the following character
+      or characters is a subscript. Example: CS_2 (the "2" is a
+      subscript).
+
+      Numbers enclosed by curly braces within the text are page
+      numbers (example: {100}), which have been incorporated to
+      enable the reader to use the index.
+
+
+
+
+
+WIRELESS TRANSMISSION OF PHOTOGRAPHS
+
+[Illustration: FIG. 10.]
+
+       *       *       *       *       *
+
+
+WIRELESS TRANSMISSION OF PHOTOGRAPHS
+
+by
+
+MARCUS J. MARTIN
+
+SECOND EDITION
+REVISED AND ENLARGED 1919
+
+
+
+
+
+
+
+The Wireless Press, Ltd.
+12-13 Henrietta Street, Strand
+London, W.C. 2
+
+
+
+{v}
+
+PREFACE TO SECOND EDITION
+
+Although during the last few years very little, in common with other
+wireless work, has been possible in connection with the practical side of
+the wireless transmission of photographs, yet, now that the prospect of
+experimental work is once again occupying the minds of all wireless
+workers, advantage has been taken of a reprint of this little volume to
+amplify a few points that were insufficiently dealt with in the first
+edition, and also to add some fresh matter.
+
+To Chapter V. has been added a short description of the Nernst lamp, and
+also some useful information regarding photographic films, and a few notes
+relating to enlarging included in the Appendix B.
+
+A fresh appendix dealing with the principles of optical lenses has also
+been added. This is a subject that plays an important part in any system of
+wireless photography, and to those experimenters whose knowledge of optics
+is limited this section should prove useful.
+
+To serious workers engaged on the problem of the wireless transmission of
+photographs, attention {vi} is called to a series of articles which are
+being published from time to time in the _Wireless World_, on the design
+and construction of wireless photographic apparatus.
+
+  M. J. M.
+
+  MAIDSTONE, 1919.
+
+{vii}
+
+PREFACE
+
+In these progressive times it is only reasonable to expect that some
+attempt would be made to utilise the ether-waves for other purposes than
+that of telegraphic communication, and already many clever minds are at
+work trying to solve the problems of the wireless control of torpedoes and
+airships, wireless telephony, and, last but not least, the wireless
+transmission of photographs.
+
+It may seem rather premature to talk about the wireless transmission of
+photographs at a time when the ordinary systems are not fully developed;
+but the prospects of wireless photography are of a very encouraging nature,
+especially for long over-water distances, as there are great difficulties
+to be overcome in long-distance transmission over ordinary land lines and
+cables which will be entirely eliminated by wireless methods.
+
+From a perusal of Chapter I. the reader will be able to understand
+something of the difficulties that are to be encountered in working over
+long distances, and he will also be able to appreciate something of the
+advantages that would be derived {viii} from a reliable wireless system.
+Apart from the value of such a system for transmitting news pictures, it
+would also be of great advantage to transmit to ships at sea photographs of
+criminals for identification purposes. In such a small volume as this it
+would be impossible to deal with the working of wireless apparatus and the
+many systems that have been devised for the transmission of photographs
+over metallic circuits. The Author has taken it for granted that other
+works have been studied in connection with these subjects, and will
+therefore only describe such apparatus as is likely to be of use in
+wireless transmission. At present the transmission of photographs by
+wireless methods is in a purely experimental stage, and this book will have
+served its purpose if it helps to put future experimenters on the right
+track and prevent them from making expensive and fruitless experiments, by
+showing them the right direction in which investigations are being carried
+out. As there is no claim to originality in respect of a good many pieces
+of apparatus, etc., described, I have not thought it necessary to state the
+various sources from which the information has been obtained.
+
+  M. J. M.
+
+  ASHFORD, 1916.
+
+       *       *       *       *       *
+
+
+{ix}
+
+CONTENTS
+
+                                                                       PAGE
+  PREFACE TO SECOND EDITION                                               v
+
+  PREFACE                                                               vii
+
+CHAPTER I
+
+  INTRODUCTORY                                                            1
+
+    Foreword--Early experiments--Advantages of
+    Radio-Photography--Difficulties in Cable working--Bernochi's
+    System--Knudsen's System.
+
+CHAPTER II
+
+  TRANSMITTING APPARATUS                                                 13
+
+    Wireless Apparatus--Preparing the Photographs--Transmitting
+    Machines--Transmitting Apparatus--Effects of
+    Arcing--Spark-Gaps--Contact Breakers--Complete Station--Professor
+    Korn's Apparatus--Poulsen Company's Photographic Recorder--Comparison
+    of various systems--Practical applications.
+
+CHAPTER III
+
+  RECEIVING APPARATUS                                                    37
+
+    Methods of Receiving--Author's Photographic Receiver--Decohering
+    Apparatus--Description of Einthoven Galvanometer--Use of Galvanometer
+    in Receiving--Belin's Application of Blondel's
+    Oscillograph--Description of Charbonelle's Receiver--Use of Telephone
+    Relay--Description of Telephone Relay--Telephotographic
+    Receiver--Polarisation Receiver--Kathode-Ray Receiver--Electrolytic
+    Receiver--Atmospherics in Long-Distance working.
+
+{x}
+
+CHAPTER IV
+
+  SYNCHRONISING AND DRIVING                                              63
+
+    Driving Motors--Isochronising the Electrolytic System--Professor Korn's
+    method--Description of Hughes Governor--Author's Speed
+    Regulator--Problem of Synchronising--Methods of Synchronising--Advances
+    made in Radio-Photography.
+
+CHAPTER V
+
+  THE "TELEPHOGRAPH"                                                     74
+
+    Author's System of
+    Radio-Photography--Requirements--Advantages--Transmitting
+    machine--Description of Differential Relay--Wireless Receiving
+    Apparatus--Photo-Telegraphic Receiving Apparatus--Circuit
+    Breaker--Friction Brake--Magnetic Clutch--Description of
+    Isochroniser--Method of working--Types of Nernst Lamp--Action of Nernst
+    Lamp--Comparison of Actinic Value--Inertia of Photographic
+    Films--Choosing Films--Speed of Films--Standard of Speed--Comparative
+    Film Speeds--Effects of Minimum Exposure--Effects of Maximum
+    Exposure--Considerations in working and choosing Films.
+
+APPENDIX A
+
+  SELENIUM CELLS                                                        109
+
+    Nature of Selenium--Preparation of Selenium--Forms of Selenium
+    Cells--Action of Selenium Cells--Characteristics of Selenium
+    Cells--Effects of Inertia in Photo-Telegraphy--Methods of counteracting
+    Inertia--Sensitiveness of Selenium to Light--Effect of Heat on
+    Selenium.
+
+APPENDIX B
+
+  PREPARING THE METAL PRINTS                                            115
+
+    Outline of Process--Line Screens--Choice of Camera--Fixing Line Screen
+    in Camera--Lenses and Stops--Taking the Photograph--Copying
+    Stands--Choice of Photographic Plates--Sources of Illumination--Metal
+    Prints--Coating the {xi} Metal Sheets--Sensitising Solution--Printing
+    Operations--Developing--Intensifying--Precautions to be observed in
+    working--Preparing Sketches on Metal--Apparatus for Reducing or
+    Enlarging--Improvements to Copying Board--Lenses for Copying--Formula
+    for Copying.
+
+APPENDIX C
+
+  LENSES                                                                126
+
+    Action of Light--Law of Refraction--Lenses--Prisms--Action of
+    Lenses--Focal Length of Lenses--Formation of Images--Apparent Magnitude
+    of Objects--Real and Virtual Images--Formation of Virtual Images--Power
+    of Magnification--Defects of Lenses--Aberration.
+
+       *       *       *       *       *
+
+
+{xiii}
+
+ILLUSTRATIONS
+
+  FIG.                                                                 PAGE
+
+  1. Diagram showing effects of capacity on an intermittent current       5
+
+  2. Bernochi's wireless apparatus                                        7
+
+  3. Knudsen's wireless apparatus                                        10
+
+  4. Wireless transmitting station                                       13
+
+  5. Diagram of experiment illustrating principle of line photograph     16
+
+  6. Drawing of transmitting machine                                     17
+
+  7. Drawing of transmitting machine                                     18
+
+  8. Drawing of stylus                                                   18
+
+  9. Electrical connections of machine                                   19
+
+  10. Photograph of Author's experimental machine            _Frontispiece_
+
+  10a. End view of Author's experimental machine  }
+                                                  }        _facing page_ 21
+  10b. View of image broken up by a "cross" screen}
+
+  11. Connections of complete transmitting apparatus                     23
+
+  12. Drawing of ordinary type of spark-gap                              27
+
+  13. Synchronous rotating spark-gap                                     28
+
+  14. Non-synchronous rotating spark-gap                                 28
+
+  15. Connections for complete wireless photographic station             30
+
+  16. Connections of Professor Korn's apparatus                          31
+
+  17. Connections of Poulsen's photographic recorder                     33
+
+  18. Author's photographic receiver                                     38
+
+  19. Enlarged drawing of cone                                           39
+
+  20. End view of Author's photographic receiver                         39
+
+  21. Connections of decohering apparatus                                41
+
+  22. Connections for complete photographic receiver                     42
+
+  {xiv}
+  23. Arrangement of Einthoven galvanometer                              45
+
+  24. Einthoven galvanometer arranged for receiving                      46
+
+  25. Connection of telephone relay                                      49
+
+  26. Drawing of Author's improved photographic receiver                 51
+
+  27. Diagram giving ratio of vibrating arm                              51
+
+  28. Arrangement of polarisation receiver                               53
+
+  29. Arrangement of kathode-ray receiver                                54
+
+  30. Connections of electrolytic receiver                               56
+
+  31. Drawing of improved stylus for receiving                           58
+
+  32. Drawing of Hughes telegraph governor                               66
+
+  33. Arrangement of simple speed regulator                              68
+
+  34. Diagram of connections of simple speed regulator                   68
+
+  35. Author's arrangement for complete radio-photographic station       77
+
+  36. Drawing of transmitting machine and circuit breaker                78
+
+  37. Drawing of special transmitting stylus showing adjusting
+      arrangements                                                       79
+
+  37a. End view of transmitting stylus                                   79
+
+  38. Connections of new type of relay designed by the Author            80
+
+  39. Arrangement of mercury containers and dipping rods for relay       82
+
+  40. Drawing of Author's receiver                                       84
+
+  41. Enlarged drawing of diaphragm and steel point                      84
+
+  41a. Drawing showing arrangement of bush and counter-weight            84
+
+  42. Optical arrangements of receiver                                   85
+
+  43. Optical arrangements of receiver                                   86
+
+  44. Drawing of circuit breaker                                         88
+
+  45. Drawing of friction brake                                          89
+
+  46. Sectional drawing of magnetic clutch                               90
+
+  47. Plan of magnetic clutch                                            90
+
+  48. Details of Isochroniser                                            92
+
+  49. Connections of Isochroniser                                        94
+
+  50. Dial of Isochroniser                                               94
+
+  51. Diagram of driving mechanism                                       96
+
+  {xv}
+  52. Diagram showing starting positions of machines                     97
+
+  52a. Arrangement of small type Nernst lamp                             99
+
+  52b. Ballasting resistances for Nernst lamps                          100
+
+  52c. Arrangement of large type Nernst lamp                            101
+
+  53. Connections of selenium cell elements                             110
+
+  53a. Form of selenium cell used by Bell and Tainter                   110
+
+  54. Diagram showing construction of modern cell                       111
+
+  55. Resistance curve of selenium cell                                 111
+
+  55a. Actual curve of selenium cell                                    112
+
+  56. Diagram of Professor Korn's method for counteracting inertia      113
+
+  57. Arrangement of plate sheath and line screen                       117
+
+  58. Details of clips to hold line screen                              118
+
+  59. Arrangement of apparatus for copying                              119
+
+  60. Drawing showing method of arranging camera and copying stand for
+     adjustment                                                         119
+
+  61. Photograph of line screen and metal print  }
+                                                 } _facing page_        124
+  62. Photograph of sketch drawn upon metal foil }
+
+  63. Method of marking out copying board                               124
+
+  64. Diagram illustrating law of refraction                            127
+
+  65. Forms of lenses                                                   128
+
+  66. Action of light passed through a prism                            129
+
+  67. Diagram illustrating action of a lens                             130
+
+  68. Formation of principal focus of a lens                            130
+
+  69. Formation of conjugate foci of a lens                             131
+
+  70. Apparatus illustrating principle of camera                        132
+
+  71. Formation of an image by a lens                                   133
+
+  72. Diagram illustrating apparent magnitude                           134
+
+  73. Formation of virtual image by a convex lens                       137
+
+  74. Formation of virtual image by a concave lens                      138
+
+  75. Diagram showing spherical aberration                              139
+
+  76. Combination of plano-convex lenses                                139
+
+  77. Combination of meniscus and convex lenses                         139
+
+       *       *       *       *       *
+
+
+{1}
+
+RADIO-PHOTOGRAPHY
+
+CHAPTER I
+
+INTRODUCTORY
+
+Those who desire to experiment on radio-photography, _i.e._ transmitting
+photographs, drawings, etc., from one place to another without the aid of
+artificial conductors, must cultivate at least an elementary knowledge of
+optics, chemistry, mechanics, and electricity; photo-telegraphy calling for
+a knowledge of all these sciences. There are, no doubt, many wireless
+workers who are interested in this subject, but who are deterred from
+experimenting owing to a lack of knowledge regarding the direction
+developments are taking, besides which, information on this subject is very
+difficult to obtain, the science of photo-telegraphy being, at the present
+time, in a purely experimental stage.
+
+The wireless transmission of photographs has, no doubt, a great commercial
+value, but for any system to be commercially practicable, it must be
+simple, rapid, and reliable, besides being able to work {2} in conjunction
+with the apparatus already installed for the purpose of ordinary wireless
+telegraphy.
+
+As far back as 1847 experiments were carried out with a view to solving the
+problem of transmitting pictures and writing by electrical methods over
+artificial conductors, but no great incentive was held forth for
+development owing to lack of possible application; but owing to the great
+public demand for illustrated newspapers that has recently sprung into
+being, a large field has been opened up. During the last ten years,
+however, development has been very rapid, and some excellent results are
+now being obtained over a considerable length of line.
+
+The wireless transmission of photographs is, on the other hand, of quite
+recent growth, the first practicable attempt being made by Mr. Hans Knudsen
+in 1908. It may seem rather premature to talk about the wireless
+transmission at a time when the systems for transmitting over ordinary
+conductors are not perfectly developed, but everything points to the fact
+that for long-distance transmission a reliable wireless system will prove
+to be both cheaper and quicker than transmission over ordinary land lines
+and cables.
+
+The effects of capacity and inductance--properties inherent to all
+telegraph systems using metallic conductors--have a distinct bearing upon
+the two questions, how far and how quickly can {3} photographs be
+transmitted? Owing to the small currents received and to prevent
+interference from earth currents it is necessary to use a complete metallic
+circuit. If an overhead line could be employed no difficulty would be
+experienced in working a distance of over 1000 miles, but a line of this
+length is impossible--at least in this country--and if transmission is
+attempted with any other country, a certain amount of submarine cable is
+essential. It has been found that the electrostatic capacity of one mile of
+submarine cable is equal to the capacity of 20 miles of overhead line, and
+as the effect of capacity is to retard the current and reduce the speed of
+working, it is evident that where there is any great length of cable in the
+circuit the distance of possible transmission is enormously reduced.
+
+If we take for an example the London-Paris telephone line with a length of
+311 miles and a capacity of 10.62 microfarads, we find that about half this
+capacity, or 5.9 microfarads,[1] is contributed by the 23 miles of cable
+connecting England with France.
+
+In practice the reduction of speed due to capacity has, to a great extent,
+been overcome by means of apparatus known as a line-balancer, which hastens
+the slow discharge of the line and {4} allows each current sent out from
+the transmitter--the current in several systems being intermittent--to be
+recorded separately on the receiver. Photographs suitable for press work
+can now be sent over a line which includes only a short length of cable for
+a distance of quite 400 miles in about ten minutes, the time, of course,
+depending upon the size of the photograph. In extending the working to
+other countries where there is need for a great length of cable, as between
+England and Ireland, or America, the retardation due to capacity is very
+great. On a cable joining this country with America the current is retarded
+four-tenths of a second. In submarine telegraphy use is made of only one
+cable with an earth return, but special means have had to be adopted to
+overcome interference from earth currents, as the enormous cost prohibits
+the laying of a second cable to provide a complete metallic circuit. The
+current available at the cable ends for receiving is very small, being only
+1/200000th part of an ampere, and this necessitates the use of apparatus of
+a very sensitive character. One system of photo-telegraphy in use at the
+present time, employs what is known as an electrolytic receiver (see
+Chapter III.) which can record signals over a length of line in which the
+capacity effects are very slight, with the marvellous speed of 12,000 a
+minute, but this speed rapidly decreases with an increase of distance
+between the {5} [Illustration] two stations. The effect of capacity upon an
+intermittent current is clearly shown in Fig. 1. If we were to send twenty
+brief currents in rapid succession over a line of moderate capacity in a
+given time, we should find that instead of being recorded separately and
+distinctly as at _a_, each mark would be pointed at both ends and joined
+together as shown at _b_, while only perhaps fifteen could be recorded. If
+the capacity be still farther increased as at _c_, only perhaps half the
+original number of currents could be recorded in the same time, owing to
+the fact that with an increase of resistance, capacity, and inductance of
+the line a longer time is required for it to charge up and discharge,
+thereby materially lessening the rate at which it will allow separate
+signals to pass; the number of signals that can therefore be recorded in a
+given time is greatly diminished. If we were to attempt to send the same
+number of signals over a line of great capacity, as could be sent, and
+recorded separately and distinctly over a line of small capacity--the time
+limit being of course the same in both instances--we should find that the
+{6} signals would be recorded practically as a continuous line. The two
+latter cases _b_, and _c_, Fig. 1, clearly shows the retardation that takes
+place at the commencement of a current and the prolongation that takes
+place at the finish. If the photo-telegraphic system previously mentioned
+could be rendered sensitive enough to work on the Atlantic cables, we
+should find that only about 1200 signals a minute could be recorded, and
+this would mean that a photograph which could be transmitted over ordinary
+land lines in about ten minutes would take at least fifty minutes over the
+cable. This would be both costly and impracticable, and time alone will
+show whether, for long-distance work, transmission by wireless will be both
+cheaper and more rapid than any other method. At present wireless
+telegraphy has not superseded the ordinary methods of communicating over
+land, but there can be no doubt that wireless telegraphy, if free from
+Government restrictions, would in certain circumstances very quickly
+supersede land-line telegraphy, while it has proved a formidable commercial
+competitor to the cable as a means of connecting this country with America.
+Likewise we cannot say that no system of radio-photography will ever come
+into general use, but where there is any great distance to be bridged,
+especially over water, wireless transmission is really the only practical
+solution. From the {7} foregoing remarks, it is evident that a reliable
+system of radio-photography would secure a great victory in the matter of
+time and cost alone, besides which, the photo-telegraphic apparatus would
+be merely an accessory to the already existing wireless installation.
+
+[Illustration: FIG. 2.]
+
+There have been numerous suggestions put forward for the wireless
+transmission of photographs, but they are all more or less impracticable.
+One of the earliest systems was devised by de' Bernochi of Turin, but his
+system can only be regarded interesting from an historical point of view,
+and as in all probability it could only have been made to work over a
+distance of a few hundred yards it is of no practical value. Fig. 2 will
+help to explain the apparatus. A glass cylinder A' is fastened at one end
+to a threaded steel shaft, which runs in two bearings, one bearing having
+an internal thread corresponding with that on the {8} shaft. Round the
+cylinder is wrapped a transparent film upon which a photograph has been
+taken and developed. Light from a powerful electric lamp L, is focussed by
+means of the lens, N, to a point upon the photographic film. As the
+cylinder is revolved by means of a suitable motor, it travels upwards
+simultaneously by reason of the threaded shaft and bearing, so that the
+spot of light traces a complete spiral over the surface of the film. The
+light, on passing through the film (the transmission of which varies in
+intensity according to the density of that portion of the photograph
+through which it is passing), is refracted by the prism P on to the
+selenium cell S which is in series with a battery B and the primary X of a
+form of induction coil. As light of different intensities falls upon the
+selenium cell,[2] the resistance of which alters in proportion, current is
+induced in the secondary Y of the coil and influences the light of an arc
+lamp of whose circuit it is shunted. This arc lamp T is placed at the focus
+of a parabolic reflector R, from which the light is reflected in a parallel
+beam to the receiving station.
+
+The receiver consists of a similar reflector R' with a selenium cell E
+placed at its focus, whose resistance is altered by the varying light
+falling upon it from the reflector R. The selenium cell {9} E is in series
+with a battery F and the mirror galvanometer H. Light falls from a lamp D
+and is reflected by the mirror of the galvanometer on to a graduated
+aperture J and focussed by means of the aplanatic lens U upon the receiving
+drum A^2, which carries a sensitised photographic film. The two cylinders
+must be revolved synchronously. The above apparatus is very clever, but
+cannot be made to work over a distance of more than 200 yards.
+
+A system based on more practical lines was that invented and demonstrated
+by Mr. Hans Knudsen, but the apparatus which he employed for receiving has
+been discarded in wireless work, as it is not suitable for working with the
+highly-tuned systems in use at the present time.
+
+Knudsen's transmitter, a diagrammatic representation of which is given in
+Fig. 3, consists of a flat table to which a horizontal to-and-fro motion is
+given by means of a clockwork motor. Upon this table is fastened a
+photographic plate which has been prepared in the following manner. The
+plate upon which the photograph is to be taken has the gelatine film from
+three to four times thicker than that commonly used in photography. In the
+camera, between the lens and this plate, a single line screen is
+interposed, which has the effect of breaking the picture up into parallel
+lines. Upon the plate being developed and before it is {10} [Illustration]
+completely dry, it is sprinkled over with fine iron dust. With this type of
+plate the transparent parts dry much quicker than the shaded or dark parts,
+and on the iron dust being sprinkled over the plate it adheres to the
+darker portions of the film to a greater extent than it does to the lighter
+portions; a picture partly composed of iron dust is thus obtained. A steel
+point attached to a flat spring rests upon this plate and is made to travel
+at right angles to the motion of the table. As the picture is partly
+composed of iron dust, and as the steel needle is fastened to a delicate
+spring it is evident that as the plate passes to and fro under the needle,
+both the spring and needle are set in a state of vibration. This vibrating
+spring makes {11} and breaks the battery circuit of a spark coil, which in
+turn sets up sparking in the spark-gap of the wireless apparatus.
+
+The receiver consists of a similar table to that used for transmitting, and
+carries a glass plate that has been smoked upon one side. A similar spring
+and needle is placed over this plate, but is actuated by means of a small
+electro-magnet in circuit with a battery and a sensitive coherer. As the
+coherer makes and breaks the battery circuit by means of the intermittent
+waves sent out from the transmitting aerial, the needle is made to vibrate
+upon the smoked glass plate in unison with the needle at the transmitting
+end. Scratches are made upon the smoked plate, and these reproduce the
+picture on the original plate. A print can be taken from this scratched
+plate in a similar manner to an ordinary photographic negative.
+
+The two tables are synchronised in the following manner. Every time the
+transmitting table is about to start its forward stroke a powerful spark is
+produced at the spark-gap. The waves set up by this spark operate an
+ordinary metal filings coherer at the receiving end which completes the
+circuit of an electro-magnet. The armature of this magnet on being
+attracted immediately releases the motor used for driving, allowing it to
+operate the table. The time taken to transmit a photograph, quarter-plate
+size, is about fifteen minutes. {12} Although very ingenious this system
+would not be practicable, as besides speed the quality of the received
+pictures is a great factor, especially where they are required for
+reproduction purposes. The results from the above apparatus are said to be
+very crude, as with the method used to prepare the photographs no very
+small detail could be transmitted.
+
+       *       *       *       *       *
+
+
+{13}
+
+CHAPTER II
+
+TRANSMITTING APPARATUS
+
+Let us now consider the requirements necessary for transmitting photographs
+by means of the wireless apparatus in use at the present time.
+
+[Illustration: FIG. 4.]
+
+The connections for an experimental syntonic wireless transmitting station
+are shown in the diagram Fig. 4. A is the aerial; T, the inductance; E,
+earth; L, hot-wire ammeter. The closed oscillatory circuit consists of an
+inductance F, spark-gap G, and a block condenser C. H is a spark-coil for
+supplying the energy, the secondary J being connected to the spark-gap. A
+{14} mercury break N and a battery B are placed in the primary circuit of
+the coil. The Morse key K is for completing the battery circuit for
+signalling purposes. When the key K is depressed, the battery circuit is
+completed, and a spark passes between the balls of the spark-gap G
+producing oscillations in the closed circuit, which are transposed to the
+aerial circuit by induction. For signalling purposes it is only necessary
+for the operator by means of the key K to send out a long or short train of
+waves in some pre-arranged order, to enable the operator at the receiving
+station to understand the message that is being transmitted.
+
+If a photograph could be prepared in such a manner that it would serve the
+purpose of the key K, and could so arrange matters that a minute portion of
+the photograph could be transmitted separately but in succession, and that
+each portion of the photograph having the same density could be given the
+same signal, then it would only be necessary to have apparatus at the
+receiving station capable of arranging the signals in proper sequence (each
+signal recorded being the same size and having the same density as the
+transmitted portion of the photograph) in order to receive a facsimile of
+the picture transmitted.
+
+The following method of preparing the photograph[3] is one that has been
+adopted in several {15} systems of photo-telegraphy, and is the only one at
+all suitable for wireless transmission. The photograph or picture which is
+to be transmitted is fastened out perfectly flat upon a copying-board. A
+strong light is placed on either side of this copying board, and is
+concentrated upon the picture by means of reflectors. The camera which is
+used for copying has a single line screen interposed between the lens and
+sensitised plate, and the effect of this screen is to break the picture up
+into parallel lines. Thus a white portion of the photograph would consist
+of very narrow lines wide apart, while the dark portion would be made up of
+wide lines close together; a black part would appear solid and show no
+lines at all. From this line negative it will be necessary to take off a
+print upon a specially prepared sheet of metal. This consists of a sheet of
+thick lead- or tinfoil, coated upon one side with a thin film of glue to
+which bichromate of potash has been added; the bichromate possessing the
+property of rendering the glue waterproof when acted upon by light. The
+print can be taken off by artificial light (arc lamps being generally
+used), but the exact time to allow for printing can only be found by
+experiment, as it varies considerably according to the thickness of the
+film. The printing finished, the metal print is washed under running water,
+when all those parts not acted upon by light, _i.e._ the parts between the
+lines, are {16} washed away, leaving the bare metal. We have now an image
+composed of numerous bands of insulating material (each band varying in
+width according to the density of the photograph at any point from which it
+is prepared) attached to a metal base, so that each band of insulating
+material is separated by a band of conducting material. It is, of course,
+obvious that the lines on the print cannot be wider apart, centre to
+centre, than the lines of the screen used in preparing it. A good screen to
+use is one having 50 lines to the inch, but one is perhaps more suitable
+for experimental work a little coarser, say 35 lines to the inch. To use a
+screen having 50 or more lines to the inch, the transmitting apparatus, as
+will be evident later on, will require to be very nearly perfect.
+
+[Illustration: FIG. 5.]
+
+Before proceeding further it will perhaps be as well to make an experiment.
+If we take one of the metal prints or, more simple, draw a sketch in
+insulating ink upon a sheet of metal A, Fig. 5, and connect a battery B and
+the galvanometer D as shown, we shall find on drawing the free end of the
+wire across the metal plate that all the time the wire is in contact with
+the lines of insulating material the needle of the galvanometer will remain
+{17} at zero, but where it is in contact with the metal plate the needle is
+deflected.
+
+From this experiment it will be seen that we have in our metal line print,
+which consists of alternate lines of insulating and conducting material, a
+method by which an electric circuit can be very easily made and broken. It
+is, of course, necessary to have some arrangement whereby the whole of the
+surface of the metal print is utilised for this purpose to the best
+advantage. One type of transmitting machine used for this purpose is
+represented by the diagram, Fig. 6. The cylinder A is fastened to the steel
+shaft B, which runs in the two bearings D and D', the bearing D' having an
+internal thread corresponding to that on the shaft. The stylus in this
+class of machine is a fixture, the cylinder being given a lateral as well
+as a revolving movement. As it is impossible to use a rigid drive, a
+flexible coupling F is employed between the shaft B and the motor.
+
+[Illustration: FIG. 6.]
+
+Another type of machine is shown in Fig. 7. The drum in this case is
+stationary, the table T moving laterally by reason of the screwed shaft
+{18} [Illustration] and half nut F. The table, shown separate in Fig. 8,
+carries a stiff brass spring A, to which is attached a holder B made to
+take a hardened steel point. The holder is provided with a set screw P for
+securing the steel point Z. The spring and needle are insulated from the
+rest of the machine, as shown in the drawing. In working, the metal print
+is wrapped tightly round the cylinder of the machine, the glue image being,
+of course, uppermost. To fasten the print a little seccotine should be
+applied to one edge, and the joint carefully smoothed down with the
+fingers. [Illustration] If there is any tendency on the part of the print
+to slip round on the drum, a couple of small spring clips placed over the
+ends of the drum will act as a preventive. It is necessary to place the
+print upon the drum in such a manner that the stylus draws away from the
+edge of the lap and not towards it, and the metal prints should be of such
+a size that when placed round the drum of the {19} machine a lap of about
+3/16ths of an inch is allowed.
+
+[Illustration: FIG. 9.]
+
+The steel point Z (ordinary gramophone needles may be used and will be
+found to answer the purpose admirably) is made to press lightly upon the
+metal print, and while the pressure should be sufficient to make good
+electrical contact, it should not be sufficient to cause the needle to
+scratch the surface of the foil. The pressure is regulated by means of the
+milled nut H. The electrical connections are given in Fig. 9. One wire from
+the battery M is taken to the terminal T, and the other wires from M and F
+lead to the relay R. The current flows from the battery M through the
+spring Y, through the drum and metal print, the stylus Z, spring A, down to
+the relay R, and from R back to the battery M. As the drum carrying the
+single line half-tone print is revolved, the stylus, by reason of the
+lateral movement given to the table or cylinder as the case may be, will
+trace a spiral path over the entire surface of the print. As the stylus
+traces over a conducting strip the circuit is completed, and the tongue of
+the relay R is attracted, making contact with the stop S. {20} On passing
+over a strip of insulation the circuit is broken and the tongue of the
+relay R returns to its normal position.
+
+As already stated, the conducting and insulating bands on the print vary in
+width according to the density of the photograph from which it is prepared,
+so that the length of time that the tongue of the relay R is held against
+the stop S, is in proportion to the width of the conducting strip which is
+passing under the stylus at any instant. The function of the transmitter is
+therefore to send to the relay R an intermittent current of varying
+duration.
+
+The two photographs Figs. 10 and 10_a_ are of a machine designed and used
+by the writer in his experiments. In this machine the drum is 3.5 inches
+long and 1.5 inches in diameter. The lead screw has 30 threads to the inch,
+and the reduction between it and the drum is 3:1, so that the table has a
+movement of 1/90th inch per revolution of the drum.
+
+From the brief description of the various types of machines that have been
+given it will be apparent that in the design of the machine proper there is
+nothing very complicated, although the addition of the driving and
+synchronising apparatus complicates matters rather considerably. The
+questions of driving and synchronising the machines at the two stations is
+fully dealt with in Chapter IV.
+
+[Illustration: FIG. 10a.]
+
+[Illustration: FIG. 10b. Enlarged view of an image broken up by a cross
+screen.]
+
+{21} Although the design of the machines is rather simple great attention
+must be paid both to accuracy of construction and accuracy of working, and
+this applies, not only to the machines (whether for transmitting or
+receiving) but for all the various pieces of apparatus that are used. Too
+much care cannot be bestowed upon this point, as in the wireless
+transmission of photographs there is a large number of instruments all
+requiring careful adjustment, and which have to work together in perfect
+unison at a high speed.
+
+The machine shown in Figs. 10 and 10_a_ was designed and used by the writer
+solely for experimental work. It will be noticed in the description given
+in the appendix of the method of preparing the metal prints that a 5" x 4"
+camera is recommended, while the machine, Fig. 10, is designed to take a
+print procured from a quarter-plate negative. This size of drum was adopted
+for several reasons, and although it will be found quite large enough for
+general experimental work the writer has come to the conclusion that for
+practical commercial work a drum to take a print 5" x 4" will give better
+results.
+
+In making a negative of a picture that is required for reproduction
+purposes, the line screen in the camera is replaced by a "cross screen,"
+_i.e._ two single line screens placed with their lines at an angle of 90deg
+to one another, and this breaks the {22} image up into small squares
+instead of lines. By looking at any ordinary newspaper or book illustration
+through a powerful magnifying glass the effects of a cross screen will
+readily be seen. With a cross screen a certain amount of detail is
+necessarily lost, but with a single line screen the amount lost is much
+greater. If there is any very small detail in the picture most of this
+would be lost in a coarse screen, hence the necessity of employing as fine
+a line screen as practicable in order to get as much detail in as possible.
+It is mainly on this account that a 5" x 4" print is recommended, as, if
+fairly bold subjects are used for copying, the small detail (this is, of
+course, a very vague and indefinable term) will not be too fine, and the
+time required for transmitting reasonable. For obvious reasons it is a
+great advantage to put the print under pressure to cause the glue image to
+sink into the soft metal base and leave a perfectly flat and smooth
+surface. It is essential that the bands on the print lie along the axis of
+the cylinder, so that the stylus traces its path across them, and not with
+them.
+
+We have now an arrangement that is capable of taking the place of the key
+K, Fig. 4, and the diagram, Fig. 11, gives the connections for the complete
+transmitter. A is the aerial, E earth, T inductance, L ammeter. The closed
+oscillatory circuit consists of a spark-gap G, inductance F, {23}
+[Illustration] and a condenser C. The secondary J of the coil H is
+connected to the spark-gap, and the primary P is in circuit with the
+mercury break N, the battery B, and the local contacts of the relay R. The
+action is as follows. When contact is made between the stylus Z and the
+drum V by means of the conducting bands on the line print, the circuit of
+the relay R and the battery M is completed. The closing of the local
+circuit of the relay R actuates the second relay R', allowing the primary
+circuit of the coil H to be closed. As soon as the primary circuit of the
+coil is completed sparks pass between the electrodes of the spark-gap G,
+causing waves to radiate from the aerial. The duration of the wave-trains
+radiated depends upon the duration of contact made by the relays {24} R and
+R', and this in turn depends upon the width of the conducting strip that is
+passing under the stylus. The battery M should be about 4 volts, and the
+battery D about 2 volts. The two-way switch X is connected up so that the
+relay R' can be thrown out and the key K switched in for ordinary
+signalling purposes. If any sparking takes place at the point of the
+stylus, a small condenser C' (about 1 microfarad capacity) should be
+connected as shown. In the present instance the condenser should be used
+more as a preventive than as a cure, as in all probability the voltage from
+M will not be sufficient to cause destructive (if any) sparking; but, as
+most wireless workers know, anything in the nature of a spark occurring in
+the neighbourhood of a detector (this, of course, only applies when the
+receiving apparatus is placed in close proximity to the transmitter) is
+liable to destroy the adjustment.
+
+In transmitting over ordinary conductors where the initial voltage is
+fairly high and the self-induction of the circuit very great, the use of
+the condenser will be found to be absolutely essential. It has also been
+noted that the angle which the stylus presents to the drum has a marked
+effect upon the sparking, an angle of about 60deg being found to give very
+good results.
+
+If the size of the single line print used is 5 inches by 4 inches, and a
+screen having 50 lines {25} to the inch is used for preparing it, then the
+stylus will have to make 250 contacts during one revolution of the drum.
+Assuming the drum to make one revolution in three seconds, then the time
+taken to transmit the complete photograph can be found from the equation T
+= w x t x s, where w is the width of the print, t the travel of the stylus
+during one revolution of the drum, and s the time required for one
+revolution of the drum. In the present instance this will be T = 4 x 90 x 3
+= 1080 seconds = 18 minutes. The number of contacts made by the stylus per
+minute is 5000, and in working at this speed the first difficulty is
+encountered in the use of the two relays. The relay R is lightly built, and
+capable of working at a fairly high speed, but R' is a heavier pattern, and
+consequently works at a slightly lower rate. This relay must necessarily be
+heavier, as more substantial contacts are needed in order to pass the heavy
+current taken by the spark-coil.
+
+Relays sensitive and accurate enough to work at this speed will in all
+probability be beyond the reach of the majority of workers, but there are
+several types of relays on the market very reasonable in price that will
+answer very well for experimental work, although the speed of working will
+no doubt be slower.
+
+For the best results the duration of the wave-trains sent out should be of
+the same duration as {26} the contact made by R, and therefore equal to the
+time taken by the stylus to trace over a conducting strip; but if the
+duration of the contact made by R is t, then that made by R' and
+consequently the duration of the groups of wave-trains would be t - v where
+v equals the extra time required by R' to complete its local circuit. The
+difference in time made by the two relays, although very slight, will be
+found to affect very considerably the quality of the received pictures.
+Renewing the platinum contacts is also a great expense, as they are soon
+burnt out where a heavy current is passed. If the distance experimented
+over is short so that the power required to operate the spark-coil is not
+very heavy, one relay will be sufficient providing the contacts are massive
+enough to carry the current safely. It is useless to expect any of the
+ordinary relays in general use to work satisfactorily at such a high speed,
+and in order to compensate for this we must either increase the time of
+transmitting, or, as already suggested, make use of a coarser line screen
+in preparing the photographs.
+
+For reasons already explained, all points of make and break should be
+shunted by a condenser. The effective working speed of an ordinary type of
+relay may be anything from 1000 to 2500 dots a minute, depending upon
+accuracy of design and construction.
+
+In the wireless transmission of photographs it {27} is absolutely essential
+to use some form of rotary spark-gap, as where sparks are passed in rapid
+succession the ordinary type of gap is worse than useless. When a spark
+passes between the electrodes of an ordinary spark-gap, Fig. 12, we find
+that for a fraction of a second after the first spark has passed, the
+normally high resistance of the gap has been lowered to less than one ohm.
+If the column of hot gas which constitutes the spark is not instantly
+dispersed, but remains between the electrodes, it will provide an easy path
+for any further discharges, and if sparks are passed at all rapidly, what
+was at first a disruptive and oscillatory discharge will degenerate into a
+hot, non-oscillatory arc.[4]
+
+[Illustration: FIG. 12.]
+
+Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are
+known as "synchronous" and "non-synchronous" gaps respectively. In the
+synchronous gap the cog-wheel is mounted on the shaft of the alternator,
+and a cog comes opposite the fixed electrode when the maximum of potential
+is reached in the condenser, thus ensuring a discharge at every alternation
+of current. With this type of gap a spark of pure tone is obtained which
+{28} [Illustration] [Illustration] is of great value where the signals are
+received by means of a telephone, but where the signals are to be
+mechanically recorded the tone of the spark is of little consequence. In a
+non-synchronous gap a separate motor is used for driving the toothed wheel,
+and can either be mounted on the motor shaft or driven by means of a band,
+there being no regard given to synchronism with the alternator. The fixed
+electrode is best made long enough to cover about two of the teeth, as this
+ensures regular sparking and a uniform sparking distance; the {29} spark
+length is double the length of the spark-gap. The toothed wheel should
+revolve at a high speed, anything from 5000 to 8000 revolutions per minute,
+or even more being required. The shaft of the toothed wheel is preferably
+mounted in ball-bearings.
+
+Owing to the large number of sparks that are required per minute in order
+to transmit a photograph at even an ordinary speed, it is necessary that
+the contact breaker be capable of working at a very high speed indeed. The
+best break to use is what is known as a "mercury jet" interrupter, the
+frequency of the interruptions being in some cases as high as 70,000 per
+second. No description of these breaks will be given, as the working of
+them is generally well understood.
+
+In some cases an alternator is used in place of the battery B, Fig. 4, and
+when this is done the break M can be dispensed with. In larger stations the
+coil H is replaced with a special transformer.
+
+The writer has designed an improved relay which will respond to currents
+lasting only 1/100th part of a second, and capable of dealing with rather
+large currents in the local circuit.[5] This relay has not yet been tried,
+but if it is successful the two relays R and R' can be dispensed with, and
+the result will be more accurate and effective transmission.
+
+{30}
+
+[Illustration: FIG. 15.]
+
+The connections for a complete experimental station, transmitting and
+receiving apparatus combined, are given in Fig. 15. The terminals W, W are
+for connecting to the photo-telegraphic receiving apparatus Q, being a
+double pole two-way switch for throwing either the transmitting or
+receiving apparatus in circuit. There is another system of transmitting
+devised by Professor Korn, which employs an entirely different method from
+the foregoing. By using the apparatus just described, the waves generated
+are what are known as "damped waves," and by using these damped waves,
+tuning, which is so essential to good commercial working, can be made to
+reach a fairly high degree of efficiency. {31}
+
+The question of damped _versus_ undamped waves is a somewhat burning one,
+and no attempt will be made here to deal with the merits or demerits of the
+claims made for the respective systems. A series of articles describing the
+production of undamped waves and their efficiency in working compared with
+damped waves will be found in the _Wireless World_, Nos. 3 and 4, 1913, and
+are well worth reading by any one interested in the subject.
+
+[Illustration: FIG. 16.]
+
+A diagrammatic representation of the apparatus as arranged by Professor
+Korn is given in Fig. 16. The undamped or "continuous" waves are generated
+by means of a high-frequency alternator or Poulsen arc. In Fig. 16, X is
+the generator, F inductance, C condenser; the aerial inductance T is
+connected by the aerial A and earth E. By this means the waves are tuned to
+a certain period. {32} A metal print, similar to what has already been
+described, is wrapped round the drum D of the machine, and when the stylus
+Z traces over an insulating strip the waves generated are in tune with the
+receiving station, but when it traces over a conducting strip, a portion of
+the inductance T is short-circuited, the period of the oscillations is
+altered, and the two stations are thrown out of tune.
+
+The receiving station is provided with an aperiodic circuit, which consists
+of an inductance F', condenser C', and a thermodetector N. A string
+galvanometer H (described in Chapter III.), and the self-induction coils B,
+B' are connected as shown, the coils B, B' preventing the high-frequency
+currents, which change their direction, from flowing through the
+galvanometer. The manner in which the string galvanometer is arranged to
+reproduce a transmitted picture is shown in Fig. 24.
+
+The connections adopted by the Poulsen Company for photographically
+recording wireless messages are given in Fig. 17, a string galvanometer of
+the Einthoven type being used. The two self-induction coils S and S' are in
+circuit with the detector D and the galvanometer G. The condenser C'
+prevents the continuous current produced by the detector from flowing
+through the high frequency circuit; P is the primary of the aerial {33}
+inductance and F the secondary. The method of transmitting adopted by
+Professor Korn appears to be a simple and reliable arrangement, provided
+that an equally reliable method of producing the undamped waves can be
+found. Owing to the absence of mechanical inertia it should be capable of
+working at a good speed, while the absence of a number of pieces of
+delicate apparatus all requiring careful adjustment add greatly to its
+reliability.
+
+[Illustration: FIG. 17.]
+
+In any spark system with a properly designed aerial a coil taking ten
+amperes is capable of transmitting signals over a distance of thirty to
+fifty miles, but where the number of interruptions of the break required
+per second is very high, as in radio-photography, it must be remembered
+that a much higher voltage is needed to drive the requisite amount of
+current through the primary winding of the coil than would be the case if
+the interruptions were slower. It is possible to use platinum {34} contacts
+for the relays, for currents up to ten amperes, but for heavier currents
+than this some arrangement where contact is made with mercury will be found
+to be more economical and reliable.
+
+In the transmitter already described and given in Fig. 11, the best results
+would be obtained by finding the speed at which the relay R' works best,
+and regulating the number of contacts made by the stylus accordingly.
+
+The method employed by De' Bernochi (see Chapter I.) of varying the
+intensity of a beam of light by passing it through a photographic film,
+which in turn alters the resistance of a selenium cell, has been very
+successfully employed in at least one system of photo-telegraphy. Its
+application has also been suggested for wireless transmission, and although
+with any system using continuous waves this would not be very difficult, it
+could hardly be adapted to work with the ordinary spark system. The
+apparatus for receiving from this type of transmitter would, on the other
+hand, necessarily be more elaborate than the methods that are described in
+the next chapter, and as far as the writer's experience goes, experiments
+along these lines would not prove very profitable, as simplicity is the
+keynote of success in any radio-photographic system.
+
+It has been suggested that in order to decrease the time of transmission a
+cylinder capable of {35} taking a print 7 inches by 5 inches be employed,
+the print being prepared from rather a coarse line screen--say 35 to the
+inch--and a traverse of about 1/50 inch given to the stylus, thus reducing
+the time of transmission to about twelve minutes. It is questionable,
+however, whether the increase in speed would compensate for the loss of
+detail, as only very bold subjects could be transmitted. As already pointed
+out, wireless transmission would only be employed for fairly long
+distances, and the extra time and expense required to receive a fairly good
+detailed picture is negligible when compared with the enormous time it
+would take to receive the original photograph by any ordinary means of
+transit.
+
+The public much prefer to have passable pictorial illustrations of current
+events than wait several days for a more perfect picture--the original, and
+the advantage of any newspaper being able to publish photographs several
+days before its rivals is obvious. There can also be no doubt but that a
+system of radio-photography, if fairly reliable and capable of working over
+a distance of say thirty miles, would be of great military use for
+transmitting maps and written matter with a great saving of time and even
+life. Written matter could be transmitted with even greater safety than
+messages which are sent in the ordinary way in Morse Code, as the signals
+received in the receiver {36} of an hostile installation would be but a
+meaningless jumble of sounds, and even were they possessed of
+radio-photographic apparatus the received message would be unintelligible,
+unless they knew the exact speed at which the machines were running and
+could synchronise accurately.
+
+       *       *       *       *       *
+
+
+{37}
+
+CHAPTER III
+
+RECEIVING APPARATUS
+
+There are only two methods available at present for receiving the
+photographs, and both have been used in ordinary photo-telegraphic work
+with great success. They have disadvantages when applied to wireless work,
+however, but these will no doubt be overcome with future improvements. The
+two methods are (1) by means of an ordinary photographic process, and (2)
+by means of an electrolytic receiver.
+
+In several photo-telegraphic systems the machine used for transmitting has
+the cylinder twice the size of the receiving cylinder, thus making the area
+of the received picture one-quarter the area of the picture transmitted.
+The extra quality of the received picture does not compensate for the
+disadvantage of having to provide two machines at each station, and in the
+writer's opinion results, quite good enough for all practical purposes, can
+be obtained by using a moderate size cylinder so that one machine answers
+for both transmitting {38} and receiving, and using as fine a line screen
+as possible for preparing the photographs.
+
+[Illustration: FIG. 18.]
+
+The writer, when first experimenting in photo-telegraphy, endeavoured to
+make the receiving apparatus "self-contained," and one idea which was
+worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and is
+placed just within the focus of a lens which has a focal length of 3/4
+inch. When a source of light is placed at some point between a lens and its
+principal focus, the light rays are not converged, but are transmitted in a
+parallel beam the same size as the lens. It has been found that this
+arrangement gives a sharper line on the drum than would be the case were
+the light focussed direct upon the hole in the cone A. An enlarged drawing
+of the cone is given in Fig. 19. The hole in the tip of the cone A is a
+bare 1/90 inch in diameter--the size of this hole depends upon the travel
+per revolution of the drum or table of the machine used--and in working,
+the cone is run as close as possible to the {39} drum without being in
+actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and the
+armature is made as light as possible. The spring to which the armature is
+attached should be of such a length that its natural period of vibration is
+equal to the number of contacts made by the transmitting stylus. The spring
+must be stiff enough to bring the armature back with a fairly crisp
+movement. The spring and armature is shown separate in Fig. 20.
+
+[Illustration: FIG. 19.]
+
+[Illustration: FIG. 20.]
+
+The shutter C is about 1/4 inch square and made from thin aluminium. The
+hole in the centre is 1/16 x 1/8 inch, and the movement of the armature is
+limited to about 3/32 inch. In all arrangements of this kind there is a
+tendency for the armature spring to vibrate, as it were, sinusoidally, if
+the coil is magnetised and demagnetised at a higher rate than the natural
+period of vibration of the spring. {40} This causes an irregularity in the
+rate of the vibrations which affects the received image very considerably.
+A photographic film is wrapped round the drum of the machine, being
+fastened by means of a little celluloid cement smeared along one edge.
+
+This device, although it will work well over artificial conductors, is not
+suitable for wireless work, as it is too coarse in its action; it can be
+made sensitive enough to work at a speed of 1000 to 1500 contacts per
+minute, with a current of .5 milliampere. It is impossible to obtain a
+current of this magnitude from the majority of the detectors in use, so
+that if any attempt is made to use this device for radio-photography it
+will be necessary to employ a Marconi coherer (filings), as this is
+practically the only coherer from which so large a current can be obtained.
+
+There have been many attempts made to receive with an ordinary filings
+coherer, but as was pointed out in Chapter I. these have now been discarded
+in serious wireless work, being only used in small amateur stations or
+experimental sets. As the reasons for this are well known to the majority
+of wireless workers there is no need to enumerate them here.
+
+A method whereby a filings coherer can be decohered, the act of decohering
+closing a local circuit which contains the photographic {41} receiving
+apparatus, is given in the diagram Fig. 21.
+
+[Illustration: FIG. 21.]
+
+In the figure, the coherer C is fixed in rigid supports, one support being
+provided with a platinum pin F. To the coherer is connected the sensitive
+electro-magnet M, which becomes magnetised as soon as the incoming waves
+act upon the coherer. To the armature B is attached a light aluminium arm
+S, pivoted at K, and carrying at the other end the striker G, which is
+fitted with a platinum contact. When the armature B is attracted the
+coherer is decohered by the force of the impact between the contacts F and
+G. To prevent damage to the coherer the force of the blow is taken off by
+the ability of the striker to work back through a hole in the arm S, the
+spring {42} N keeping it normally in a fixed position. T and P are
+adjusting screws, and the terminals J are for connecting to the receiving
+apparatus. With this arrangement a very short wave-train causes only one
+tap of the contacts, so that only one mark is registered on the receiving
+drum for every contact made on the transmitter.
+
+[Illustration: FIG. 22.]
+
+The drawing, Fig. 22, gives a diagrammatic representation of apparatus
+arranged for another photographic method of receiving. The machine shown in
+Fig. 6 is used in this case. A is the aerial, E earth, P primary of
+oscillation-transformer, S secondary of transformer, C variable condenser,
+C' block condenser, D detector, X two-way switch, T telephone.
+
+A De' Arsonval galvanometer H is also connected to the switch X, so that
+either the telephone or the galvanometer can be switched in. The {43}
+galvanometer can be made sensitive enough to work with a current as small
+as 10^{-7} of an ampere, with a period of about 1/150th of a second. The
+screen J has a small hole about 1/8 inch diameter drilled in the centre.
+Under the influence of the brief currents which pass through the detector
+every time a group of waves is received, the mirror of the galvanometer
+swings to-and-fro in front of the screen J, and allows the light reflected
+from the source of light M to pass through the aperture in the screen, on
+to the lens N.
+
+Round the drum V of the machine is wrapped a sensitive photographic film,
+and this records the movements of the mirror which correspond to the
+contacts on the half-tone print used in transmitting. Every time current
+passes through the galvanometer, the light that is received from M,[6]
+passes through the aperture in the screen J, and is focussed by the lens N
+to a point upon the revolving film. As soon as the current ceases, the
+mirror swings back to its original position, and the film is again in
+darkness. Upon being developed a photograph, similar to the negative used
+for preparing the metal print is obtained. If desired the apparatus can be
+so arranged that the received picture is a positive instead of a negative.
+
+{44}
+
+The detector used should be a Lodge wheel-coherer or a Marconi
+valve-receiver, as these are the only detectors that can be used with a
+recording instrument. If the swing of the galvanometer mirror is too great,
+a small battery with a regulating resistance can be inserted in order to
+limit the movement of the mirror to a very short range; the current of
+course flowing in an opposite direction to the current flowing through the
+coherer.
+
+In this, as in all other methods of receiving, the results obtained depend
+upon the fineness of the line screen used in preparing the metal prints;
+and as already shown the fineness of the screen that can be used is
+dependent upon the mechanical efficiency of the entire apparatus.
+
+Another system, and one that has been tried as a possible means of
+recording wireless messages, is as follows. The wireless arrangements
+consist of apparatus similar to that shown in Fig. 22, but instead of a
+Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is
+substituted for the reflecting galvanometer. The Einthoven galvanometer
+consists of a very powerful electro-magnet, the pole pieces of which
+converge almost to points. A very fine silvered quartz thread is stretched
+between the pole pieces, as shown in Fig. 23, the tension being adjustable.
+The period of swing is about 1/250th of a second. A hole is bored through
+the poles, and one of them is fitted {45} [Illustration] with a sliding
+tube which carries a short focus lens N. The light from M passes through
+the magnets, and a magnified image of the quartz thread is thrown upon the
+ebonite screen J. This screen is provided with a fine slit, and when the
+galvanometer is at rest the shadow of the thread just covers the slit in
+the screen and prevents any light from M reaching the photographic film.
+Upon signals being received the shadow of the thread moves to one side for
+a long or short period, uncovering the slit, and allowing light to pass
+through. The lens R concentrates the collected light to a point upon the
+revolving film. The connections for the complete receiver are given in Fig.
+24.
+
+The modified form of the Einthoven galvanometer, as arranged by Professor
+Korn for use with his selenium machines for photo-telegraphy over ordinary
+land lines, consists of two fine silver wires which are displaced in a
+lateral direction between the pole pieces when traversed by a current; the
+current passing through both wires in the same {46} direction. A small
+shutter of aluminium foil is attached to the wires at the optical centre.
+The silver wires used are 1/1000 inch in diameter, with a natural period of
+about 1/120th of a second; the length of wires free to swing being usually
+about 5 cm.
+
+[Illustration: FIG. 24.]
+
+The period of the wires depends to a great extent upon their length and
+diameter, and also upon their tension. By using short fine wires the period
+can be made much smaller, but a greater current is required to produce a
+similar displacement. Where the current available, as in wireless
+telegraphy, is very small, and a definite displacement of the wires is
+required, it is at once apparent that with wires of a given diameter there
+is a limit to their length and therefore to the period. Finer wires can be
+used, but here again there is a practical limit to their fineness, although
+galvanometers have been constructed with a single silvered quartz thread
+1/12000th of an inch diameter, which, when placed in a powerful field, will
+give a good displacement with a current as small as 10^{-8} ampere. {47}
+
+With the apparatus arranged by the Poulsen Company, given in the diagram,
+Fig. 17, for photographically recording wireless signals, the current
+required to operate the galvanometer for signals transmitted at the rate of
+1500 a minute is 1 x 10^{-6} ampere, while for signals up to 2500 a minute
+a current about 5 x 10^{-6} ampere is necessary.
+
+Another very sensitive instrument, employed by M. Belin, and known as
+Blondel's oscillograph, consists of two fine wires stretched between the
+poles of a powerful electro-magnet, a small and very light mirror being
+attached to the centre of the wires. The current passes down one wire and
+up the other, and the wires, together with the mirror, are twisted to a
+degree depending upon the strength of the received current. In order to
+render the instrument dead-beat the moving parts are arranged to work in
+oil. The light reflected from the mirror is made use of in a manner similar
+to that shown in Fig. 22.
+
+In all photographic methods of receiving, the apparatus must be enclosed in
+some way to prevent any extraneous light from reaching the film, or better
+still placed in a room lighted only by means of a ruby light.
+
+The following method is given more as a suggestion than anything else, as I
+do not think it has been tried for wireless receiving, although it is
+stated to have given some good results over {48} ordinary land lines. It is
+the invention of Charbonelle, a French engineer, and is quite an original
+idea. His method consists of placing a sheet of carbon paper between two
+sheets of thin white paper, and wrapping the whole tightly round the drum
+of the machine. A hardened steel point is fastened to the diaphragm of a
+telephone receiver, and this receiver is placed so that the steel point
+presses against the sheets of paper. As the diaphragm and steel point
+vibrates under the influence of the received currents marks are made by the
+carbon sheet on the bottom paper.
+
+Over a line where a fair amount of current is available at the receiver,
+the diaphragm would have sufficient movement to mark the paper, but the
+movement would be very small with the current received from a detector.
+This difficulty could no doubt be overcome to a certain extent by making a
+special telephone receiver, with a large and very flexible diaphragm, and
+wound for a very high resistance. The movement of an ordinary telephone
+diaphragm for a barely audible sound is, measured at the centre, about
+10^{-6} of a c.m. With a unit current the movement at the centre is about
+1/700th of an inch. Greater movement of the diaphragm could be obtained by
+connecting a _Telephone relay_ to the detector, and using the magnified
+current from the relay to operate the telephone. {49}
+
+[Illustration: FIG. 25.]
+
+The telephone relay consists of a microphone C, Fig. 25, formed of the two
+pieces of osmium iridium alloy. The contact is separated to a minute degree
+partly by the action of the local current from F, which flows through it
+and also through the winding W of the two magnet coils. The local current
+from F assists in forming the microphone by rendering the space between the
+contacts conductive. The vibrating reed P is fastened to the metal frame
+(not shown) which carries a micrometer screw by which the distance between
+the contacts can be accurately regulated. It will be seen from Fig. 25 that
+the local circuit consists of a battery F (about 1.5 volts), the microphone
+contacts C, the windings W, milliampere meter B, and the terminals T, for
+connecting to the galvanometer or telephone, all in {50} series. On the top
+of the magnet cores N, S is a smaller magnet D, wound with fine wire for a
+resistance of about 4935 ohms, the free ends of the coils being connected
+to the detector terminals. The working is as follows. Supposing the current
+from the detector flows through D in such a way that its magnetism is
+increased, the reed P will be attracted, the contacts opened, and their
+resistance increased. It will be seen that the current from F is passed
+through the coils W, in such a way as to increase the magnetism of the
+permanent magnet, so that any opening of the microphone contact increases
+their resistance, causes the current to fall, and weakens the magnets to
+such an extent that the reed P can spring back to its normal position. On
+the other hand, if the detector current flows through D in such a direction
+as to decrease the magnetism in the permanent magnets, the reed P will rise
+and make better contact owing to the removal of the force opposing the
+stiffness of the reed. Owing to the decrease in the resistance of the
+microphone, the strength of the local current will be increased, the
+magnets strengthened, and the reed P will be pulled back to its original
+position. This relay gives a greatly magnified current when properly
+adjusted, the current being easily increased from 10^{-4} to 10^{-2}
+amperes. It is also very sensitive, but needs careful adjustment in order
+that the best results may {51} be obtained. A greater range of
+magnification can be obtained by placing two or more relays in series.
+
+[Illustration: FIG. 26.]
+
+A very sensitive receiver designed by the writer is given in the figures 26
+and 27. To the centre of a telephone diaphragm is fastened a light steel
+point P, and the movement of this point is communicated to the aluminium
+arm D, which is pivoted at C. As will be seen the telephone receiver is of
+special construction, it containing only one coil and therefore only one
+core; by this means the movement of the diaphragm is centralised. The coil
+is wound for a resistance of about 200 ohms, and the diaphragm should be
+fairly thin but very resillient.
+
+[Illustration: FIG. 27.]
+
+To the free end of D is fastened the mirror T, made from thin diaphragm
+glass about 1-1/2 centimetres diameter, and having a focal length of 40
+inches. Light from the lamp L is transmitted by the lens N in a parallel
+beam to the mirror which {52} concentrates it to a point upon a hole
+1/100th of an inch in diameter in the screen J. As the telephone diaphragm
+vibrates under the influence of the received signals the arm, and
+consequently the mirror, vibrates also, and the hole in the screen J is
+constantly being covered and uncovered by the spot of light. It will be
+seen from Fig. 27 that the ratio between the centre of the mirror and the
+pivot C, and C and the steel point P is 10:1, so that if a movement of
+1/20000th of an inch is obtained at the centre of the diaphragm the mirror
+will move 1/2000th of an inch; and as the focal length of the mirror is 40
+inches a movement of 1/50th inch is given to the spot of light.
+
+This receiver is capable of working at a fairly high speed, as the inertia
+of the moving parts is practically negligible; the weight of the arm and
+mirror being less than 20 grains. The hole in the screen is made slightly
+less in diameter than the traverse of the revolving cylinder, the slight
+distance between the cylinder and the screen allowing the light to disperse
+sufficiently to produce a line on the film of about the right thickness.
+
+There are two other possible means of photographically receiving the
+picture that upon investigation may yield some results; but it is doubtful
+whether the current available, even that obtained from a telephone relay,
+will be sufficient to produce the desired magnetic effect, and the {53}
+insertion of a second relay would detract greatly from the efficiency by
+decreasing the speed of working. If rays of monochromatic light from a lamp
+L, Fig. 28, pass through a Nicol prism P (polarising prism), then through a
+tube containing CS_2 (carbon bisulphide), afterwards passing through the
+second prism P' (analysing prism), and if the two Nicol prisms are set at
+the polarising angle, no light from L would reach the photographic film
+wrapped round the drum V of the machine. Upon the tube being subjected to a
+field produced by a current passing through the coil C, the refractive
+index of the liquid will be changed, and light from L will reach the
+photographic film.[7]
+
+[Illustration: FIG. 28.]
+
+The second method is rather more complicated, and is based upon the fact
+that the kathode rays in a Crookes' tube can be deflected from their course
+by means of a magnet. In Fig. 29 the kathode K of the X-ray tube sends a
+kathode ray discharge through an aperture in the anode A, through a small
+aperture in the ebonite screen J {54} on to the drum V of the machine,
+round which is wrapped a photographic film; A and K being connected to
+suitable electrical apparatus. Upon the coil M being energised, the
+kathode-ray is deflected from its straight-line course, and the drum V is
+left in darkness.
+
+[Illustration: FIG. 29.]
+
+The method which is now going to be described is very ingenious, as it
+makes use of what is known as an electrolytic receiver. This method of
+receiving has proved to be the most practical and simple of all the
+photo-telegraphic systems that have been devised.
+
+The application of this system to wireless reception is as follows. The
+aerial A, and the earth E, are joined to the primary P of a transformer,
+the secondary S being connected to a Marconi valve receiver C. The valve
+receiver is connected to the battery B and silvered quartz thread K of an
+Einthoven galvanometer (already described). The thread is 1/12000th of an
+inch in diameter, and will respond to currents as small as 10^{-8} of {55}
+an ampere. The light from M throws an enlarged shadow of the thread over a
+slit in the screen J, and as the thread moves to one side under the
+influence of a current, the slit in J is uncovered, and the light from M is
+thrown upon a small selenium cell R. In the dark the selenium cell has a
+very high resistance, and therefore no current can flow from the battery D
+to the relay F. When the string of the galvanometer moves to one side and
+uncovers the slit in the screen J, a certain amount of light is thrown upon
+the selenium cell lowering its resistance, allowing sufficient current to
+pass through to operate the relay.
+
+Round the drum of the machine (shown in Fig. 7) is wrapped a sheet of paper
+that has been soaked in certain chemicals that are decomposed on the
+passage of an electric current through them. As soon as the local circuit
+of the relay is closed, the current from the battery Z (about 12 volts)
+flows through the paper and produces a coloured mark. The picture,
+therefore, is composed of long or short marks which correspond to the
+varying strips of conducting material on the single line print. In order to
+render the marks short and crisp, a small battery Y, and regulating
+resistance L, is placed across the drum and stylus. The diagram, Fig. 30,
+gives the connections for the complete receiver. {56}
+
+The paper used is soaked in a solution consisting of
+
+  Ferrocyanide of potassium             1/4 oz.
+  Ammoniac Nitrate                      1/2 oz.
+  Distilled water[8]                      4 oz.
+
+[Illustration: FIG. 30.]
+
+The paper has to be very carefully chosen, as besides being absorbent
+enough to remain moist during the whole of the receiving, the surface must
+also remain fairly smooth, as with a rough paper the grain shows very
+distinctly, and if there is an excess of solution the electrolytic marks
+are inclined to spread and so cause a blurred image. The writer tried
+numerous specimens of paper before one could be found that gave really
+satisfactory results. It was also found that when working in a warm room
+the paper became nearly {57} dry before the receiving was finished, and the
+resistance of the paper being greatly increased (this may be anything up to
+1000 ohms), the marking became very faint. A sponge moistened with the
+solution and applied to the undecomposed portion of the paper, while still
+revolving, was found to help matters considerably.
+
+Another experience which happened during the writer's early experiments,
+the cause of which I am still unable to explain, occurred in connection
+with the stylus. The stylus used consisted of a sharply pointed steel
+needle, and after working for about three minutes it was noticed that the
+lines were becoming gradually wider, finally running into each other. Upon
+examination it was found that the point of the needle had worn away
+considerably, becoming in fact, almost a chisel point. Almost every needle
+tried acted in a similar manner, and to overcome this difficulty the stylus
+shown in Fig. 31 was devised.
+
+It will be seen that it consists of a holder A, somewhat resembling a drill
+chuck, fastened to the flat spring B in such a manner that the angle the
+stylus makes to the drum can be altered. The needle consists of a length of
+36-gauge steel wire, and as this wears away slowly the jaws of the holder
+can be loosened and a fresh length pushed through. The wire should not
+project beyond the face of the holder more than 1/8th inch. The gauge {58}
+of wire chosen would not suit every machine, the best gauge to use being
+found by trial, but in the writer's machine the pitch of the decomposition
+marks is much finer than of those made by the commercial machines, and this
+gauge, with the slight but unavoidable spreading of the marks, will produce
+a mark of just the right thickness. As already mentioned, no explanation of
+this peculiarity on the part of the stylus can be given, as there is
+nothing very corrosive in the solution used, and the pressure of the stylus
+upon the paper is so slight as to be almost negligible.
+
+[Illustration: FIG. 31.]
+
+No special means are required for fastening the paper to the drum, the
+moist paper adhering quite firmly. Care should be taken, however, to fasten
+the paper--which should be long enough to allow for a lap of about 1/4
+inch--in such a manner that when working the stylus draws away from the
+edge of the lap and not towards it.
+
+The current required to produce electrolysis is very small, about one
+milliampere being sufficient. {59} Providing that the voltage is
+sufficiently high, decomposition will take place with practically "no
+current," it being possible to decompose the solution with the discharge
+from a small induction coil. The quantity of an element liberated is by
+weight the product of time, current, and the electro-chemical equivalent of
+that element, and is given by the equation W = zct, where
+
+  W = quantity of element liberated in grammes.
+  z = electro-chemical equivalent,
+  c = current in amperes,
+  t = time in seconds.
+
+The chemical action that takes place is therefore very small, as the
+intermittent current sent out from the transmitter in some cases only lasts
+from 1/50th to 1/100th a second.
+
+The decomposed marks on the paper are blue, and, as photographers know,
+blue is reproduced in a photograph as a white, so that a photograph taken
+of our electrolytic picture, which will of course be a blue image upon a
+white ground, will be reproduced almost like a blank sheet of paper. If,
+however, a yellow contrast filter is placed in front of the camera lens,
+and an orthochromatic plate used, the blue will be reproduced in the
+photograph as a dead black.
+
+There is one other point that requires attention. It will be noticed that
+the metal print used for {60} transmitting is a positive, since it is
+prepared from a negative. The received picture will therefore be a
+negative, making the final reproduction, if it is to be used for newspaper
+work, a negative also. Obviously this is no good. The final reproduction
+must be a positive, therefore the received picture must be also a positive.
+To overcome this difficulty matters must be so arranged at the receiving
+station that in the cases of Figs. 17, 18, 22, and 24, the film is kept
+permanently illuminated while the stylus on the transmitter is tracing over
+an insulating strip, and in darkness when tracing over a conducting strip.
+In Fig. 30 the relay F should allow a continuous current from Z to flow
+through the electrolytic paper, and only broken when the resistance of the
+selenium cell is sufficiently reduced to allow the current from D to
+operate the relay.
+
+The author has endeavoured to make direct positives on glass of the picture
+to be transmitted, so that a negative metal print could be prepared. The
+results obtained were not very satisfactory, but the method tried is given,
+as it may perhaps be of interest. The plate used in the camera has to be
+exposed three or four times longer than is required for an ordinary
+negative. The exposed plate is then placed in a solution of protoxalate of
+iron (ferrous oxalate) and left until the image shows plainly through the
+back of the plate. It {61} is then washed in water and placed in a solution
+consisting of
+
+  Distilled water                      1000 cc.
+  Nitric acid                             2 cc.
+  Sulphuric acid                          3 cc.
+  Bichromate of potash                  105 grammes.
+  Alum                                   80    "
+
+After being in this bath for about fifteen minutes the plate is again well
+washed in water, and developed in the ordinary way. The first two
+operations should be performed in the dark room, but the remaining
+operations can be performed in daylight, once the plate has been placed in
+the bichromate bath. As already stated, the results obtained were not very
+satisfactory, and such a method is not now worth following up, as it is
+comparatively easy so to arrange matters at the receiving station that a
+positive or negative image can be received at will.
+
+It is necessary to connect the stylus of the receiving machine to the
+positive pole of the battery Z, otherwise the marks will be made on the
+underside of the paper. The electrolytic receiver, owing to the absence of
+mechanical and electro-magnetic inertia, is capable of recording signals at
+a very high speed indeed.
+
+"Atmospherics," which are such a serious nuisance in long-distance wireless
+telegraphy, will also prove a nuisance in wireless photography, {62} but
+their effects will not be so serious in a photographic method of receiving
+as they would be in the electrolytic system. In a photographic receiver
+where the film is, under normal conditions, constantly illuminated, the
+received signals (both the transmitted signals and the atmospheric
+disturbances) will be recorded, after development, as transparent marks
+upon the film, the remainder of the film being, of course, perfectly
+opaque. By careful retouching the marks due to the disturbances can be
+eradicated, a print upon sensitised paper having been first obtained to act
+as a guide during the process.
+
+       *       *       *       *       *
+
+
+{63}
+
+CHAPTER IV
+
+SYNCHRONISING AND DRIVING
+
+Clockwork and electro-motors are the source of driving power that are most
+suitable for photo-telegraphic work, and each has its superior claims
+depending on the type of machine that is being used. For general
+experimental work, however, an electro-motor is perhaps the most
+convenient, as the speed can be regulated within very wide limits. For a
+constant and accurate drive a falling weight has no equal, but the
+apparatus required is very cumbersome and the work of winding both tedious
+and heavy. This method of driving was at one time universally employed with
+the Hughes printing telegraph, but it has now been discarded in favour of
+electro-motors, which are more compact, besides being cheaper to instal in
+the first instance.
+
+Synchronising and isochronising the two machines are the most difficult
+problems that require solving in connection with wireless photography, and
+as previously mentioned, the {64} synchronising of the two stations must be
+very nearly perfect in order to obtain intelligible results. The limit of
+error in synchronising must be about 1 in 500 in order to obtain results
+suitable for publication.
+
+The electrolytic system is perhaps the easiest to isochronise, as the
+received picture is visible. On the metal print used for transmitting, and
+at the commencing edge a datum line is drawn across in insulating ink. The
+reproduction of this line is carefully observed by the operator in charge
+of the receiving instrument, and the speed of the motor is regulated until
+this line lies close against a line drawn across the electrolytic paper.
+Although this may seem an ideal method there are one or two considerations
+to be taken into account. Unless the decomposition marks are made the
+correct length and are properly spaced, however good the isochronising may
+be, the result will be a blurred image. Any one who has worked with a
+selenium cell, will know that it cannot change from its state of high
+resistance to that of low resistance with infinite rapidity, and the
+effects of this inertia, or "fatigue" as it has been called, are more
+pronounced when working at a high speed. In working, the effects of this
+inertia would be to increase the time of contact of the relay F (Fig. 30)
+as the current from D would flow for a slightly longer period through R to
+F than the period of {65} illumination allowed by K. This, of course, would
+mean a lengthening of the marks on the paper; results would also differ
+greatly with different selenium cells. There is a method of compensation by
+which the inertia of a cell can almost entirely be overcome, but it would
+add greatly to the complicacy of the receiving apparatus.
+
+In using an electro-motor with any optical method of receiving there are
+two methods available. The first is an arrangement similar to that used by
+Professor Korn in his early experiments with his selenium machines. The
+motor used for driving has several coils in the armature connected with
+slip rings, from which an alternating current may be tapped off; the motor
+acting partially as a generator, besides doing good work as a motor in
+driving the machine. This alternating current is conducted to a frequency
+meter, which consists of a powerful electro-magnet, over which are placed
+magnetised steel springs, having different natural periods of vibration. By
+means of a regulating resistance the motor is run until the spring which
+has the same period as the desired armature speed vibrates freely. The
+speed of the motors at both stations can thus be adjusted with a fair
+amount of accuracy. Another method is to make use of a governor similar to
+those employed in the Hughes printing telegraph system. A drawing of the
+governor is given in Fig. 32. It consists of a
+
+[Illustration] {67} metal frame which supports an upright steel bar S,
+whose ends turn on pivots. This bar is rectangular in section. The
+gear-wheel G is fastened near the bottom of this rod and gears with a
+similar wheel on the shaft of the driving motor (not shown). Suspended from
+the broader sides of S are the two flexible arms D, each carrying a brass
+ball T. These balls are not fastened to the arms, but can slide up and
+down, being held in position by the wire springs M, one end of each spring
+being fastened to the screws C. These screws work in a slot cut in the
+upper part of S, and are connected to the adjusting screw E. When E is
+turned the screws are raised or lowered accordingly, and also the balls on
+the arms D.
+
+Fastened to the arms are two brushes of tow B, and these revolve inside but
+just clearing the inner surface of the steel ring Z. Upon the motor speed
+increasing above the normal the arms D, and consequently the balls T, swing
+out, making a larger circle, causing the brushes B to press against the
+steel ring Z, setting up friction which, however, is reduced as soon as the
+motor regains its ordinary working speed. By careful adjustment the speed
+of the motors can be kept perfectly constant. The object of having the
+balls T adjustable on D, is to provide a means of altering the motor speed,
+as the lower the balls on D the slower the mechanism runs, and _vice
+versa_. {68}
+
+[Illustration]
+
+A simple and effective speed regulator devised by the writer is given in
+drawings 33 and 34. It comprises two parts, A and B, the part A being
+connected to the driving motor, and the part B working independently. The
+independent portion B consists of an ordinary clock movement M, a steel
+spindle J being geared to one of the slower moving wheels, so that it makes
+just one revolution in two seconds. This spindle, which runs in two coned
+bearings, carries at its outer end a light [Illustration] pointer D, about
+two inches long, to the underside of which is fastened the thin brass
+contact spring S, which presses lightly upon the ebonite ring N. {69} The
+portion A comprises a spindle, pointer, and contact spring similar to those
+employed in B, the spindle J' being geared to the driving motor by means of
+F, so that the pointer D' makes a little more than one revolution in two
+seconds. By means of a special form of brake on the driving motor, the
+speed is reduced, so that both pointers travel at the same rate, viz. one
+revolution in two seconds. By careful adjustment the two pointers can be
+made to revolve in synchronism,[9] and when this is obtained the contact
+springs S, S', pass over the contacts C, C', completing the circuit of the
+battery B and lamp L. When working properly the lamp L lights up regularly
+once every second. This regulator is an excellent one to use for
+experimental work, although it depends a great deal upon the skill of the
+operator, but good adjustment should be obtained in about two minutes. It
+is a good plan to insert a clutch of some description between the driving
+motor and the machine, so that the regulator can be adjusted prior to the
+act of receiving or transmitting, the machine being prevented from
+revolving by means of a catch. The motor used should be powerful enough to
+take up the work of driving the machine without any reduction in speed. The
+clocks M can be regulated so that they only gain or lose a few seconds in
+{70} twenty-four hours, which gives an accuracy in working sufficient for
+all practical purposes.
+
+Connection is made with the contact springs S, S', by means of the springs
+T, T', which press against the spindles J, J'.
+
+Another important point is the correct placing of the picture upon the
+receiving drum. It is necessary that the two machines besides revolving in
+perfect isochronism should synchronise as well, _i.e._ begin to transmit
+and record at exactly the same position on the cylinders, viz. at the edge
+of the lap, so that the component parts of the received image shall occupy
+the same position on the paper or film as they do on the metal print. If
+the receiving cylinder had, let us suppose, completed a quarter of a
+revolution before it started to reproduce, the reproduction when removed
+from the machine and opened out will be found to be incorrectly placed; the
+bottom portion of the picture being joined to the top portion, or _vice
+versa_, and this means that perhaps an important piece of the picture would
+be rendered useless even if the whole is not spoilt. It is evident,
+therefore, that some arrangement must be employed whereby synchronism, as
+well as isochronism of the two instruments can be maintained.
+
+There are several methods of synchronising that are in constant use in
+high-speed telegraphy, in which the limit of error is reduced to a minimum,
+{71} and some modification of these methods will perhaps solve the problem,
+but it must be remembered that synchronism is far easier to obtain where
+the two stations are connected by a length of line than where the two
+stations are running independently.
+
+In one system of ordinary photo-telegraphy synchronism is obtained in the
+following manner. The receiving cylinder travels at a speed slightly in
+excess of the transmitting cylinder, and as its revolution is finished
+first is prevented from revolving by a check, and when in this position the
+receiving apparatus is thrown out of circuit and an electro-magnet which
+operates the check is switched in. When the transmitting cylinder has
+completed its revolution (about 1/100th of a second later) the transmitting
+apparatus, by means of a special arrangement, is thrown out of circuit for
+a period, just long enough for a powerful current to be sent through the
+line. This current actuates the electro-magnet. The check is withdrawn and
+the receiving cylinder commences a fresh revolution in perfect synchronism
+with the transmitting cylinder. As soon as the check is withdrawn the
+receiving apparatus is again placed in circuit until another revolution is
+completed. As the receiver cannot stop and start abruptly at the end of
+each revolution a spring clutch is inserted between the driving motor and
+the machine. {72}
+
+Although a method of synchronising similar to this may later on be devised
+for wireless photography, the writer, from the result of his own
+experiments, is led to believe that results good enough for all practical
+purposes can be obtained by fitting a synchronising device whereby the two
+machines are started work at the same instant, and relying upon the perfect
+regulation of the speed of the motors for correct working.
+
+The method of isochronism must, however, be nearly perfect in its action,
+as it is easy to see that with only a very slight difference in the speed
+of either machine this error will, when multiplied by 40 or 50 revolutions,
+completely destroy the received picture for practical purposes.
+
+From what has been written in this and in the preceding chapters it will be
+evident that the successful solution of transmitting photographs by
+wireless methods will necessitate the use of a great many pieces of
+apparatus all requiring delicate adjustment, and depending largely upon
+each other for efficient working. As previously stated, there is at present
+no real system of wireless photography, the whole science being in a purely
+experimental stage, but already Professor Korn has succeeded in
+transmitting photographs between Berlin and Paris, a distance of over 700
+miles. If such a distance could be worked over successfully, there is no
+reason to doubt that before long {73} we shall be able to receive pictures
+from America with as great reliability and precision as we now receive
+messages.
+
+In nearly all wireless photographic systems devised up to the present the
+chief portion of the receiver consists of a very sensitive galvanometer,
+and although very good results have been obtained by their use they are
+more or less a nuisance, as the extreme delicacy of their construction
+renders them liable to a lot of unnecessary movement caused by external
+disturbances. A galvanometer of the De' Arsonval pattern, used by the
+writer, was constantly being disturbed by merely walking about the room,
+although placed upon a fairly substantial table; and for the same reason it
+was impossible to attempt to place the driving motor of the machine on the
+same table as the galvanometer. For ship-board work it will be evident that
+the use of such a sensitive instrument presents a great difficulty to
+successful working, and a good opening exists for some piece of
+apparatus--to take the place of the galvanometer--that will be as sensitive
+in its action but more robust in its construction.
+
+       *       *       *       *       *
+
+
+{74}
+
+CHAPTER V
+
+THE "TELEPHOGRAPH"
+
+In the present chapter it is proposed to give a brief description of a
+system of radio-photography devised by the author, and which includes a
+greatly improved method of transmitting and receiving, as well as an
+ingenious arrangement for synchronising the two stations; the whole being
+an attempt to produce a system that would be capable of working
+commercially over fairly long distances.
+
+The system about to be described, and which I have designated the
+"telephograph," is the outcome of several years' original experimental
+work, many difficulties that were manifest in the working of the earlier
+systems having been overcome by apparatus that has been expressly designed
+for the purpose.
+
+In any practical system of radio-photography the following points are of
+great importance: (1) the speed of transmission; (2) the quality of the
+received picture; (3) the method of synchronising {75} the two machines so
+that transmission and reception begin simultaneously; (4) the correct
+regulation of the speed of the driving motors; (5) the simplicity and
+reliability of the entire arrangement. Points 1 and 2 are dependent upon
+several factors; the number of contacts made by the stylus per minute; the
+size of the metal print used; the number of lines per inch on the screen
+used in preparing the print; and the accurate and harmonious working of the
+various pieces of apparatus employed.
+
+In the system under discussion the size of the metal print used is 5 inches
+by 7 inches, and a screen having 50 lines to the inch is used for preparing
+it. With the drum of the machine making one revolution in four seconds, the
+stylus makes 87 contacts per second, or 5220 a minute, the time for
+complete transmission being twenty-five minutes. By the use of ordinary
+relays not more than 2000 contacts a minute can be obtained, and in the
+present system it is only by means of a specially designed relay that such
+a high rate of working has been made possible. Similarly, too, with the
+receiving of such a large number of signals transmitted at such a high
+speed, a special instrument has been devised that can record this number of
+signals without any trouble, and could even record up to 8000 signals a
+minute, provided that a suitable transmitter could be designed. {76}
+
+In the present system the writer does not claim to have completely solved
+the problem of the wireless transmission of photographs, but it is a great
+advance on any system previously described, and the following advantages
+are put forward for recognition: (1) a greatly improved method of
+transmitting and receiving; (2) a simple method of regulating the speed of
+the driving motors and maintaining isochronism with a limit of error of
+less than 1 in 800; (3) an arrangement for synchronising the two machines
+whereby transmitting and receiving begin simultaneously; (4) the use of one
+machine only at each station.
+
+TRANSMITTING APPARATUS
+
+A diagrammatic representation of the apparatus required for a complete
+station, transmitting and receiving combined, is given in Fig. 35, the
+usual wireless equipment having been omitted from the diagram to avoid
+confusion.
+
+_The Machine._--This, as will be seen from Fig. 36, consists of a
+base-plate M, to which are attached the two bearings B and B'. The bearing
+B' is fitted with an internal thread to correspond with the threaded
+portion of the shaft D. The drum V is a brass casting, being fastened to
+the shaft by set screws. The shaft is threaded 75 to the inch. The bearings
+are preferably of the concentric type. The circuit breaker C is so arranged
+that when {77} the drum has traversed the required distance, the end of the
+shaft pushes back the spring M, breaking the circuit of the driving gear
+and stopping the machine. The machine is connected to the driving gear by
+the flexible coupling A.
+
+[Illustration: FIG. 35.
+
+M, motor; Y, isochroniser; F, clutch; A, machine; R, stylus; S, relay; X,
+gearing; O, circuit breaker; T, receiver; C, condenser; U, telephone relay;
+K, polarised relay; L, contact breaker; D, D^1, D^2, D^3, batteries; P,
+friction brake; B, B^1, double-pole two-way switches; N, N^1, N^2, single
+switches; W, key; E, electric clock; J, telephones.]
+
+The drum measures 5 inches long by 2-1/8 inches diameter, and this takes a
+metal print 5 inches by 7 inches, which allows for a lap of about 1/4 inch.
+In working, the print is wrapped tightly round the drum, being secured by
+means of a little seccotine smeared along one edge. Care must be taken that
+the edge of the lap draws away from the point of {78} the stylus and not
+towards it. A margin of bare foil, about 1/8 inch wide, should be left on
+the print at the commencing edge, the purpose of which will be explained
+later.
+
+[Illustration: FIG. 36.]
+
+_The Stylus._--As the drum of the machine travels laterally, by reason of
+the threaded shaft and bearing, the stylus must necessarily be a fixture.
+It consists of a holder B, drilled to take a hardened steel point S,
+attached to the spring M. The spring is arranged to work in the guide F,
+which is provided with an adjusting screw W for regulating the pressure of
+the stylus upon the print; the pressure being sufficient to enable good
+contact to be made, but must not be heavy enough to scratch the soft foil.
+The needle should present an angle of about 60deg to the surface of the
+print, as this angle has been found to give the best results in working.
+
+To eliminate any sparking that may take place at the point of make and
+break, due to the self-induction of the relay coils, a condenser C, about 1
+microfarad capacity, should be connected across {79} the drum and stylus.
+The complete stylus is given in the drawings, Figs. 37, 37_a_, and also in
+the diagrams Figs. 8 and 9.
+
+[Illustration: FIG. 37.
+
+Showing the arrangement for sliding the stylus to or from the machine.]
+
+[Illustration: FIG. 37a.]
+
+_The Relay._--As will be seen from the diagram, Fig. 38, this consists of
+two electro-magnets having very soft iron cores, the magnet M being wound
+in the usual manner, while the magnet N is wound differentially. The
+armature A is made as light as possible, and is pivoted at P, and when
+there is no current flowing through any of the coils, is held midway
+between the magnet cores by the two spiral springs S and T, which are under
+slight but equal tension. The connections are as follows. The wires from
+the winding on M are connected directly to the relay terminals F and H, as
+are also the wires from one winding on N. The other winding on N is
+connected in series with the battery C, ammeter B, and regulating
+resistance R. {80}
+
+[Illustration: FIG. 38.]
+
+When the circuit of the battery C is completed, the coil of N, to which it
+is connected, is energised, and the armature A is attracted against the
+stop V. When in this position the tension of the spring S is released,
+while the tension of the spring T is increased. As soon as the circuit of
+the battery D is completed by means of the metal line print on the
+transmitting machine, the current divides at the terminals F and H, a
+portion flowing through the magnet coil M, and a portion through the
+remaining winding on N. The current which flows through the winding on N
+produces a magnetising effect equal to that caused by the other winding on
+N, but since the two windings are of equal length and resistance, and since
+the current flowing through the two windings is of equal strength but in
+opposite directions, the result is to neutralise {81} the magnetising
+effects produced by each winding, and consequently no magnetism is produced
+in the cores.
+
+The other portion of the current from D flows through the coil M, and it
+becomes magnetised at the same time that the coil N becomes demagnetised.
+The armature A is attracted by M against the stop X, and this attraction is
+assisted by the spring T, which was under increased tension. The conditions
+of the springs are now reversed, the spring S being under increased
+tension, while the tension of the spring T is released.
+
+As soon as the current from D is broken, the magnetism disappears from M,
+the neutralising current in N ceases, and N once more becomes magnetised,
+owing to the current which still flows through one winding from C; the
+armature is therefore again attracted by N, assisted by the spring S. The
+current flowing through the two windings of N must be perfectly equal, and
+the regulating resistance R, and ammeters B and B', are inserted for
+purposes of adjustment. The current from C must flow in a direction
+opposite to that which flows from D.
+
+[Illustration: FIG. 39.
+
+H, H', containers; M, mercury; E, paraffin oil; T, T', terminals; C,
+suspending rod; D, base; F, F', dipping rods.]
+
+The local circuit of the relay is completed by means of a copper dipper in
+mercury, somewhat resembling an ordinary mercury break, but modified to
+suit the present requirements. The arrangement will be seen from Fig. 39.
+The whole of the {82} moving parts are made as light as possible, and for
+this reason the rod C and the dippers F, F' should be made as short as
+convenient. The containers H, H' are separate, of cast iron, and
+rectangular in shape. The dipper is of very thin copper tube--an advantage
+where alternating current is to be used--and is made adjustable for height
+on the suspending rod C. The leg F is of such a length that permanent
+contact is made with the mercury in the container H, while the leg F'
+clears the surface of the mercury by about 1/4 inch, when the armature of
+the relay is in its normal position. To prevent undue churning of the
+mercury, which would necessarily take place if the dipper entered and left
+the mercury at each movement of the armature, a pointed ebonite plug is
+inserted in the end of the tube. This will be found to give good results at
+a high speed, the mercury being practically undisturbed, and the production
+of "sludge" reduced to a minimum. To prevent oxidation of the mercury, and
+to prevent arcing, the surface is covered with paraffin oil. If this is not
+sufficient to prevent arcing a condenser should be shunted across the {83}
+containers. The volume of mercury, and the area of the dippers, should be
+sufficient to carry the current used for a considerable period without
+heating up to any extent. An adjustable weight J is provided in order to
+balance the armature and dipping rod.
+
+The remaining transmitting apparatus consists of the battery D^2 and the
+usual wireless apparatus. The double-pole two-way switch B' is to enable
+the photo-telegraphic set to be switched out and the hand key W switched in
+for ordinary signalling purposes. The battery D^2 should be about 12 volts.
+
+RECEIVING APPARATUS
+
+The wireless portion of the receiver is similar to that given in Fig. 22,
+is of the usual syntonic type, and comprises an oscillation transformer, S
+being the secondary, and P the primary; C' is a block condenser, and C a
+variable condenser. The detector D is of the carborundum crystal or
+electrolytic pattern. A two-way switch B is provided so that the relay U
+can be switched out and the telephones J switched in for ordinary receiving
+purposes. The relay U is a Brown's telephone relay.
+
+[Illustration: FIG. 40.]
+
+_The Receiver._--The magnified current from the relay U is taken to a
+special telephone receiver, the construction of which is given in Fig. 40.
+The diaphragm F is about 2-1/2 inches diameter, and should be fairly thin
+but very resilient. Only one {84} [Illustration] [Illustration] coil is
+provided, and this should be wound with No. 47 S.S.C. copper wire for a
+resistance of about 2000 ohms. By using only one coil and therefore only
+one core, the movement of the diaphragm is centralised. To the centre of
+the diaphragm a light steel point is fastened, about 1/2 inch long, and
+provided with a projecting hook H. An enlarged view of this pin is given in
+Fig. 41. The movement of the diaphragm and consequently of the steel point
+P is communicated to a pivoted rod R, which is of special construction. A
+piece of aluminium tube 3-3/4 inches long, and of the section given at B,
+is bushed at one end with a piece of brass of the shape shown in Fig. 41a.
+A stiff steel wire T about 1 inch long (20 gauge) is screwed into the end
+of Z, and carries a counterbalance weight C. A hardened {85} steel spindle,
+pointed at both ends, is fastened at D, and runs between two coned
+bearings, one of which is adjustable. The underside of Z is flattened, and
+a small coned depression is made for the reception of the pointed end of
+the pin. By means of the spring J the two pieces, Z and P, are held firmly
+together, at the same time allowing perfect freedom of movement. The bridge
+G is made from a piece of sheet aluminium placed in a slot cut in the tube
+R, the end of the tube being pressed tight upon G, and secured by means of
+a small rivet.
+
+The optical arrangements are as follows. By means of the Nernst lamp L, and
+the lenses B and B', Figs. 42 and 43, a magnified shadow of G is thrown
+upon the screen J. When the shutter G is in its normal position (_i.e._ at
+rest), its shadow is just above the small hole in J, and light from L
+reaches the photographic film wrapped round the drum V of the machine.
+
+[Illustration: FIG. 42.
+
+J, screen; L, Nernst lamp; G, shutter; B, condensing lens; B_1, focussing
+lens.]
+
+When, however, signals are sent out from the transmitting apparatus, the
+magnified current from the relay U energises the coil of the special
+telephone S, attracting the diaphragm F, and consequently giving movement
+to the pivoted rod R. As by means of the optical arrangements a {86}
+magnified movement as well as a magnified image of G is thrown upon the
+screen J, the shadow of G will, when the telephone S is actuated, cover the
+hole in the screen, and prevent any light from reaching the film on V,
+until current from the relay U ceases to flow. Therefore, when the stylus
+of the transmitter traces over a conducting strip on the metal print, no
+light reaches the film on V, but when tracing over an insulating strip the
+shadow of G on the screen J rises, and the light from L reaches the film.
+By this means a positive picture is received, which is a great advantage
+where the photographs are required for reproduction. Atmospherics would be
+represented by irregular transparent marks on the film after development,
+and these can be easily eradicated by retouching.
+
+[Illustration: FIG. 43. E, ebonite screen; F, focussing lens; G, shutter;
+O, condensing lens; L, Nernst lamp.]
+
+The drum of the machine moves laterally 1/75th of an inch per revolution,
+and the hole in the screen is 1/90th of an inch in diameter. As the screen
+J is not in direct contact with the film, the slight diffusion of the light
+that takes place will produce {87} a mark of about the right thickness.
+With a movement of the diaphragm of only 1/40000th of an inch, the actual
+movement of G will be 1/4000th of an inch. If the optical arrangements have
+a magnifying power of 100, then the movement of the shadow upon the screen
+will be 1/40th of an inch, which will be ample to cover the aperture.
+
+The aluminium rod R, minus the counter-weight, can be made to weigh not
+more than 12 grains. It is necessary to enclose the optical parts in a
+light tight box, indicated by the dotted lines in Fig. 43, in order to
+prevent any extraneous light from reaching the film.
+
+_The Contact Breaker._--The contact breaker (L, Fig. 35), as will be seen
+from Fig. 44, consists of an electro-magnet N, the windings of which are
+connected with the battery B and the polarised relay K. The armature which
+is supported by the spring G carries a contact arm A, which in its normal
+position makes permanent contact with the contact screw T, and completes
+the circuit between the relay K and the telephone relay U (Fig. 35). As
+soon as the transmitter sends out the first signal, the magnified current
+from the telephone relay actuates the relay K, which in turn completes the
+circuit of the contact breaker. Directly the armature M has been attracted,
+the contact with T is broken, and A makes fresh contact with the screw H,
+by means of the spring Z {88} fastened to the underside of A. The armature,
+once it has been attracted, is held in permanent contact with H by the
+catch S, independent of the magnets N. As soon as contact is made with H,
+the clutch (F, Fig. 35) circuit is completed, and the circuit of the relay
+K is broken. When the circuit of the clutch F is broken by means of the
+circuit breaker C on the machine (Fig. 36), the stop S is pulled back by
+hand, allowing the contact arm A to rise, and again make fresh contact with
+the contact screw T.
+
+[Illustration: FIG. 44.]
+
+DRIVING APPARATUS
+
+_The Friction Brake._--This consists of a steel disc A, Fig. 45, about
+2-1/2 inches diameter and 3/8 inch or 1/2 inch wide on the face, secured to
+the main shaft of the driving motor. The arm H, pivoted at C, carries at
+one end the curved block B, which is faced with a pad of tow F. The other
+extremity is pivoted to the steel rod P, which slides {89} [Illustration]
+in holes bored in the standards J. One end of the rod P is screwed with a
+fine thread, about 75 to the inch, and is fitted with a regulating wheel T,
+by means of which the block B can be made to press upon the disc A with any
+required degree of pressure. A fairly stiff steel spring R is placed upon
+the rod P, between one standard J and the collar N. As the speed of the
+driving motor is slightly in excess of that required by the machine, the
+block B, by means of the wheel, is made to press upon the disc A, setting
+up friction which reduces the motor speed until the isochroniser indicates
+that the correct working speed has been attained.
+
+_The Clutch_.--The details of this will be seen from Figs. 46 and 47. It
+consists of a steel shaft coned at both ends running between two
+countersunk bearings, one of which is adjustable. This shaft carries the
+two portions of the clutch A and B, the portion A being a fixture on the
+shaft, and the portion B running free upon it. The portion B is a gun-metal
+casting bored to run accurately upon the steel shaft. A soft iron annular
+ring is fastened to the face.
+
+[Illustration: FIG. 46.
+
+E, spindle; R, bobbins; P, iron cores; D, copper rings; T, brushes; N, back
+plate; V, front plate; J, gearing; S, spring; H, collar; Z, iron ring; F,
+fixed bearing; C, insulating bush.]
+
+The portion A consists of a gun-metal casting {90} [Illustration] bored a
+tight fit for the shaft E, secured by means of a set screw. The two magnet
+cores P are screwed into the front plate V, which is also of gun-metal, and
+after the bobbins R have been slipped on, the shanks of the cores are
+passed through holes drilled in the flange N of the main casting and held
+in place with nuts. The faces of both A and B must be turned perfectly
+square with the shaft, so that they run accurately together. The portion B
+is {91} kept in contact with A by means of a spring S, the pressure being
+regulated by the collar H. Current is taken to the magnets by means of the
+two insulated copper rings D mounted upon the body of A. The gear-wheels on
+both portions have teeth of very fine pitch, the number of teeth on each
+being regulated by the speed of the driving motor and the required machine
+speed. Connection with the circuit breaker L and the battery B^2 is made
+with the collecting rings D by the brushes T. The complete connections are
+given in the diagram Fig. 51.
+
+_The Isochroniser._--This is a device for ensuring the correct speed
+regulation of the driving motors, and is shown in detail in Fig. 48. It
+comprises two portions, one portion being rotated at a definite speed by
+electrical means, and the other portion rotated by the driving motor.
+
+The main portion consists of a metal tube N, bushed at both ends, the
+bottom end of the tube being arranged to work on ball-bearings. An ebonite
+bush C carries three copper rings T, T^1, T^2, and the brushes R, R^1, R^2
+are in electrical contact with them. The ebonite plate J, 3-1/2 inches
+diameter, is secured to the top end of N, and carries a contact piece Q,
+shown separate at E. As will be seen this is a block of ebonite with three
+contacts arranged on the top surface. The middle contact P is 1/64th of an
+inch wide, and the contacts P^1 {92} and P^2 are placed on either side at a
+distance of 1/16 inch; the contact strips P^1, P^2 carry the brass pins D,
+which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting
+wire is carried from the contact P to the copper ring T, another from P^1
+to T^1, and one from P^2 to T^2.
+
+[Illustration: FIG. 48.
+
+N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T^1, T^2,
+copper rings; C, insulating block; R, R^1, R^2, brushes; J, ebonite disc;
+Q, contact block; D, metal pins; O, pulley, P, P^1, P^2, contact plates; K,
+needle; Z, spring; W, steel rod; E, countersunk bearing.]
+
+The bushes S are bored a running fit for the steel rod W (shown separate at
+A), which is coned at both ends, and runs between two countersunk bearings,
+the bottom bearing E being fixed while {93} the top bearing (not shown) is
+adjustable. A needle K is fastened near the end of the rod W, and attached
+to this needle is the spring Z, which presses lightly but firmly upon the
+contact block Q. To provide a level surface for Z to work over, the spaces
+between the contact pieces are filled in with an insulating material, and
+the whole surface finished off perfectly smooth. The spring Z is 1/8 inch
+wide for portion of its length, but at the point where it presses upon Q it
+is reduced in width to 1/64th of an inch (see Fig. 48). The driving
+arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a
+grooved pulley, is run in bearings parallel with the shaft W, and is
+connected by suitable gearing to the shaft of the driving motor, so that
+the needle K makes one revolution in about 2-1/2 seconds. A belt passing
+over the pulleys connects the two shafts, and the tension of the belt is
+regulated by means of an adjustable jockey pulley.
+
+The tube N, carrying the disc J, must be rotated at a fixed speed, and this
+is accomplished in the following manner. An ordinary electric clock impulse
+dial, actuated from a master clock, is connected by suitable gearing H, so
+that the tube N makes exactly one revolution in 2 seconds; it being
+possible to adjust an electric clock of the "Synchronome" type, so that it
+only gains or loses about 1 second in 24 hours, and this provides {94} an
+accuracy sufficient for all practical purposes. The connections are given
+in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that
+a connecting wire is run from the steel spindle W to one terminal each of
+the lamps L, L^1, L^2, and from the other terminal of the lamps to one
+terminal of the batteries J, the battery comprising a set of three 4-volt
+accumulators. The other terminals of the batteries are joined one to each
+of the brushes R, R^1, R^2.
+
+[Illustration: FIG. 49.]
+
+[Illustration: FIG. 50.
+
+M, terminals for connecting to electric clock; L, white lamp; L^1, blue
+lamp; L^2, red lamp.]
+
+The lamps are coloured, the lamp L being white, and the lamps L^1 and L^2
+blue and red respectively, and care must be taken in connecting up that
+when the needle K makes contact with the stud P the white lamp L is in
+circuit. When the machines are working, the operator, by means of the brake
+(already described), reduces the speed of the driving motor until the
+needle K travels in unison with the disc J, making permanent contact with P
+on the contact {95} block Q, which is evidenced by the lamp L remaining
+alight. If, however, the needle travels faster than the disc J, contact
+with P is broken and fresh contact is made with P^2, the lamp L is
+extinguished and the red lamp L^2 lights up, and remains alight until the
+operator reduces the speed. Similarly, too, if the needle travels slower
+than J, contact is made with P^1, and the circuit of the blue lamp L^1 is
+completed. When the speed is either above or below the normal, the needle K
+engages with one or the other of the pins D, and as the tension of the
+driving belt is only such as is required to drive the needle, the belt
+slips on the pulleys until the normal speed is regained.
+
+METHOD OF WORKING
+
+The clockwork motor M, Fig. 51, should be capable of running for several
+hours with one winding, and powerful enough to take up the work of driving
+the machine without any appreciable effort. The main spindle of the motor
+is so arranged that it makes one revolution in two minutes, and the
+reduction in speed between the motor shaft and the shaft to which the
+coupling A is attached is 30:1. The metal line print having been wrapped
+round the drum of the machine, the stylus is put into position, at the edge
+of the lap, and with the needle resting about half-way on {96} the margin
+of the bare foil left at the commencing edge of the print. Now, when the
+two stations are in perfect readiness for work, the motors are started and
+the speed adjusted; the speed of the machine being just under one
+revolution in four seconds.
+
+[Illustration: FIG. 51.
+
+M, clockwork motor; S, isochroniser; E, friction break; T, brushes; F,
+electric clutch; X, gearing; D, D^1, switches; A, flexible coupling; K,
+polarised relay; L, circuit breaker; B_1, B_2, B_3, batteries; P, electric
+clock; W, terminals for connection to telephone relay; H, terminals for
+connection to terminals J, on transmitting machine.]
+
+The switch D is then closed, and the arm of the switch D^1 placed on the
+contact stud (1), at the transmitting station only. As soon as the switches
+are closed the clutch F comes into action, and the transmitting machine
+begins to revolve. When the whole of the line print wrapped round the drum
+of the machine has passed under the stylus, the end of the shaft D, Fig.
+36, engages {97} with the spring _m_, breaking the clutch circuit and
+allowing the motor to run free. As soon as the machine stops, the switch D
+is opened and the machine run back to its starting position by hand.
+
+At the receiving station the switch D is also closed, and the arm of the
+switch D^1 placed on the contact stud (2). The closing of these switches
+does not bring the clutch F into operation until current from the telephone
+relay U connected to the wireless receiving apparatus works the sensitive
+polarised relay K, which in turn completes the circuit of the
+circuit-breaker L. When the armature of L is attracted, the circuit of the
+relay K is broken, the circuit of the clutch F is completed, and the
+machine starts revolving.
+
+[Illustration: FIG. 52.]
+
+The current from the relay U, due to the transmitting stylus passing over
+_one_ contact strip on the metal print, is too brief to actuate the heavier
+mechanism of the relay K, hence the need of the margin of bare foil at the
+commencing edge of the metal print, so that a practically continuous
+current will flow to the relay K until the armature is attracted. As,
+however, the relay is not actuated at the receipt of the first signal, and
+as it is necessary for the machine to start recording at a certain point on
+the film, viz. {98} at the edge of the lap--the reason for this was given
+in Chapter IV.--the starting position of the receiving drum will be similar
+to that given in the diagram Fig. 52, where X indicates the lap of the
+photographic film, and the arrow the direction of rotation.
+
+It is, of course, obvious that a somewhat similar adjustment must be made
+with regard to the position of the stylus on the metal print at the
+transmitting machine.
+
+In the present system, as in almost every photographic method of receiving
+that has been described, the Nernst lamp is invariably mentioned as the
+source of illumination. Since the advent of the high-voltage metal-filament
+lamps the Nernst lamp has fallen somewhat into disuse for commercial
+purposes, but it possesses certain characteristics that render it eminently
+suitable for the purpose under discussion.
+
+The main principle of this type of lamp depends upon the discovery made by
+Professor Nernst in 1898, after whom the lamp is named, that filaments of
+certain earthy bodies when raised to a red heat became conductive
+sufficiently well to pass a current which raised it to a white heat, and
+furthermore that the glowing filament emitted a brighter light for a given
+amount of current than carbon filaments.
+
+[Illustration: FIG. 52a.]
+
+Nernst lamps are made in two sizes, the larger {99} being intended for the
+same work as usually done by arc lamps, and the smaller to replace
+incandescent lamps; the smaller type being made to fit into the ordinary
+bayonet lampholders. The principal parts of a Nernst lamp consist of the
+filament, the heater, the automatic cut-out, and the resistance, and their
+arrangement in the smaller type of lamp is given in the diagram, Fig. 52a.
+The current enters at the positive terminal, passes through the heater M,
+and out through the negative terminal. The filament B, which consists of a
+short length of an infusible earth made of the oxides of several rare
+minerals, of which zirconia is one, is a non-conductor at first, but
+becomes a conductor upon being raised to a high temperature by means of the
+heater M. As soon as the filament becomes conductive the current then
+passes through the automatic cut-out H, and the armature D is attracted,
+thus breaking the heater circuit. The current then flows from the positive
+terminal {100} [Illustration] through the cut-out H, resistance J, and
+filament B, and from thence out of the lamp. Since the resistance of the
+filament decreases the hotter it gets, it is necessary to insert a
+ballasting resistance in series with it which has the opposite property of
+increasing its resistance as it gets hotter, to prevent the filament taking
+too much current and destroying itself. Such a resistance, J, consists of a
+filament of fine iron wire, which, to prevent oxidation from exposure to
+the air, is enclosed in a glass bulb filled with hydrogen gas. Fig. 52_b_
+shows the form of ballast resistance used in the small and large type of
+lamp respectively.
+
+Either direct or alternating current can be used with these lamps, and with
+direct current the polarity must be strictly observed, and that the
+positive wire is connected to the positive and the {101} negative wire to
+the negative terminal. With the smaller type of lamp once it has been
+correctly placed in its holder it is essential that it should not be
+turned, as a change in the direction of the current will rapidly destroy
+the filament.
+
+[Illustration: FIG. 52c.]
+
+The arrangement of the larger type of Nernst lamp can be readily seen from
+the drawing, Fig. 52c.
+
+Care must be taken to see that the voltage required by the burner and
+resistance equals the voltage of the supply circuit, and that only parts of
+the same amperage are used together on the same lamp. No advantage is
+obtained by over-running a Nernst lamp, this only shortening its life
+without increasing the light. Under normal conditions the average life of
+the burner is about 700 hours.
+
+The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75
+watts per c.p. The light given is remarkably steady, and the lamps are
+adaptable for all voltages from 100 to 300. In one of the large type of
+lamps for use on a 235-volt {102} circuit the burner takes 0.5 ampere at
+215 volts, and the resistance 0.5 ampere at 20 volts, while one of the
+smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts
+and 0.25 ampere at 20 volts for the burner and resistance respectively. The
+burner and heater are very fragile, and should never be handled except by
+the porcelain plate to which they are attached. The lamps burn in air and
+emit a brilliant white light of high actinic power, the intrinsic
+brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with
+1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon
+filament lamps.
+
+The chief advantage of the Nernst lamp from a photographic point of view
+lies in the fact that it produces abundantly the blue and violet rays which
+have the greatest chemical effect upon a photographic plate or film. These
+rays are known as chemical or actinic rays, and are only slightly produced
+in some types of incandescent electric lamps. Carbon-filament lamps are
+very poor in this respect.
+
+Because a light is visually brilliant it must by no means be assumed that
+it is the best to use for purposes of photography, and this is a point over
+which many photographers stumble when using artificial light. Many sources
+of light, while excellent for illumination, have very low actinic powers,
+while others may have low illuminating but high {103} actinic powers. A
+lamp giving a light yellowish in colour has usually low actinic power,
+while all those lamps giving a soft white light are generally found to be
+highly actinic.
+
+In addition to the actinic value of the source of illumination, the
+photographic film used must be very carefully chosen, as the chemical
+inertia of the sensitised film plays an important part in the successful
+reproduction of the picture, and also, to a certain extent, affects the
+speed of transmission. The length of exposure, the amount of light admitted
+to the film, and the characteristics of the film itself, are all factors
+which have a decided bearing upon the quality of the results obtained, and
+the film found to be most suitable in one case will perhaps give very
+unsatisfactory results in another.
+
+In photo-telegraphy the length of exposure is determined by the time taken
+by the transmitting stylus to trace over a conducting strip on the metal
+print, and this time, of course, varies with the density of the image and
+also with the speed of transmission.
+
+The film in ordinary photography is chosen with regard to the subject and
+the existing light conditions, and the amount of light admitted to the film
+and the length of exposure are regulated accordingly. No such latitude is,
+however, possible in photo-telegraphy. With each set of apparatus {104} the
+various factors, such as the light value, the amount of light admitted to
+the film, and the length of exposure, will be practically fixed quantities,
+and the film that will give the most satisfactory results under these fixed
+conditions can only be found by the rough-and-ready method of "trial and
+error."
+
+The films in common use are manufactured in four qualities, namely,
+ordinary, studio, rapid, and extra rapid. These terms should really relate
+to the light sensitiveness of the film (or, as it is technically termed,
+the speed), but at the best they are a rough and very unsatisfactory guide,
+for the reason that some unscrupulous makers, purely for business purposes,
+do not hesitate to label their films and plates as slow, rapid, etc.,
+without troubling to make any tests for correct classification.
+
+The speed of photographic films and plates is generally indicated by a
+number, and the system of standardisation adopted by the majority of makers
+in this country is that originated by Messrs. Hurter & Driffield,
+abbreviated H. & D. In their system the speed of the film and the exposure
+varies in geometrical proportion, a film marked H. & D. 50 requiring double
+the exposure of one marked H. & D. 100. The highest number always denotes
+the highest speed, and the exposure varies inversely with the speed.
+
+Besides the Hurter & Driffield method of {105} obtaining the speed numbers
+of plates and films adopted by a large number of makers in this country,
+there are also two standard English systems known as the W.P. No. (Watkin's
+power number) and Wynne F. No., both of which are used to a fair extent.
+
+The "Actinograph" number or speed number of a plate in the H. & D. system
+is found by dividing 34 by a number known as the Inertia, the Inertia,
+which is a measure of the insensitiveness of the plate, being determined
+according to the directions laid down by Hurter & Driffield--that is, by
+using pyro-soda developer and the straight portion only of the density
+curve. If, for instance, the Inertia was found to be one-fifth, then the
+speed number would be 34 / 1/5 = 170, and the plate is H. & D. 170. The
+W.P. No. is found by dividing 50 by the Inertia. Thus 50 / 1/5 = 250, and
+the plate is W.P. 250, but for all practical purposes the W.P. No. can be
+taken as one and a half times H. & D. The Wynne F. numbers may be found by
+multiplying the square root of the Watkins number by 6.4. Thus
+
+  [sqrt]250 = 15.81, and 15.81 x 6.4 = W.F. 101.
+
+For those photographers who are in the habit of using an actinometer giving
+the plate speeds in H. & D. numbers, the following table, taken from the
+_Photographer's Daily Companion_, is given, {106} which shows at a glance
+the relative speed numbers for the various systems. The Watkins and Wynne
+numbers only hold good, however, when the inertia has been found by the H.
+& D. method.
+
+TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS
+
+  ------------------------------------------------------
+  |H. & D.|W.P. No.|W.F. No.||H. & D.|W.P. No.|W.F. No.|
+  --------+--------+-----------------+--------+---------
+  |   10  |   15   |   24   ||  220  |  323   |  114   |
+  |   20  |   30   |   28   ||  240  |  352   |  120   |
+  |   40  |   60   |   49   ||  260  |  382   |  124   |
+  |   80  |  120   |   69   ||  280  |  412   |  129   |
+  |  100  |  147   |   77   ||  300  |  441   |  134   |
+  |  120  |  176   |   84   ||  320  |  470   |  138   |
+  |  140  |  206   |   91   ||  340  |  500   |  142   |
+  |  160  |  235   |  103   ||  380  |  558   |  150   |
+  |  200  |  294   |  109   ||  400  |  588   |  154   |
+  ------------------------------------------------------
+
+Although theoretically the higher the speed of the film the less the
+duration of exposure required, there is a practical limit, as besides the
+intensity and actinic value of the light admitted to the film a definite
+time is necessary for it to overcome the chemical inertia of the sensitised
+coating and produce a useful effect. With every make of film it is possible
+to give so short an exposure that although light does fall upon the film it
+does no work at all--in other words, we can say that for every film there
+is a minimum amount of light action, and anything below this is of no use.
+The exposure that enables the smallest amount of light action to take place
+is termed the limit of the smallest useful exposure. {107}
+
+There is also a maximum exposure in which the light affects practically all
+the silver in the film, and any increased light action has no increased
+effect. This is the limit of the greatest useful exposure.
+
+In photo-telegraphy the duration of exposure, as already pointed out, is
+determined by certain conditions connected with the transmitting apparatus,
+and with conditions similar to those mentioned on page 75 the length of
+exposure will vary roughly from 1-50th to 1-150th of a second.
+
+The most suitable film to use for purposes of photo-telegraphy is one
+having a fairly slow speed in which the range of exposure required comes
+well within the limits of the film. There is no advantage in using a film
+having a speed of, say, H. & D. 300 if good results can be obtained from
+one with a speed of, say, H. & D. 200, as the use of the higher speed
+increases the risk of overexposure. With the high-speeded films the
+difficulties of development are also greatly increased, there being more
+latitude in both exposure and development with the slower speeds, and
+consequently a better chance of obtaining a good negative.
+
+Another point, often puzzling to the beginner, and which increases the
+difficulty of choosing a suitable make of film, is that, although one make
+of film marked H. & D. 100 will give good results, another make, also
+marked H. & D. 100, will give {108} very poor results. This is owing, not
+to a poor quality film, as many suppose, but to the almost insurmountable
+difficulty of makers being able to employ exactly the same standard of
+light for testing purposes, so that although various makes may all be
+standardised by the H. & D. method, films bearing the same speed numbers
+may vary in their actual speed by as much as 30 to 50 per cent.
+
+       *       *       *       *       *
+
+
+{109}
+
+APPENDIX A
+
+SELENIUM CELLS
+
+Selenium is a non-metallic element, and was first discovered by Berzelius
+in 1817, in the deposit from sulphuric acid chambers, which still continues
+the source from which it is obtained for commercial purposes, although it
+is found to a small extent in native sulphur. Its at. wt. is 79.2, and its
+sp. gr. 4.8. Symbol, Se.
+
+In its natural state selenium is practically a non-conductor of
+electricity, its resistance being forty thousand million times greater than
+copper. Its practical value lies in the property which it possesses, that
+when in a prepared condition it is capable of varying its electrical
+resistance according to the amount of light to which it is exposed, the
+resistance decreasing as the light increases.
+
+Selenium is prepared by heating it to a temperature of 120deg C., keeping
+it there for some hours, and allowing it to cool slowly, when it assumes a
+crystalline form and changes from a bluish grey to a dull slate colour. A
+selenium cell in its simplest form consists merely of some prepared
+selenium placed between two or more metal electrodes, the selenium acting
+as a high resistance conductor between them. The form given by Bell and
+Tainter to the cells used in their experiments is given in Figs. 53 and
+53a. It consists of a number of rectangular brass plates P, P', separated
+by very thin sheets of mica M, the mica sheets being slightly narrower than
+the brass plates, the whole being clamped together in the frame F by the
+two bolts B. {110} By means of a sand-bath the cell is raised to the
+desired temperature, and selenium is rubbed over the surface, which melts
+and fills the small spaces between the brass plates. All the plates P are
+connected together to form one terminal, and the plates P' to form the
+other. By using very thin mica sheets, and a large number of elements, a
+very narrow transverse section of selenium, together with a large active
+surface, can be obtained.
+
+The cell used for commercial purposes is usually constructed as follows. A
+small rectangular piece of porcelain, slate, mica, or other insulator, is
+wound with many turns of fine platinum wire. The wire is wound double, as
+shown in Fig. 54, the spaces between the turns being filled with prepared
+selenium. A thin glass cover is sometimes placed over the cell to protect
+the surface from injury.
+
+[Illustration: FIG. 53.
+
+P, P', plates; M, mica; S, selenium.]
+
+[Illustration: FIG. 53a.]
+
+A strong light falling upon a cell lowers its resistance, and _vice versa_,
+the resistance of a cell being at its highest when unexposed to light; the
+light is apparently absorbed and made to do work by varying the electrical
+resistance of the selenium. Selenium cells vary very considerably as
+regards their quality as well as in their electrical resistance, it being
+possible to obtain cells of the same size for any resistance between 10 and
+1,000,000 ohms, and also, a cell may remain in good working condition for
+several months, while another will become useless in as many weeks.
+
+The ability of a cell to respond to very rapid changes in the illumination
+to which it is exposed is determined largely upon its inertia, it being
+taken as a general rule {111} that the higher the resistance of a cell the
+less the inertia, and _vice versa_, and also, that the higher the
+resistance the greater the ratio of sensitiveness. Inertia plays an
+important part in the working of a cell, slightly opposing the drop in
+resistance when illuminated, and opposing to a [Illustration] much greater
+degree the return to normal for no-illumination. The effects of inertia or
+"lag," as it is termed, can readily be seen by reference to Fig. 55. It
+will be noticed that the current value rapidly increases when the cell is
+first illuminated, but if after a short time _t_ the light is cut off, the
+current value, instead of returning at once to normal for no-illumination,
+only partially rises owing to the interference of the inertia, and some
+time elapses before the cell returns to its normal condition; the time
+varying from a few seconds to several minutes, depending upon the
+characteristics of the cell and the amount of light to which it is exposed.
+An actual curve is given in Fig. 55a. The inertia or "lag" of a cell
+produces upon an intermittent current an effect similar to that produced by
+the capacity [Illustration] of a line, as was noted in Chapter I.,
+preventing the incoming signals from being recorded separately, and
+distinctly. To obtain the best results in photo-telegraphy, the resistance
+of a cell should only be decreased to an extent sufficient to pass the
+current required to operate the recording apparatus, and the illumination
+should be regulated so that this condition of the cell takes place.
+
+The comparative slowness of selenium in responding to {112} any great
+changes in the illumination offers a serious difficulty to its use in
+photo-telegraphy, but various methods have been devised whereby the effects
+of inertia can be counteracted. In the system of De' Bernochi (see Chapter
+I.) the changes in the illumination are neither very rapid nor very great,
+and the inertia effects would therefore be very slight; but in any
+photo-telegraphic system in which a metal line print is used for
+transmitting, where the source of illumination is constant and the
+resistance of the cell is required to drop to a definite value and return
+to normal instantly, many times in succession, the inertia effects are very
+pronounced. The most successful method of counteracting the inertia is that
+adopted by Professor Korn of always keeping the cell sufficiently
+illuminated to overcome it, so that any additional light acts very rapidly.
+Another method worked out and patented by Professor Korn, and known as the
+"compensating cell" method, gives a practically dead beat action, the
+resistance returning to its normal condition as soon as the illumination
+ceases. The arrangement is given in the diagram Fig. 56.
+
+[Illustration: FIG. 55a.]
+
+Light from the transmitting or receiving apparatus, as the case may be,
+falls upon the selenium cell S^1, which is {113} placed on one arm of a
+Wheatstone bridge, a second cell S^2 being placed on the opposite arm. The
+selenium cell S^1 should have great sensitiveness and small inertia, the
+compensating cell S^2 having proportionally small sensitiveness and large
+inertia. Two batteries B, B', of about 100 volts, are connected as shown, B
+being provided with a compensating variable resistance W; W' is also a
+regulating resistance. When no light is falling upon the cell S^1, light
+from L is prevented from reaching the second cell S^2 by a small shutter
+which is fastened to the strings of the Einthoven galvanometer (described
+in Chapter III.), and the piece of apparatus C--relay or galvanometer as
+the case may be--remains in a normal condition. When, however, light falls
+upon the cell S^1, the balance of the bridge is upset, and light from L
+falls a fraction of a second later upon the second cell S^2, and the
+current flowing through C completes the circuit. Needless to say it is
+necessary that the two cells be well matched, as it is very easy to have
+over-compensation, in which case the current is brought below zero.
+
+[Illustration: FIG. 56.]
+
+It is also stated that by enclosing the cells in exhausted glass tubes,
+their inertia can be greatly reduced and their life considerably prolonged.
+The sensitiveness of a cell is the ratio between its resistance in the dark
+and its resistance when illuminated. The majority of cells have a ratio
+between 2:1 and 3:1, but Professor Korn has shown mathematically that by
+conforming to certain conditions regarding the construction the ratio of
+sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms
+can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only
+a short distance away; the resistance may be still {114} further decreased
+by continuing the illumination, but this produces a permanent defect in the
+cells termed "fatigue," the cells becoming very sluggish in their action
+and their sensitiveness gradually becoming less, the ratio between their
+resistance in the dark and their resistance when illuminated being reduced
+by as much as 30 per cent.
+
+Excessive illumination will also produce similar results. The inertia of a
+cell is practically unaffected by the wavelength of the light used, but the
+maximum sensitiveness of a cell is towards the yellow-orange portion of the
+spectrum.
+
+In addition to light, heat has also been found to vary the electrical
+resistance of selenium in a very remarkable manner. At 80deg C. selenium is
+a non-conductor, but up to 210deg C. the conductivity gradually increases,
+after which it again diminishes.
+
+       *       *       *       *       *
+
+
+{115}
+
+APPENDIX B
+
+PREPARING THE METAL PRINTS
+
+Electricians who desire to experiment in photo-telegraphy, but who have no
+knowledge of photography, may perhaps find the following detailed
+description of preparing the metal prints of some value. The would-be
+experimenter may feel somewhat alarmed at the amount of work entailed, but
+once the various operations are thoroughly grasped, and with a little
+patience and practice, no very great difficulty should be experienced. The
+simpler photographic operations, such as developing, fixing, etc., cannot
+be described here, and the beginner is advised to study a good text-book on
+the subject.
+
+The method to be given of preparing the photographs is practically the only
+one available for wireless transmission, and although the manner given of
+preparing is perhaps not strictly professional, having been modified in
+order to meet the requirements of the ordinary amateur experimenter, the
+results obtained will be found perfectly satisfactory.
+
+As will have been gathered from Chapter II., the camera used for copying
+has to have a single line screen placed a certain distance in front of the
+photographic plate, and the object of this screen is to break the image up
+into parallel bands, each band varying in width according to the density of
+the photograph from which it has been prepared. Thus a white portion of the
+photograph would consist of very narrow lines wide apart, while a dark
+portion would be made up of wide lines close together; a black part would
+appear solid and show no lines at all. It is, of course, obvious {116} that
+the lines on the negative cannot be wider apart, centre to centre, than the
+lines of the screen. A good screen distance has been found to be 1 to 64,
+_i.e._ the diameter of the stop is 1/64th of the camera extension, and the
+distance of the screen lines from the photographic plate is 64 times the
+size of the screen opening. The following table shows what this distance is
+for the screen most likely to be used. The line screens used consist of
+glass plates upon which a number of lines are accurately ruled, the width
+of the lines and the spaces between being equal; the lines are filled in
+with an opaque substance. These ruled screens are very expensive, and are
+only made to order,[10] a screen half-plate size costing from 21s. to 27s.
+6d. An efficient substitute for a ruled screen can be made by taking a
+rather large sheet of Bristol board and ruling lines across in pure black
+drawing ink, the width of the lines and the spaces between being 1/12th of
+an inch respectively. A photograph must be taken of this card, the
+reduction in size determining the number of lines to the inch. A card 20 x
+15 inches, with 12 lines to the inch, would, if reduced to 5 x 4 inches,
+make a screen having 48 lines to the inch. Preparing the board is rather a
+tedious operation, but the line negative produced will be found to give
+results almost as good as those obtained from a purchased screen.
+
+DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.
+
+  --------------------------------------------------------------
+  |Screen ruling  |Actual space| Distance of |In 1/32|In milli-|
+  |lines per inch.| in inches. |screen ruling| inches|  metres.|
+  |               |            |  in inches. |       |         |
+  |---------------+------------+-------------+-------+---------|
+  |       35      |    1/70    |      .91    |  28.8 |   21.8  |
+  |       50      |    1/100   |      .64    |  20.5 |   16.2  |
+  |       65      |    1/130   |      .49    |  15.7 |   12.4  |
+  --------------------------------------------------------------
+
+As it is impossible for many to have the use of professional apparatus
+designed for this particular kind of work, {117} the fixing of the screen
+into an ordinary camera must be left to the ingenuity of the worker. A
+half-plate back focussing camera will be found suitable for general
+experimental work, but if this is not available, a large box camera can be
+pressed into service.
+
+[Illustration: FIG. 57.]
+
+The writer has never seen a half-plate box camera, but one taking a 5 x 4
+inch plate can be obtained second-hand very cheaply. It is a comparatively
+simple matter to fix the line screen into a camera of this description, the
+drawings Figs. 57 and 58 showing the method adopted by the writer. The two
+clips D, made from fairly stout brass about 1/2 inch wide, are bent to the
+shape shown (an enlarged section is given at C) and soldered at the top and
+bottom of one of the metal sheaths provided for holding the plates. The
+distance between the front of the photographic plate (the film side) and
+the back of the line screen (also the film side), indicated by the arrow at
+A, is determined by the number of lines on the screen. As will be seen from
+the table given, the distance for a screen having 50 lines to the inch will
+be 41/64ths of an inch.
+
+[Illustration: FIG. 58.
+
+M, sheath; P, photographic plate; D, clips; S, line screen.]
+
+In all probability there will be enough clearance between the top of the
+sheath and the top of the camera to allow for the thickness of the clip,
+but if not, a shallow groove a little wider than the clip should be
+carefully cut in the top of the camera, so that it will slide in easily.
+The screen should be placed between the clips, the film side on the {118}
+inside, _i.e._ facing the photographic plate. As with a box camera the
+extension is a fixture, the size of stop to be used is a fixture also. The
+extension of a camera (this term really applies to a bellows camera) is
+measured from the front of the photographic plate to the diaphragm, and if
+this distance in our camera is 8 inches, then the diameter of the stop to
+give the best results would be 1/64th of this, or 1/8th inch. Although for
+all ordinary experimental work the lens fitted to the camera will be
+suitable, the best type of lens for process work of all kinds is the
+"Anastigmat."
+
+The picture or photograph from which it is desired to make a print should
+be fastened out perfectly flat upon a board with drawing pins, and if a
+copying stand is not available it must be placed upright in some convenient
+position. The diagram Fig. 59 gives the disposition of the apparatus
+required for copying. A simple and inexpensive copying stand is shown in
+Fig. 60. The blackboard A should be about 30 inches square, and must be
+fastened perfectly upright upon the base-board B. The stand C should be
+made so that it slides without any side play between the guides D, and
+should be of such a height that the lens of the camera comes exactly
+opposite the {119} [Illustration] [Illustration] centre of the board A. The
+camera, if of the box type, can be secured to the stand by means of a screw
+and wingnut, the screw being passed from the inside as shown. The beginner
+is advised to photograph only very bold and simple subjects, such as black
+and white drawings or enlargements. It is not safe to trust to the
+view-finders as to whether the whole of the picture is included on the
+plate, a piece of ground glass the same size as the plate sheaths, and used
+as a focussing screen, being much more reliable. It is a good plan to focus
+the camera for a number of different-sized pictures, marking the board A,
+and the {120} guides D, so that adjustment is afterwards a very simple
+matter.
+
+The make of plate used is also a great factor in getting a good negative,
+and Wratten Process Plates will be found excellent. As already mentioned,
+such subjects as the exposure and the development of the plate cannot be
+dealt with here, these subjects having been exhaustively treated in several
+text-books on photography. With an arc lamp the exposure is about twice as
+long as in daylight, but the exposure varies with the amount of light
+admitted to the plate, character of the source of light, and the
+sensitiveness of the plate used, etc. The writer has used acetylene gas
+lamps for this purpose with great success. The beginner is advised to use
+artificial light, as this can be kept perfectly even. With daylight,
+however, the light is constantly fluctuating, and this renders the use of
+an actinometer a necessity for correct exposure. After development, if the
+plate is required for immediate use, it can be quickly dried by soaking for
+a few minutes in methylated spirit.
+
+Having obtained a good negative, the next operation is to prepare what is
+known as a metal print. For this we shall require some stout tin-foil or
+lead-foil, about 12 or 15 square feet to the pound, and this should be cut
+into pieces of such a size that it allows a lap of 3/16 inch when wrapped
+round the drum of the transmitting machine. Obtain some good fish-glue and
+add a saturated solution of bichromate of potash in the proportion of 4
+parts of potash to 40 or 50 parts of glue. Pour a little of this glue into
+a shallow dish, lay a sheet of foil upon a flat board, and with a fairly
+stiff brush (a flat hog's-hair as wide as possible) proceed to coat the
+sheet of foil with a thin but perfectly even coating of glue. The thickness
+of the coating can only be found by trial, for if the coating is too thick
+a longer time will be required for printing; but it must not be thin enough
+to show interference colours. After the coating has been laid on, a soft
+brush, such as photographers use for dusting dry {121} plates, should be
+passed up and down, and across and across, with light, even strokes to
+remove any unevenness. A glue solution used by professional photo-engravers
+is as follows:
+
+  Fish-glue               12 oz.
+  Bichromate of Ammonia  3/4 oz.
+  Water                   18 to 24 oz.
+  Ammonia .880            30 minims.
+
+The bichromate should be dissolved in the water, and, when added to the
+glue, stir very thoroughly in order that complete mixing may take place.
+The coating may be done in a good light, not bright sunlight, but _it must
+be dried in the dark_, because, although insensitive while in a moist
+condition, it becomes sensitive immediately on desiccation. If allowed to
+dry in the light the whole coating will become insoluble, and for this
+reason the brushes used should be washed out as soon as they are finished
+with. The sheets will take about 15 minutes to dry in a perfectly dry room,
+but it is not advisable to prepare many sheets at once, as they will not
+keep for more than two or three days.
+
+The prepared negative must now be placed in an ordinary printing frame, and
+a print taken off upon one of the metal sheets in the same way as a print
+is taken off upon ordinary sensitised paper. In daylight the exposure
+varies from 5 to 20 minutes, but in artificial light various trials will
+have to be made in order to get the best results, the exposure varying with
+the amount of bichromate in the coating; the proportion of the bichromate
+to the glue should remain about 6 per cent. Light from a 25 ampere arc lamp
+for 2 to 5 minutes, at a distance of 18 inches, will generally suffice to
+"print" the impression on the metal sheets. The printing finished, the
+metal print should be laid upon a sheet of glass and held under a running
+stream of water. The washing is complete as soon as the unexposed parts of
+the glue coating have been entirely washed away leaving the bare metal, and
+this will take anything from 3 to 7 {122} minutes, depending upon the
+thickness of the film. As soon as it is dry the print is ready for use.
+
+As already mentioned, the negative from which the metal print is made
+requires that the lines be perfectly sharp and opaque, and the spaces
+between perfectly transparent. Ordinary dry plates are too rapid, a rather
+slow plate being required. Wratten Process Plates give excellent results,
+and the following is a good developer to use with them:
+
+  Glycin                15 grammes       1     oz.
+  Sulphite of Soda      40  ,,           2-1/2 ,,
+  Carbonate of Potash   80  ,,           5     ,,
+  Water               1000 c.c.         60     ,,
+
+This developer should be used for 6 minutes at a temperature of 50deg F.,
+3-1/2 minutes at 65deg, and 1-3/4 minutes at 80deg. It is best only used
+once. If an intensifier is required, the following formula will be found to
+give satisfactory results:
+
+  Bichloride of Mercury   1 oz.    60 grammes.
+  Hot Water              16 ,,   1000 c.c.
+
+Allow to cool, completely pour off from any crystals, and add:
+
+  Hydrochloric Acid      30 minims   4 c.c.
+
+Allow negative to bleach thoroughly, wash well in water, and blacken in 10
+per cent ammonia .880, or 5 per cent sodium sulphide.
+
+In preparing the negatives and metal prints the following points should be
+observed:
+
+A good negative should have the lines perfectly sharp and opaque; there
+should be no "fluff" between the lines even when they are close together.
+
+A properly exposed and developed negative should not require any reducing
+or intensifying.
+
+If the lamps used for illuminating the copying board are placed 2 feet
+away, and the exposure required is 5 minutes, the exposure, if the lamps
+are placed 4 feet away, will be {123} 20 minutes, as the amount of light
+which falls upon an object decreases as the inverse square of the distance.
+
+Get the coating on the foil as thin as possible, and err on the side of
+over-exposure, for if the coating is thick and has been under-exposed,
+excessive washing will dissolve the whole coating; for, unless
+insolubilisation has taken place right up to the metal base, the under
+parts will remain in a more or less soluble condition.
+
+On no account must the unexposed sheets be placed near a fire, otherwise
+they will be spoilt, the whole coating becoming insoluble; heat acting in
+the same manner as light.
+
+In washing, keep the print moving so that the stream of water does not fall
+continually in one place. It is best to hold the print so that the water
+runs off in the direction of the lines.
+
+To dry the prints after washing they can be laid out flat in a moderately
+warm oven, or before a stove, the heat of course not being sufficient to
+cause the coating to peel.
+
+To render the glue image more distinct the print should be immersed for a
+few seconds in an aniline dye solution, the glue taking up the colour
+readily. These dyes are soluble in either water or alcohol. A dye known as
+"magenta" is very good.
+
+The process of coating the metal sheets must be performed as quickly as
+possible (about 10 seconds), as owing to the peculiar nature of the
+bichromated glue it soon sets, and once this has taken place it is
+impossible to smooth down any unevenness.
+
+See that the negative and metal sheet make good contact while printing.
+
+If the glue solution does not adhere to the surface of the foil in a
+perfectly even film, but assumes a streaky appearance, a little liquid
+ammonia, or a weak solution of nitric acid, rubbed over the surface of the
+foil, which is afterwards gently scoured with precipitated chalk on a tuft
+of cotton {124} wool, will remove the grease which is the cause of the
+difficulty.
+
+A photograph of a picture prepared from a line negative is given in Fig.
+61. For a great many experiments, and in order to save time, trouble, and
+expense, sketches drawn upon stout lead-foil in an insulating ink will
+answer the purpose admirably, but if any exact work is to be done a single
+line print is of course absolutely necessary. The insulating ink can be
+prepared by dissolving shellac in methylated spirit, or ordinary gum can be
+used. A very fine brush should be used in place of a pen, as the gum will
+not flow freely from an ordinary nib unless greater pressure than the foil
+will safely stand be applied. A sketch prepared in this manner is shown in
+Fig. 62. A little aniline dye should be added to the gum to render it more
+visible, or a mixture of gum and liquid indian ink will be found suitable.
+
+[Illustration: FIG. 63.]
+
+With the copying arrangement already described it is only possible to
+employ it for reducing, it being necessary to employ a bellows camera with
+a back focussing attachment for purposes of enlarging, and this constitutes
+the chief drawback to the use of a fixed focus camera. By replacing the box
+camera with a focussing camera of the same size, we shall have a piece of
+apparatus capable of reducing or enlarging, only in this case the camera
+should be a fixture and the board, A, arranged to slide backwards and
+forwards instead.
+
+[Illustration: FIG. 61.
+
+Portions of photographs (full size) of single line screen, and single line
+print. Screen 40 lines to the inch.]
+
+[Illustration: FIG. 62.]
+
+{125} An extra improvement would be to rule the surface of the copying
+board, A, in a manner similar to that shown in the diagram, Fig. 63. The
+rulings should be marked off from the centre of the board, and should
+enclose parallelograms of the various plate sizes ranging from 3-1/4 x
+4-1/4 inches up to the full size of the board. By fastening the picture or
+photograph to be copied in the space on the board corresponding in size, we
+can ensure that it is in the correct position for the whole to be included
+on the photographic plate, providing, of course, that the centre of lens
+and board coincide.
+
+With regard to the lens required, the practice adhered to by most
+photographers is to use a lens having a focal length equal to the diagonal
+of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be
+used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 x 4 inch
+camera a 6-inch lens will be required. The following is a simple rule for
+finding the conjugate foci of a lens, and is useful in obtaining the
+distance from the lens to the photographic plate and the picture to be
+copied. Let us suppose that we wish to make a 1-1/2 times enlarged line
+negative from a 4-1/4 x 3-1/4 inch print. Add 1 to the number of times it
+is required to enlarge and multiply the result by the focal length of the
+lens in inches. In the present case this will be 1-1/2 + 1 = 2-1/2; and if
+a 6-inch lens is used, 2-1/2 x 6 = 15 inches will be the distance of the
+lens from the plate. Divide this number by the number of times it is
+desired to enlarge, and the distance of the lens from the picture to be
+copied is obtained; in this instance 15 / 1-1/2 = 10 inches. The same rule
+can be followed when it is required to reduce any given number of times,
+only in this case the greater number will represent the distance between
+the lens and the picture to be copied, and the lesser number the distance
+between the lens and the plate.
+
+In reducing, a 1/4-plate lens will be found to fully cover a 5 x 4 inch
+plate, providing the reduction is not greater than three to one.
+
+       *       *       *       *       *
+
+
+{126}
+
+APPENDIX C
+
+LENSES
+
+In this small volume it is not desirable, neither is it intended, to give
+an exhaustive treatment on the subject of lenses and their action, but as
+optics plays an important part in the transmission of photographs, both by
+wireless and over ordinary conductors, the following notes relating to a
+few necessary principles have been included as likely to prove of interest.
+
+Light always travels in straight lines when in a medium of uniform density,
+such as water, air, glass, etc., but on passing from one medium to another,
+such as from air to water, or air to glass, the direction of the light rays
+is changed, or, to use the correct term, _refracted_. This refraction of
+the rays of light only takes place when the incident rays are passed
+obliquely; if the incident rays are perpendicular to the surface separating
+the two media they are not refracted, but continue their course in a
+straight line.
+
+All liquid and solid bodies that are sufficiently transparent to allow
+light rays to pass through them possess the power of bending or refracting
+the rays, the degree of refraction, as already explained, depending upon
+the nature of the body.
+
+The law relating to refraction will perhaps be better understood by means
+of the following diagram. In Fig. 64 let the line AB represent the surface
+of a vessel of water. The line CD, which is perpendicular to the surface of
+the {127} water, is termed the _normal_, and a ray of light passed in this
+direction will continue in a straight line to the point E. If, however, the
+ray is passed in an oblique direction, such as ND, it will be seen that the
+ray is bent or refracted in the direction DM; if the ray of light is passed
+in any other oblique direction, such as JD, the refracted ray will be in
+the direction DK. The angle NDC is called the _angle of incidence_ and MDE
+the _angle of refraction_. If we measure accurately the line NC, we shall
+find that it is 1-1/3, or more exactly 1.336, times greater than the line
+EM. If we repeat this measurement with the lines JH and PK we shall find
+that the line JH also bears the proportion of 1.336 to the line PK. The
+line NC is called the _sine of the angle of incidence_ NDC, and EM the
+_sine of the angle of refraction_ MDE.
+
+[Illustration: FIG. 64.]
+
+Therefore in water the sine of the angle of incidence is to the sine of the
+angle of refraction as 1.336 is to 1, and this is true whatever the
+position of the incident ray with respect to the surface of the water. From
+this we say that _the sines of the angles of incidence and refraction have
+a constant proportion or ratio to one another_.
+
+The number 1.336 is termed the _refractive index_, or _coefficient_, or the
+_refractive power_ of water. The refractive power varies, however, with
+other fluids and solids, and a complete table will be found in any good
+work on optics.
+
+Glass is the substance most commonly used for refracting the rays of light
+in optical work, the glass being worked up into different forms according
+to the purpose for which it {128} is intended. Solids formed in this way
+are termed _lenses_. A lens can be defined as a transparent medium which,
+owing to the curvature of its surfaces, is capable of converging or
+diverging the rays of light passed through it. According to its curvature
+it is either spherical, cylindrical, elliptical, or parabolic. The lenses
+used in optics are always exclusively spherical, the glass used in their
+construction being either crown glass, which is free from lead, or flint
+glass, which contains lead and is more refractive than crown glass. The
+refractive power of crown glass is from 1.534 to 1.525, and of flint glass
+from 1.625 to 1.590. Spherical surfaces in combination with each other or
+with plane surfaces give rise to six different forms of lenses, sections of
+which are given in Fig. 65.
+
+[Illustration: FIG. 65.]
+
+All lenses can be divided into two classes, convex or converging, or
+concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses,
+being thicker at the middle than at the borders, and _d_, _e_, _f_, which
+are thinner at the middle, being diverging lenses. The lenses _e_ and _g_
+are also termed meniscus lenses, and _a_ represents a prism. The line XY is
+the axis or _normal_ of these lenses to which their plane surfaces are
+perpendicular.
+
+Let us first of all notice the action of a ray of light when passed through
+a prism. The prism, Fig. 66, is represented by the triangle BBB, and the
+incident ray by the line TA. {129} Where it enters the prism at A its
+direction is changed and it is bent or refracted towards the base of the
+prism, or towards the normal, this being always the case when light passes
+from a rare medium to a dense one, and where the light leaves the opposite
+face of the prism at D it is again refracted, but away from the normal in
+an opposite direction to the incident ray, since it is passing from a dense
+to a rare medium. The line DP is called the _emergent_ or refracted ray. If
+the eye is placed at T, and a bright object at P, the object is seen not at
+P, but at the point H, since the eye cannot follow the course taken by the
+refracted rays. In other words, objects viewed through a prism always
+appear deflected towards its summit.
+
+[Illustration: FIG. 66.]
+
+In considering the action of a lens we can regard any lens as being built
+up of a number of prisms with curved faces in contact. Such a lens is shown
+in Fig. 67, the light rays being refracted towards the base of the prisms
+or towards the normal, as already explained; while the top half of the lens
+will refract all the light downwards, the bottom half will act as a series
+of inverted prisms and refract all the light upwards.
+
+[Illustration: FIG. 67.]
+
+[Illustration: FIG. 68.]
+
+If a beam of parallel light--such as light from the sun--be passed through
+a double convex lens L, Fig. 68, we shall find that the rays have been
+refracted from their parallel course and brought together at a point F.
+This point F is {130} termed the principal focus of the lens, and its
+distance from the lens is known as the focal length of that lens. In a
+double and equally convex lens of glass the focal length is equal to the
+radius of the spherical surfaces of the lens. If the lens is a plano-convex
+the focal length is twice the radius of its spherical surfaces. If the lens
+is unequally convex the focal length is found by the following rule:
+multiply the two radii of its surfaces and divide twice that product by the
+sum of the two radii, and the quotient will {131} be the focal length
+required. Conversely, by placing a source of light at the point F the rays
+will be projected in a parallel beam the same diameter as the lens. If,
+however, instead of being parallel, the rays proceed from a point farther
+from the lens than the principal focus, as at A, Fig. 69, they are termed
+divergent rays, but they also will be brought to a focus at the other side
+of the lens at the point a. If the source of light A is moved nearer to the
+principal focus of the lens to a point A^1 the rays will come to a focus at
+the point _a_^1, and similarly when the light is at A^2 the rays will come
+to a focus at the point _a_^2. It can be found by direct experiment that
+the distance _fa_ increases in the same proportion as AF diminishes, and
+diminishes in the same proportion as AF increases. The relationship which
+exists between pairs of points in this manner is termed the _conjugate
+foci_ of a lens, and though every lens has only one principal focus, yet
+its conjugate foci are innumerable.
+
+[Illustration: FIG. 69.]
+
+The formation of an image of some distant object in its principal focus is
+one of the most useful properties of a convex lens, and it is this property
+that forms the basis of several well-known optical instruments, including
+the camera, telescope, microscope, etc.
+
+If we take an oblong wooden box, AA, and substitute a sheet of ground
+glass, C, for one end, and drill a small pinhole, H, in the centre of the
+other end opposite the {132} glass plate, we shall find that a tolerably
+good image of any object placed in front of the box will be formed upon the
+glass plate. The light rays from all points of the object, BD, Fig. 70,
+will pass straight through the hole H, and illuminate the ground glass
+screen at points immediately opposite them, forming a faint inverted image
+of the object BD. The purpose of the hole H is to prevent the rays from any
+one point of the object from falling upon any other point on the glass
+screen than the point immediately opposite to it, therefore the smaller we
+make H, the more distinct will be the image obtained. Reducing the size of
+H in order to produce a more distinct image has the effect of causing the
+image to become very faint, as the smaller the hole in H, the smaller the
+number of rays that can pass through from any point of the object. By
+enlarging the hole H gradually, the image will become more and more
+indistinct until such a size is reached that it disappears altogether.
+
+[Illustration: FIG. 70.]
+
+If in this enlarged hole we place a double convex lens, LL, Fig. 71, whose
+focal length suits the length of the box, the image produced will be
+brighter and more distinct than that formed by the aperture, H, since the
+rays which proceed from any point of the object will be brought by the lens
+to a focus on the glass screen, forming a bright {133} distinct image of
+the point from which they come. The image owes its increased distinctness
+to the fact that the rays from any one point of the object cannot interfere
+with the rays from any other point, and its increased brightness to the
+great number of rays that are collected by the lens from each point of the
+object and focussed in the corresponding point of the image. It will be
+evident from a study of Fig. 71 that the image formed by a convex lens must
+necessarily be inverted, since it is impossible for the rays from the end,
+M, of the object to be carried by refraction to the upper end of the image
+at _n_. The relative positions of the object and image when placed at
+different distances from the lens are exactly the same as the conjugate
+foci of light rays as shown in Fig. 69.
+
+[Illustration: FIG. 71.]
+
+The length of the image formed by a convex lens is to the length of the
+object as the distance of the image is to the distance of the object from
+the lens. For example, if a lens having a focal length of 12 inches is
+placed at a distance of 1000 feet from some object, then the size of the
+image will be to that of the object as 12 inches to 1000 feet, or 1000
+times smaller than the object; and if the length of the object is 500
+inches, then the length of the image will be the 1/1000th part of 500
+inches, or 1/2 inch. {134}
+
+The image formed by the convex lens in Fig. 71 is known as a _real image_,
+but in addition convex lenses possess the property of forming what are
+termed _virtual images_. The distinction can be expressed by saying, _real
+images are those formed by the refracted rays themselves, and virtual
+images those formed by their prolongations_. While a real image formed by a
+convex lens is always inverted and smaller than the object, the virtual
+image is always erect and larger than the object. The power possessed by
+convex lenses of forming virtual images is made use of in that useful but
+common piece of apparatus known as a reading or magnifying glass, by which
+objects placed within its focus are made larger or magnified when viewed
+through it; but in order to properly understand how objects seem to be
+brought nearer and apparently increased in size, we must first of all
+understand what is meant by the expression, _the apparent magnitude of
+objects_.
+
+[Illustration: FIG. 72.]
+
+The apparent magnitude of an object depends upon the angle which it
+subtends to the eye of the observer. The image at A, Fig. 72, presents a
+smaller angle to the eye than the angle presented by the object when moved
+to B, and the image therefore appears smaller. When the object is moved to
+either B or C, it is viewed under a much {135} greater angle, causing the
+image to appear much larger. If we take a watch or other small circular
+object and place it at A, which we will suppose is a distance of 50 yards,
+we shall find that it will be only visible as a circular object, and its
+apparent magnitude or the angle under which it is viewed is then stated to
+be very small. If the object is now moved to the point B, which is only 5
+feet from the eye, its apparent magnitude will be found to have increased
+to such an extent that we can distinguish not only its shape, but also some
+of the marking. When moved to within a few inches from the eye as at C, we
+see it under an angle so great that all the detail can be distinctly seen.
+By having brought the object nearer the eye, thus rendering all its parts
+clearly visible, we have actually magnified it, or made it appear larger,
+although its actual size remains exactly the same. When the distance
+between the object and the observer is known, the apparent magnitude of the
+object varies inversely as the distance from the observer.
+
+Let us suppose that we wish to produce an image of a tree situated at a
+distance of 5000 feet. At this distance the light rays from the tree will
+be nearly parallel, so that if a lens having a focal length of 5 feet is
+fastened in any convenient manner in the wall of a darkened room the image
+will be formed 5 feet behind the lens at its principal focus. If a screen
+of white cardboard be placed at this point we shall find that a small but
+inverted image of the tree will be focussed upon it. As the distance of the
+object is 5000 feet, and as the size of the received image is in proportion
+to this distance divided by the focal length of the lens, the image will be
+as 5000 / 5, or 1000 times smaller than the object.
+
+If now the eye is placed six inches behind the screen and the screen
+removed, so that we can view the small image distinctly in the air, we
+shall see it with an apparent magnitude as much greater than if the same
+small image were equally far off with the tree, as 6 inches is to 5000
+{136} feet, that is 10,000 times. Thus we see that although the image
+produced on the screen is 1000 times less than the tree from one cause, yet
+on account of it being brought near to the eye it is 10,000 times greater
+in apparent magnitude; therefore its apparent magnitude is increased as
+10,000 / 1000, or 10 times. This means that by means of the lens it has
+actually been magnified 10 times. This magnifying power of a lens is always
+equal to the focal length divided by the distance at which we see small
+objects most distinctly, viz. 6 inches, and in the present instance is 60 /
+6, or 10 times.
+
+When the image is received upon a screen the apparatus is called a _camera
+obscura_, but when the eye is used and sees the inverted image in the air,
+then the apparatus is termed a _telescope_.
+
+The image formed by a convex lens can be regarded as a new object, and if a
+second lens is placed behind it a second image will be formed in the same
+manner as if the first image were a real object. A succession of images can
+thus be formed by convex lenses, the last image being always treated as a
+fresh object, and being always an inverted image of the one before. From
+this it will be evident that additional magnifying power can be given to
+our telescope with one lens by bringing the image nearer the eye, and this
+is accomplished by placing a short focus lens between the image and the
+eye. By using a lens having a focal length of 1 inch, and such a lens will
+magnify 6 times, the total magnifying power of the two lenses will be 10 x
+6 = 60 times, or 10 times by the first lens and 6 times by the second. Such
+an instrument is known as a _compound or astronomical telescope_, and the
+first lens is called the object glass and the second lens the magnifying
+glass, or eye-piece.
+
+We are now in a position to understand how virtual images are formed, and
+the formation of a virtual image by means of a convex lens will be readily
+followed from a {137} study of Fig. 73. Let L represent a double convex
+lens, with an object, AB, placed between it and the point F, which is the
+principal focus of the lens. The rays from the object AB are refracted on
+passing through the lens, and again refracted on leaving the lens, so that
+an image of the object is formed at the eye, N. As it is impossible for the
+eye to follow the bent rays from the object, a virtual image is formed and
+is seen at A^1B^1, and is really a continuation of the emergent rays. The
+magnifying power of such a lens may be found by dividing 6 inches by the
+focal length of the lens, 6 inches being the distance at which we see small
+objects most distinctly. A lens having a focal length of 1/4 inch would
+magnify 24 times, and one with a focal length of 1/100th of an inch 600
+times, and so on. The magnifying power is greater as the lens is more
+convex and the object near to the principal focus. When a single lens is
+applied in this manner it is termed a _single microscope_, but when more
+than one lens is employed in order to increase the magnifying power, as in
+the telescope, then the apparatus is termed a _compound microscope_.
+
+[Illustration: FIG. 73.]
+
+Unlike a convex lens, which can form both real and virtual images, a
+concave lens can only produce a virtual image; and while the convex lens
+forms an image larger {138} than the object, the concave lens forms an
+image smaller than the object. Let L, Fig. 74, represent a double concave
+lens, and AB the object. The rays from AB on passing through the lens are
+refracted, and they diverge in the direction RRRR, as if they proceeded
+from the point F, which is the principal focus of the lens, and the
+prolongations of these divergent rays produce a virtual image, erect and
+smaller than the object, at A^1B^1. The principal focal distance of concave
+lenses is found by exactly the same rule as that given for convex lenses.
+
+[Illustration: FIG. 74.]
+
+Up to the present we have assumed that all the rays of light passed through
+a convex lens were brought to a focus at a point common to all the rays,
+but this is really only the case with a lens whose aperture does not exceed
+12deg. By aperture is meant the angle obtained by joining the edges of a
+lens with the principal focus. With lenses having a larger aperture the
+amount of refraction is greater at the edges than at the centre, and
+consequently the rays that pass through the edges of the lens are brought
+to a focus nearer the lens than the rays that pass through the centre.
+Since this defect arises from the spherical form of the lens it is termed
+_spherical aberration_, and in lenses that {139} are used for photographic
+purposes the aberration has to be very carefully corrected.
+
+The distortion of an image formed by a convex lens is shown by the diagram,
+Fig. 75. If we receive the image upon a sheet of white cardboard placed at
+A, we shall find that while the outside edges will be clear and distinct,
+the inside will be blurred, the reverse being the case when the cardboard
+is moved to the point B.
+
+[Illustration: FIG. 75.]
+
+[Illustration: FIG. 76.]
+
+[Illustration: FIG. 77.]
+
+Aberration is to a great extent minimised by giving to the lens a meniscus
+instead of a biconvex form, but as it is desirable to reduce the aberration
+to below once the {140} thickness of the lens, and as this cannot be done
+by a single lens, we must have recourse to two lenses put together. The
+thickness of a lens is the difference between its thickness at the middle
+and at the circumference. In a double convex lens with equal convexities
+the aberration is 1-67/100ths of its thickness. In a plano-convex lens with
+the plane side turned towards parallel rays the aberration is 4-1/2 times
+its thickness, but with the convex side turned towards parallel rays the
+aberration is only 1-17/100ths of its thickness.
+
+By making use of two plano-convex lenses placed together as at Fig. 76, the
+aberration will be one-fourth of that of a single lens, but the focal
+length of the lens, L^1, must be half as much again as that of L. If their
+focal lengths are equal the aberration will only be a little more than half
+reduced. Spherical aberration, however, may be entirely destroyed by
+combining a meniscus and double convex lens, as shown in Fig. 77, the
+convex side being turned to the eye when used as a lens, and to parallel
+rays when used as a burning glass or condenser.
+
+       *       *       *       *       *
+
+
+{141}
+
+INDEX
+
+  Aberration, 139
+    spherical, 138, 140
+  Accuracy of working, 70, 72
+  Acetylene gas lamps, 120
+  Actinic power, 102
+  Actinograph, 105
+  Actinometer, 120
+  Alternating current, 82, 100
+  Ammonia, 123
+  Angle of stylus, 24, 78
+  Aniline dye, 123
+  Arcing, 27, 82
+  Arc lamps, 15, 120, 121
+  Atmospherics, 61, 85
+
+  Ballasting resistance, 100
+  Belin, 47
+  Bernochi, 7, 112
+    system of, 7, 34
+  Berzelius, 109
+  Bichromate of potash, 120
+  Blondel's oscillograph, 47
+
+  Camera obscura, 136
+    extension, 116, 118
+    choice of, 117
+  Capacity of condenser, 24, 78
+    electrostatic, 3, 5
+    of cable, 3
+    of London-Paris telephone line, 3
+  Carbon bisulphide, 53
+  Charbonelle, 48
+    receiver of, 48
+  Chemical solution, 56
+  Circuit breaker, 76
+  Clutch, details of, 88, 89, 91
+    spring, 71
+  Coating the metal sheets, 120
+  Coherer, 11, 40
+  Collecting rings, 91
+  Commercial value of photo-telegraphy, 1
+  Compensating selenium cell, 112
+  Contact breaker, 37
+  Copying arrangements, 118, 125
+  Cross screen, 21
+
+  De' Arsonval galvanometer, 47, 73
+  Decoherer, 41
+  Design of machines, 21
+  Detectors, 83
+  Developing solutions, 105, 122
+  Diaphragm, movement of, 48, 52, 84, 87
+  Dipping rods, 81, 83
+  Distance of transmission, 33
+  Duration of wave-trains, 22, 25
+
+  Early experiments, 2
+  Einthoven galvanometer, 32, 44, 45, 54, 113
+  Electric clock, 93
+  Electrolytic receiver, 4, 37, 54, 61, 64
+  Enlarging arrangements, 124, 125
+  Experimental machine, 20
+  Extraneous light, 47
+
+  Fastening electrolytic paper, 58
+  Fatigue of selenium cell, 64, 114
+  Fish glue, 120
+  Flexible couplings, 77
+  Frequency meter, 65
+  Friction brake, 88
+
+  {142}
+  High speed telegraphy, 70
+  Hughes governor, 65
+  Hughes printing telegraph, 63
+  Hurter and Driffield, 104
+  Hydrogen, 100
+
+  Incidence, angle of, 127
+  Inertia, 64, 65, 111
+    effects in photo-telegraphy, 110
+    method of counteracting, 103, 112, 113
+    effect of wave-length of light on, 114
+  Intensifying solution, 122
+  Isochroniser, 89, 91
+    details of, 91, 92, 95
+  Isochronism, 64, 69, 70, 71
+
+  Kathode rays, 53
+  Knudsen, 2
+    apparatus of, 9
+  Korn, 30, 33, 45, 65, 72
+    apparatus of, 31
+
+  Lamps, coloured, 94
+  Lenses, 85, 125, 128
+    principal focus of, 130
+    conjugate foci of, 131
+    action of, 129
+    convex, 128, 131, 136
+    concave, 128, 138
+    focal length of, 130, 138
+    aperture, 138
+    meniscus, 139
+  Light, diffusion of, 86
+    extraneous, 87
+  Limit of error in synchronising, 64
+  Line balancer, 3
+  Line screens, 9, 15, 16, 116
+    making, 116
+
+  Magnifying power, 136, 137
+  Marconi valve, 44, 54
+    coherer, 40
+  Mechanical inertia, 33
+  Mercury break, 81
+    churning of, 82
+    containers, 82
+  Mercury jet interrupter, 29
+  Metal prints, 15, 18, 32, 59, 64, 95, 120, 124
+    drying the, 121, 123
+    exposure of, 121
+    size of, 22, 24, 75, 77
+    pressing the, 22
+  Microscope, 131, 137
+  Military uses, 35
+  Mirror galvanometer, 9, 42, 73
+  Mirror, 47, 51
+  Morse code, 35
+  Motor speed, 89, 95
+    driving, 91, 93, 95
+    clockwork, 63
+    electric, 63
+
+  Nernst lamps, 43, 85, 98
+    heater of, 99
+    filament of, 99
+    principle of, 98
+    resistance of, 100
+    efficiency of, 101, 102
+    overrunning, 101
+  Nicol prism, 53
+
+  Paper for electrolytic receiver, 56
+  Parabolic reflector, 8
+  Period of galvanometer, 43, 44, 46
+  _Photographic Daily Companion_, 105
+  Photographic films, 40, 43, 45, 53, 54, 62, 85, 86, 98
+    process, 37
+    chemical inertia, 103
+    exposure of, 103, 107
+    speed of, 104, 105
+    plates, orthochromatic, 59
+    plates, 120
+  Points to be observed in preparing metal prints, 123
+  Poulsen Company, 32, 47
+    arc, 31
+  Preparing selenium, 109
+    photographs for transmitting, 15, 115
+    sketches on metal foil, 124
+  Prism, 128
+    action of, 129
+  Process plates, 122
+  Professor Nernst, 98
+
+  {143}
+  Radio-photography, requirements of, 74
+  Refraction, angle of, 127
+  Refractive power, 127
+  Relay, 25, 39, 49, 53, 55, 60, 75
+    differential, 79
+    polarised, 97
+    working speed of, 26, 75
+  Reproducing for newspapers, 60
+  Resistance of selenium, 109
+    of selenium cells, 110
+    regulating, 113
+  Retardation of current, 6
+  Retouching, 62
+  Rotary spark-gap, 28
+
+  Selenium, 99
+    cells, 8, 34, 55, 60, 64, 109, 110
+    machines, 45
+  Self-induction, 24, 78
+  Sensitiveness of selenium cells, 113
+    ratio of, 113
+  Silvered quartz threads, 44, 46
+  Spark-gap, 27
+  Speed regulator, 68
+    adjustments of, 69
+  Spring clutch, 71
+  Starting position of machines, 98
+  String galvanometer, 32
+  Stylus, 17, 18, 57, 61, 78, 95, 103
+    sparking at, 24
+  Stylus, angle of, 24, 78
+    defects of, 57
+  Submarine cable, 4
+  Synchronism, 11, 20, 36, 64, 69, 71
+
+  Telephograph, 74
+    advantages of, 76
+    method of working, 96
+  Telephone receiver, 83, 85
+    diaphragm, 48
+    improved, 51
+  Telephone relay, 48, 50, 52, 83, 85, 97
+  Telescope, 131, 136
+  Thermodetector, 32
+  Tow, 88
+  Transmission, distance of, 35, 72
+    speed of, 25, 35, 75
+
+  Vibration, natural period of, 39
+
+  Watkins, 105
+    power number, 105
+  Waves, damped, 30
+    undamped, 30, 31
+  Wheatstone bridge, 113
+  Wireless apparatus, 13
+  _Wireless World_, 31
+  Wynne, 105
+
+  Zirconia, 99
+
+
+
+THE END
+
+
+
+_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.
+
+       *       *       *       *       *
+
+
+PUBLICATIONS OF
+
+THE WIRELESS PRESS, LTD.
+
+12 AND 13 HENRIETTA STREET,
+STRAND, LONDON, W.C.2.
+
+THE YEAR BOOK OF WIRELESS TELEGRAPHY AND TELEPHONY.
+
+With Map of the World, showing Wireless Stations; British, Colonial and
+foreign "Wireless" Laws and Regulations. Price 10S. 6D. net. (POST FREE,
+11S. INLAND; 11S. 4D. ABROAD.)
+
+THE WIRELESS TELEGRAPHISTS' POCKET BOOK OF NOTES, FORMULAE AND
+CALCULATIONS.
+
+By Dr. J. A. FLEMING, M.A., D.Sc., F.R.S., M.Inst.E.E., etc. A valuable
+compendium for Wireless Engineers and Operators. Price 9S. net. (POSTAGE
+5D.)
+
+THE HANDBOOK OF TECHNICAL INSTRUCTION FOR WIRELESS TELEGRAPHISTS.
+
+By J. C. HAWKHEAD and H. M. DOWSETT, M.I.E.E. Provides a complete
+theoretical course for the Postmaster-General's certificate of proficiency.
+310 pages. 240 Diagrams and Illustrations. Price 7S. net. (POSTAGE 6D.)
+
+MANUAL DE INSTRUCCION TECNICA PARA OPERADORES DE TELEGRAFIA SIN HILOS.
+
+Por J. C. HAWKHEAD y H. M. DOWSETT, M.I.E.E. Precio: Espana, 10 pesetas;
+Franqueo, 1 peseta extra. America Latina, $2.25, oro, neto; Franqueo, 25
+cents extra. (Great Britain, 9S.; POSTAGE 6D.)
+
+THE ELEMENTARY PRINCIPLES OF WIRELESS TELEGRAPHY.
+
+By R. D. BANGAY. In two Parts. Price 3S. each. (POSTAGE 4D.) Or in one
+Volume, price 7S. net. (POSTAGE 6D.) Used by H.M. Government for
+instructional purposes.
+
+PRINCIPIOS ELEMENT ALES DE TELEGRAFIA SIN HILOS.
+
+Por R. D. BANGAY. (Partes 1a y 2a en un Volumen.) PRECIO: Espana, 10
+pesetas; Franqueo, 1 peseta extra. America Latina, $2.25, oro, neto;
+Franqueo, 25 cents extra. (Great Britain, 9S.; POSTAGE 6D.)
+
+PRINCIPES ELEMENTAIRES DE TELEGRAPHIE SANS FIL.
+
+Par R. D. BANGAY. (Great Britain, 9S.; POSTAGE 6D.)
+
+MAGNETISM AND ELECTRICITY FOR HOME STUDY.
+
+By H. E. PENROSE. Crown 8vo. Over 500 pages. Price 5S. net, (POSTAGE 6D.)
+Contains fifty complete lessons.
+
+THE CALCULATION AND MEASUREMENT OF INDUCTANCE AND CAPACITY.
+
+By W. H. NOTTAGE, B.Sc. Invaluable to all engaged in Telegraph Engineering.
+Indispensable to the Wireless Engineer, Student and Experimenter. Price 3S.
+6D. net. (POSTAGE 5D.)
+
+A SHORT COURSE IN ELEMENTARY MATHEMATICS AND THEIR APPLICATION TO WIRELESS
+TELEGRAPHY.
+
+By S. J. WILLIS. To Students in Wireless Telegraphy, as well as those
+engaged in the practical application of this Science, this book should
+prove of real value. Price 3S. 6D. net. (POSTAGE 6D.)
+
+THE MARCONI OFFICIAL GRAMOPHONE RECORDS.
+
+For self-tuition in receiving Morse Signals. Price 4S. each, double-sided.
+(POSTAGE 9D.) Set of Six Records, 24S. post FREE.
+
+THE MAINTENANCE OF WIRELESS TELEGRAPH APPARATUS.
+
+By P. W. HARRIS. An up-to-date Manual, full of practical hints and
+explanations. Diagrams of all ship installations, from 1/4 kw. to 5 kw.
+Price 2S. 6D. net. (POSTAGE 4D.)
+
+DICTIONARY OF TECHNICAL TERMS USED IN WIRELESS TELEGRAPHY.
+
+By HAROLD WARD. Vest Pocket Edition. 2nd Edition, revised and enlarged.
+Contains over 1500 definitions. Price 2S. 6D. net. (POSTAGE 2D.)
+
+ARMATURE MODEL FOR 1-1/2 KW. ROTARY CONVERTER.
+
+Shows every Winding of the Converter Armature from start to finish. Price
+1S. net. (POSTAGE 3D.)
+
+MORSE MADE EASY.
+
+By A. L. RYE. Linen backed, for rapidly learning the Morse Code. Price 3D.
+net, or post free 3-1/2D.
+
+MORSE CODE CARD.
+
+Contains full alphabet, with punctuation marks, figures, abbreviations and
+contractions. Price 2D., post free.
+
+PRACTICAL WIRELESS TELEGRAPHY.
+
+By E. E. BUCHER. 352 pages. 340 Illustrations. Price 12S. 6D. (POSTAGE 6D.)
+
+RADIO-TELEPHONY.
+
+By ALFRED N. GOLDSMITH, Ph.D. 256 pages. 226 Illustrations. Price 15S. net.
+(POSTAGE 6D.)
+
+STANDARD TABLES AND EQUATIONS IN RADIO-TELEGRAPHY.
+
+By BERTRAM HOYLE, M.Sc.Tech., A.M.I.E.E. 159 pages. Price 9S. net. (POSTAGE
+6D.)
+
+VACUUM TUBES IN WIRELESS COMMUNICATION.
+
+By E. E. BUCHER. Deals with the Oscillation Valve. 178 pages. 130
+Illustrations. Price 12S. 6D. net. (POSTAGE 6D.)
+
+USEFUL NOTES ON WIRELESS TELEGRAPHY. (Students' Library.)
+
+By HAROLD E. PENROSE. Price 1S. 4D. net each. (POSTAGE 2D.)
+
+  Book  I. DIRECT CURRENT.
+  Book II. ALTERNATING CURRENT.
+  Book III. HIGH-FREQUENCY CURRENT AND WAVE PRODUCTION.
+  Book IV. THE 1-1/2 KW. SHIP SET.
+  Book V. THE OSCILLATION VALVE.
+
+THE OSCILLATION VALVE: THE ELEMENTARY PRINCIPLES OF ITS APPLICATION TO
+WIRELESS TELEGRAPHY.
+
+By R. D. BANGAY. 215 pages. Price 5S. (POSTAGE 3D.)
+
+THE THERMIONIC VALVE AND ITS DEVELOPMENTS IN RADIO-TELEGRAPHY AND
+TELEPHONY.
+
+By Dr. J. A. FLEMING, M.A., D.Sc., F.R.S., M.Inst.E.E., etc. 279 pages.
+Price 15S. (POSTAGE 6D.)
+
+ALTERNATING CURRENT WORK: AN OUTLINE FOR STUDENTS OF WIRELESS TELEGRAPHY.
+
+By A. SHORE. 163 pages. Price 3S. 6D. (POSTAGE 4D.)
+
+TELEPHONY WITHOUT WIRES.
+
+By PHILIP R. COURSEY, B.Sc., A.M.I.E.E., F.P.S.L. 414 pages. Price 15S.
+(POSTAGE 6D.)
+
+THE WIRELESS WORLD.
+
+A Monthly Magazine devoted to Wireless Telegraphy and Telephony. Price 9D.
+(POSTAGE 3D.) Annual Subscription, 11S. post free.
+
+THE RADIO REVIEW.
+
+A Monthly Record of Scientific Progress in Radio-telegraphy and Telephony.
+Price 2S. 6D. (POSTAGE 3D.) Annual Subscription, 30S. post free.
+
+CONQUEST.
+
+A Popular Illustrated Monthly Magazine dealing with Science, Industry and
+Invention. Price 1S. (POSTAGE 3D.) Annual Subscription, 15S. post free.
+
+CONTINUOUS WAVE WIRELESS TELEGRAPHY. Part I.
+
+By Dr. W. H. ECCLES, D.Sc., A.R.C.S., M.I.E.E. [_In the Press._
+
+       *       *       *       *       *
+
+COMPLETE CATALOGUE POST FREE.
+
+       *       *       *       *       *
+
+
+Notes
+
+[1] These measurements only apply to a single line. Where a double line is
+employed the capacity is halved.
+
+[2] See Appendix A.
+
+[3] See Appendix B.
+
+[4] In wireless telegraphy "arcing" is principally caused by the
+continuation of the supply current in the spark-gap after the capacity has
+been charged to a potential sufficient to break down the insulation of the
+gap.
+
+[5] See Chapter V.
+
+[6] Nernst lamps are the best to use, as they produce abundantly the blue
+and violet rays which have the greatest chemical effect upon a photographic
+film. Carbon filament lamps are very poor in this respect.
+
+[7] A description of the apparatus required will be found in Ganot's
+_Physics_.
+
+[8] Great care must be exercised in using this solution, as it is
+exceedingly poisonous.
+
+[9] Two clocks would isochronise if their hands travelled at precisely the
+same rate round the dials, but would not synchronise unless they both
+registered the same time as well.
+
+[10] Line screens can be obtained from Messrs. Penrose, 109 Farringdon
+Street, London; or Messrs. Fallowfield, 146 Charing Cross Road, London.
+
+
+
+***END OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF
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