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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-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_.
+
+ * * * * *
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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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