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+ border-color: #000000; + clear: both; } + </style> +</head> +<body> +<h1>The Project Gutenberg eBook, Wireless Transmission of Photographs, by +Marcus J. Martin</h1> +<pre> +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at <a href = "http://www.gutenberg.org">www.gutenberg.org</a></pre> +<p>Title: Wireless Transmission of Photographs</p> +<p> Second Edition, Revised and Enlarged 1919</p> +<p>Author: Marcus J. Martin</p> +<p>Release Date: October 9, 2010 [eBook #34052]</p> +<p>Language: English</p> +<p>Character set encoding: ISO-8859-1</p> +<p>***START OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF PHOTOGRAPHS***</p> +<p> </p> +<h4>E-text prepared by Robert Cicconetti, Keith Edkins,<br /> + and the Online Distributed Proofreading Team<br /> + (<a href="http://www.pgdp.net">http://www.pgdp.net</a>)<br /> + from page images generously made available by<br /> + Internet Archive/Canadian Libraries<br /> + (<a href="http://www.archive.org/details/toronto">http://www.archive.org/details/toronto</a>)</h4> +<p> </p> +<table border="0" style="background-color: #ccccff;margin: 0 auto;" cellpadding="10"> + <tr> + <td valign="top"> + Note: + </td> + <td> + Images of the original pages are available through + Internet Archive/Canadian Libraries. See + <a href="http://www.archive.org/details/wirelesstransmis00martuoft"> + http://www.archive.org/details/wirelesstransmis00martuoft</a> + </td> + </tr> +</table> +<p> </p> +<hr class="pg" /> +<p> </p> +<p> </p> +<p> </p> + +<h3>WIRELESS TRANSMISSION OF PHOTOGRAPHS</h3> + + <div class="figcenter" style="width:58%;"> + <a href="images/illo-fig10.png"><img style="width:100%" src="images/illo-fig10.png" + alt="Fig. 10." title="Fig. 10." /></a> + <span class="sc">Fig.</span> 10. + </div> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<h2>WIRELESS TRANSMISSION</h2> + +<p class="cenhead">OF</p> + +<h2>PHOTOGRAPHS</h2> + + <p> </p> + +<p class="cenhead">BY</p> + +<h3>MARCUS J. MARTIN</h3> + + <p> </p> + +<p class="cenhead"><i>SECOND EDITION<br /> +REVISED AND ENLARGED 1919</i></p> + + <p> </p> + <p> </p> + <p> </p> + +<h3>THE WIRELESS PRESS, LTD.</h3> + +<p class="cenhead">12-13 HENRIETTA STREET, STRAND</p> + +<p class="cenhead">LONDON, W.C. 2</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p> + +<h3>PREFACE TO SECOND EDITION</h3> + + <p>Although during the last few years very little, in common with other + wireless work, has been possible in connection with the practical side of + the wireless transmission of photographs, yet, now that the prospect of + experimental work is once again occupying the minds of all wireless + workers, advantage has been taken of a reprint of this little volume to + amplify a few points that were insufficiently dealt with in the first + edition, and also to add some fresh matter.</p> + + <p>To Chapter V. has been added a short description of the Nernst lamp, + and also some useful information regarding photographic films, and a few + notes relating to enlarging included in the Appendix B.</p> + + <p>A fresh appendix dealing with the principles of optical lenses has + also been added. This is a subject that plays an important part in any + system of wireless photography, and to those experimenters whose + knowledge of optics is limited this section should prove useful.</p> + + <p>To serious workers engaged on the problem of the wireless transmission + of photographs, attention <!-- Page vi --><span class="pagenum"><a + name="pagevi"></a>{vi}</span>is called to a series of articles which are + being published from time to time in the <i>Wireless World</i>, on the + design and construction of wireless photographic apparatus.</p> + + <div class="poem"> + <div class="stanza"> + <p>M. J. M.</p> + </div> + + <div class="stanza"> + <p><span class="sc">Maidstone</span>, 1919.</p> + </div> + </div> + +<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>{vii}</span></p> + +<h3>PREFACE</h3> + + <p>In these progressive times it is only reasonable to expect that some + attempt would be made to utilise the ether-waves for other purposes than + that of telegraphic communication, and already many clever minds are at + work trying to solve the problems of the wireless control of torpedoes + and airships, wireless telephony, and, last but not least, the wireless + transmission of photographs.</p> + + <p>It may seem rather premature to talk about the wireless transmission + of photographs at a time when the ordinary systems are not fully + developed; but the prospects of wireless photography are of a very + encouraging nature, especially for long over-water distances, as there + are great difficulties to be overcome in long-distance transmission over + ordinary land lines and cables which will be entirely eliminated by + wireless methods.</p> + + <p>From a perusal of Chapter I. the reader will be able to understand + something of the difficulties that are to be encountered in working over + long distances, and he will also be able to appreciate something of the + advantages that would be derived <!-- Page viii --><span + class="pagenum"><a name="pageviii"></a>{viii}</span>from a reliable + wireless system. Apart from the value of such a system for transmitting + news pictures, it would also be of great advantage to transmit to ships + at sea photographs of criminals for identification purposes. In such a + small volume as this it would be impossible to deal with the working of + wireless apparatus and the many systems that have been devised for the + transmission of photographs over metallic circuits. The Author has taken + it for granted that other works have been studied in connection with + these subjects, and will therefore only describe such apparatus as is + likely to be of use in wireless transmission. At present the transmission + of photographs by wireless methods is in a purely experimental stage, and + this book will have served its purpose if it helps to put future + experimenters on the right track and prevent them from making expensive + and fruitless experiments, by showing them the right direction in which + investigations are being carried out. As there is no claim to originality + in respect of a good many pieces of apparatus, etc., described, I have + not thought it necessary to state the various sources from which the + information has been obtained.</p> + + <div class="poem"> + <div class="stanza"> + <p>M. J. M.</p> + </div> + + <div class="stanza"> + <p><span class="sc">Ashford</span>, 1916.</p> + </div> + </div> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page ix --><span class="pagenum"><a name="pageix"></a>{ix}</span></p> + +<h3>CONTENTS</h3> + +<table class="nobctr" summary="Contents." title="Contents."> +<tr><td class="nspacsingle"> </td><td class="nspacsingle" style="text-align:right;"> PAGE</td></tr> +<tr><td class="nspacsingle"> <span class="sc">Preface to Second Edition</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#pagev">v</a></td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Preface</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#pagevii">vii</a></td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER I</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Introductory</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page1">1</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Foreword—Early experiments—Advantages of Radio-Photography—Difficulties +in Cable working—Bernochi's +System—Knudsen's System.</td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER II</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Transmitting Apparatus</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page13">13</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Wireless Apparatus—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.</td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER III</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Receiving Apparatus</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page37">37</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Methods of Receiving—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.</td></tr> +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> +<!-- Page x --><span class="pagenum"><a name="pagex"></a>{x}</span> +CHAPTER IV</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Synchronising and Driving</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page63">63</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Driving Motors—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.</td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> CHAPTER V</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">The "Telephograph"</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page74">74</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Author's System of Radio-Photography—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.</td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> APPENDIX A</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Selenium Cells</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page109">109</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Nature of Selenium—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.</td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> APPENDIX B</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Preparing the Metal Prints</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page115">115</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Outline of Process—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 +<!-- Page xi --><span class="pagenum"><a name="pagexi"></a>{xi}</span> +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.</td></tr> + +<tr><td class="nspacsingle" style="text-align:center; padding-top:1em;" colspan="2"> APPENDIX C</td></tr> + +<tr><td class="nspacsingle"> <span class="sc">Lenses</span> </td><td class="nspacsingle" style="text-align:right;"> <a href="#page126">126</a></td></tr> + +<tr><td class="nspacsingle" style="padding-left:2em"> Action of Light—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.</td></tr> +</table> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page xiii --><span class="pagenum"><a name="pagexiii"></a>{xiii}</span></p> + +<h3>ILLUSTRATIONS</h3> + +<table class="nobctr" summary="Illustrations." title="Illustrations."> +<tr><td class="nspacsingle"> FIG. </td><td class="nspacsingle" style="text-align:right;"> PAGE</td></tr> + +<tr><td class="nspacsingle"> 1. Diagram showing effects of capacity on an intermittent current </td><td class="nspacsingle" style="text-align:right;"> <a href="#page5">5</a></td></tr> + +<tr><td class="nspacsingle"> 2. Bernochi's wireless apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page7">7</a></td></tr> + +<tr><td class="nspacsingle"> 3. Knudsen's wireless apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page10">10</a></td></tr> + +<tr><td class="nspacsingle"> 4. Wireless transmitting station </td><td class="nspacsingle" style="text-align:right;"> <a href="#page13">13</a></td></tr> + +<tr><td class="nspacsingle"> 5. Diagram of experiment illustrating principle of line photograph </td><td class="nspacsingle" style="text-align:right;"> <a href="#page16">16</a></td></tr> + +<tr><td class="nspacsingle"> 6. Drawing of transmitting machine </td><td class="nspacsingle" style="text-align:right;"> <a href="#page17">17</a></td></tr> + +<tr><td class="nspacsingle"> 7. Drawing of transmitting machine </td><td class="nspacsingle" style="text-align:right;"> <a href="#page18">18</a></td></tr> + +<tr><td class="nspacsingle"> 8. Drawing of stylus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page18">18</a></td></tr> + +<tr><td class="nspacsingle"> 9. Electrical connections of machine </td><td class="nspacsingle" style="text-align:right;"> <a href="#page19">19</a></td></tr> + +<tr><td class="nspacsingle"> 10. Photograph of Author's experimental machine </td><td class="nspacsingle" style="text-align:right;"> <i>Frontispiece</i></td></tr> + +<tr><td class="nspacsingle"> +<table class="nob" style="margin-left: -0.25em"> +<tr><td class="nspacsingle"> 10<i>a</i>. End view of Author's experimental machine</td><td class="spacsingle" rowspan="2"> <a href="images/$rbrace.png"><img src="images/$rbrace.png" class="middle" style="height:6.5ex; width:0.75em" alt="brace" /></a> </td></tr> +<tr><td class="nspacsingle"> 10<i>b</i>. View of image broken up by a "cross" screen</td></tr> +</table> +</td><td class="nspacsingle" style="text-align:right;"> <i>facing page</i> <a href="#page21">21</a></td></tr> + +<tr><td class="nspacsingle"> 11. Connections of complete transmitting apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page23">23</a></td></tr> + +<tr><td class="nspacsingle"> 12. Drawing of ordinary type of spark-gap </td><td class="nspacsingle" style="text-align:right;"> <a href="#page27">27</a></td></tr> + +<tr><td class="nspacsingle"> 13. Synchronous rotating spark-gap </td><td class="nspacsingle" style="text-align:right;"> <a href="#page28">28</a></td></tr> + +<tr><td class="nspacsingle"> 14. Non-synchronous rotating spark-gap </td><td class="nspacsingle" style="text-align:right;"> <a href="#page28">28</a></td></tr> + +<tr><td class="nspacsingle"> 15. Connections for complete wireless photographic station </td><td class="nspacsingle" style="text-align:right;"> <a href="#page30">30</a></td></tr> + +<tr><td class="nspacsingle"> 16. Connections of Professor Korn's apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page31">31</a></td></tr> + +<tr><td class="nspacsingle"> 17. Connections of Poulsen's photographic recorder </td><td class="nspacsingle" style="text-align:right;"> <a href="#page33">33</a></td></tr> + +<tr><td class="nspacsingle"> 18. Author's photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page38">38</a></td></tr> + +<tr><td class="nspacsingle"> 19. Enlarged drawing of cone </td><td class="nspacsingle" style="text-align:right;"> <a href="#page39">39</a></td></tr> + +<tr><td class="nspacsingle"> 20. End view of Author's photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page39">39</a></td></tr> + +<tr><td class="nspacsingle"> 21. Connections of decohering apparatus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page41">41</a></td></tr> + +<tr><td class="nspacsingle"> 22. Connections for complete photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page42">42</a></td></tr> + +<tr><td class="nspacsingle"> +<!-- Page xiv --><span class="pagenum"><a name="pagexiv"></a>{xiv}</span> +23. Arrangement of Einthoven galvanometer </td><td class="nspacsingle" style="text-align:right;"> <a href="#page45">45</a></td></tr> + +<tr><td class="nspacsingle"> 24. Einthoven galvanometer arranged for receiving </td><td class="nspacsingle" style="text-align:right;"> <a href="#page46">46</a></td></tr> + +<tr><td class="nspacsingle"> 25. Connection of telephone relay </td><td class="nspacsingle" style="text-align:right;"> <a href="#page49">49</a></td></tr> + +<tr><td class="nspacsingle"> 26. Drawing of Author's improved photographic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page51">51</a></td></tr> + +<tr><td class="nspacsingle"> 27. Diagram giving ratio of vibrating arm </td><td class="nspacsingle" style="text-align:right;"> <a href="#page51">51</a></td></tr> + +<tr><td class="nspacsingle"> 28. Arrangement of polarisation receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page53">53</a></td></tr> + +<tr><td class="nspacsingle"> 29. Arrangement of kathode-ray receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page54">54</a></td></tr> + +<tr><td class="nspacsingle"> 30. Connections of electrolytic receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page56">56</a></td></tr> + +<tr><td class="nspacsingle"> 31. Drawing of improved stylus for receiving </td><td class="nspacsingle" style="text-align:right;"> <a href="#page58">58</a></td></tr> + +<tr><td class="nspacsingle"> 32. Drawing of Hughes telegraph governor </td><td class="nspacsingle" style="text-align:right;"> <a href="#page66">66</a></td></tr> + +<tr><td class="nspacsingle"> 33. Arrangement of simple speed regulator </td><td class="nspacsingle" style="text-align:right;"> <a href="#page68">68</a></td></tr> + +<tr><td class="nspacsingle"> 34. Diagram of connections of simple speed regulator </td><td class="nspacsingle" style="text-align:right;"> <a href="#page68">68</a></td></tr> + +<tr><td class="nspacsingle"> 35. Author's arrangement for complete radio-photographic station </td><td class="nspacsingle" style="text-align:right;"> <a href="#page77">77</a></td></tr> + +<tr><td class="nspacsingle"> 36. Drawing of transmitting machine and circuit breaker </td><td class="nspacsingle" style="text-align:right;"> <a href="#page78">78</a></td></tr> + +<tr><td class="nspacsingle"> 37. Drawing of special transmitting stylus showing adjusting + arrangements </td><td class="nspacsingle" style="text-align:right;"> <a href="#page79">79</a></td></tr> + +<tr><td class="nspacsingle"> 37<i>a</i>. End view of transmitting stylus </td><td class="nspacsingle" style="text-align:right;"> <a href="#page79">79</a></td></tr> + +<tr><td class="nspacsingle"> 38. Connections of new type of relay designed by the Author </td><td class="nspacsingle" style="text-align:right;"> <a href="#page80">80</a></td></tr> + +<tr><td class="nspacsingle"> 39. Arrangement of mercury containers and dipping rods for relay </td><td class="nspacsingle" style="text-align:right;"> <a href="#page82">82</a></td></tr> + +<tr><td class="nspacsingle"> 40. Drawing of Author's receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr> + +<tr><td class="nspacsingle"> 41. Enlarged drawing of diaphragm and steel point </td><td class="nspacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr> + +<tr><td class="nspacsingle"> 41<i>a</i>. Drawing showing arrangement of bush and counter-weight </td><td class="nspacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr> + +<tr><td class="nspacsingle"> 42. Optical arrangements of receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page85">85</a></td></tr> + +<tr><td class="nspacsingle"> 43. Optical arrangements of receiver </td><td class="nspacsingle" style="text-align:right;"> <a href="#page86">86</a></td></tr> + +<tr><td class="nspacsingle"> 44. Drawing of circuit breaker </td><td class="nspacsingle" style="text-align:right;"> <a href="#page88">88</a></td></tr> + +<tr><td class="nspacsingle"> 45. Drawing of friction brake </td><td class="nspacsingle" style="text-align:right;"> <a href="#page89">89</a></td></tr> + +<tr><td class="nspacsingle"> 46. Sectional drawing of magnetic clutch </td><td class="nspacsingle" style="text-align:right;"> <a href="#page90">90</a></td></tr> + +<tr><td class="nspacsingle"> 47. Plan of magnetic clutch </td><td class="nspacsingle" style="text-align:right;"> <a href="#page90">90</a></td></tr> + +<tr><td class="nspacsingle"> 48. Details of Isochroniser </td><td class="nspacsingle" style="text-align:right;"> <a href="#page92">92</a></td></tr> + +<tr><td class="nspacsingle"> 49. Connections of Isochroniser </td><td class="nspacsingle" style="text-align:right;"> <a href="#page94">94</a></td></tr> + +<tr><td class="nspacsingle"> 50. Dial of Isochroniser </td><td class="nspacsingle" style="text-align:right;"> <a href="#page94">94</a></td></tr> + +<tr><td class="nspacsingle"> 51. Diagram of driving mechanism </td><td class="nspacsingle" style="text-align:right;"> <a href="#page96">96</a></td></tr> + +<tr><td class="nspacsingle"> +<!-- Page xv --><span class="pagenum"><a name="pagexv"></a>{xv}</span> +52. Diagram showing starting positions of machines </td><td class="nspacsingle" style="text-align:right;"> <a href="#page97">97</a></td></tr> + +<tr><td class="nspacsingle"> 52<i>a</i>. Arrangement of small type Nernst lamp </td><td class="nspacsingle" style="text-align:right;"> <a href="#page99">99</a></td></tr> + +<tr><td class="nspacsingle"> 52<i>b</i>. Ballasting resistances for Nernst lamps </td><td class="nspacsingle" style="text-align:right;"> <a href="#page100">100</a></td></tr> + +<tr><td class="nspacsingle"> 52<i>c</i>. Arrangement of large type Nernst lamp </td><td class="nspacsingle" style="text-align:right;"> <a href="#page101">101</a></td></tr> + +<tr><td class="nspacsingle"> 53. Connections of selenium cell elements </td><td class="nspacsingle" style="text-align:right;"> <a href="#page110">110</a></td></tr> + +<tr><td class="nspacsingle"> 53<i>a</i>. Form of selenium cell used by Bell and Tainter </td><td class="nspacsingle" style="text-align:right;"> <a href="#page110">110</a></td></tr> + +<tr><td class="nspacsingle"> 54. Diagram showing construction of modern cell </td><td class="nspacsingle" style="text-align:right;"> <a href="#page111">111</a></td></tr> + +<tr><td class="nspacsingle"> 55. Resistance curve of selenium cell </td><td class="nspacsingle" style="text-align:right;"> <a href="#page111">111</a></td></tr> + +<tr><td class="nspacsingle"> 55<i>a</i>. Actual curve of selenium cell </td><td class="nspacsingle" style="text-align:right;"> <a href="#page112">112</a></td></tr> + +<tr><td class="nspacsingle"> 56. Diagram of Professor Korn's method for counteracting inertia </td><td class="nspacsingle" style="text-align:right;"> <a href="#page113">113</a></td></tr> + +<tr><td class="nspacsingle"> 57. Arrangement of plate sheath and line screen </td><td class="nspacsingle" style="text-align:right;"> <a href="#page117">117</a></td></tr> + +<tr><td class="nspacsingle"> 58. Details of clips to hold line screen </td><td class="nspacsingle" style="text-align:right;"> <a href="#page118">118</a></td></tr> + +<tr><td class="nspacsingle"> 59. Arrangement of apparatus for copying </td><td class="nspacsingle" style="text-align:right;"> <a href="#page119">119</a></td></tr> + +<tr><td class="nspacsingle"> 60. Drawing showing method of arranging camera and copying stand for + adjustment </td><td class="nspacsingle" style="text-align:right;"> <a href="#page119">119</a></td></tr> + +<tr><td class="nspacsingle"> +<table class="nob" style="margin-left: -0.25em"> +<tr><td class="nspacsingle"> 61. Photograph of line screen and metal print </td><td class="spacsingle" rowspan="2"> <a href="images/$rbrace.png"><img src="images/$rbrace.png" class="middle" style="height:6.5ex; width:0.75em" alt="brace" /></a></td></tr> +<tr><td class="nspacsingle"> 62. Photograph of sketch drawn upon metal foil</td></tr> +</table> +</td><td class="nspacsingle" style="text-align:right;"> <i>facing page</i> <a href="#page124">124</a></td></tr> + +<tr><td class="nspacsingle"> 63. Method of marking out copying board </td><td class="nspacsingle" style="text-align:right;"> <a href="#page124">124</a></td></tr> + +<tr><td class="nspacsingle"> 64. Diagram illustrating law of refraction </td><td class="nspacsingle" style="text-align:right;"> <a href="#page127">127</a></td></tr> + +<tr><td class="nspacsingle"> 65. Forms of lenses </td><td class="nspacsingle" style="text-align:right;"> <a href="#page128">128</a></td></tr> + +<tr><td class="nspacsingle"> 66. Action of light passed through a prism </td><td class="nspacsingle" style="text-align:right;"> <a href="#page129">129</a></td></tr> + +<tr><td class="nspacsingle"> 67. Diagram illustrating action of a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page130">130</a></td></tr> + +<tr><td class="nspacsingle"> 68. Formation of principal focus of a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page130">130</a></td></tr> + +<tr><td class="nspacsingle"> 69. Formation of conjugate foci of a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page131">131</a></td></tr> + +<tr><td class="nspacsingle"> 70. Apparatus illustrating principle of camera </td><td class="nspacsingle" style="text-align:right;"> <a href="#page132">132</a></td></tr> + +<tr><td class="nspacsingle"> 71. Formation of an image by a lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page133">133</a></td></tr> + +<tr><td class="nspacsingle"> 72. Diagram illustrating apparent magnitude </td><td class="nspacsingle" style="text-align:right;"> <a href="#page134">134</a></td></tr> + +<tr><td class="nspacsingle"> 73. Formation of virtual image by a convex lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page137">137</a></td></tr> + +<tr><td class="nspacsingle"> 74. Formation of virtual image by a concave lens </td><td class="nspacsingle" style="text-align:right;"> <a href="#page138">138</a></td></tr> + +<tr><td class="nspacsingle"> 75. Diagram showing spherical aberration </td><td class="nspacsingle" style="text-align:right;"> <a href="#page139">139</a></td></tr> + +<tr><td class="nspacsingle"> 76. Combination of plano-convex lenses </td><td class="nspacsingle" style="text-align:right;"> <a href="#page139">139</a></td></tr> + +<tr><td class="nspacsingle"> 77. Combination of meniscus and convex lenses </td><td class="nspacsingle" style="text-align:right;"> <a href="#page139">139</a></td></tr> +</table> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p> + +<h3>RADIO-PHOTOGRAPHY</h3> + +<h3>CHAPTER I</h3> + +<p class="cenhead">INTRODUCTORY</p> + + <p>Those who desire to experiment on radio-photography, <i>i.e.</i> + transmitting photographs, drawings, etc., from one place to another + without the aid of artificial conductors, must cultivate at least an + elementary knowledge of optics, chemistry, mechanics, and electricity; + photo-telegraphy calling for a knowledge of all these sciences. There + are, no doubt, many wireless workers who are interested in this subject, + but who are deterred from experimenting owing to a lack of knowledge + regarding the direction developments are taking, besides which, + information on this subject is very difficult to obtain, the science of + photo-telegraphy being, at the present time, in a purely experimental + stage.</p> + + <p>The wireless transmission of photographs has, no doubt, a great + commercial value, but for any system to be commercially practicable, it + must be simple, rapid, and reliable, besides being able to work <!-- Page + 2 --><span class="pagenum"><a name="page2"></a>{2}</span>in conjunction + with the apparatus already installed for the purpose of ordinary wireless + telegraphy.</p> + + <p>As far back as 1847 experiments were carried out with a view to + solving the problem of transmitting pictures and writing by electrical + methods over artificial conductors, but no great incentive was held forth + for development owing to lack of possible application; but owing to the + great public demand for illustrated newspapers that has recently sprung + into being, a large field has been opened up. During the last ten years, + however, development has been very rapid, and some excellent results are + now being obtained over a considerable length of line.</p> + + <p>The wireless transmission of photographs is, on the other hand, of + quite recent growth, the first practicable attempt being made by Mr. Hans + Knudsen in 1908. It may seem rather premature to talk about the wireless + transmission at a time when the systems for transmitting over ordinary + conductors are not perfectly developed, but everything points to the fact + that for long-distance transmission a reliable wireless system will prove + to be both cheaper and quicker than transmission over ordinary land lines + and cables.</p> + + <p>The effects of capacity and inductance—properties inherent to + all telegraph systems using metallic conductors—have a distinct + bearing upon the two questions, how far and how quickly can <!-- Page 3 + --><span class="pagenum"><a name="page3"></a>{3}</span>photographs be + transmitted? Owing to the small currents received and to prevent + interference from earth currents it is necessary to use a complete + metallic circuit. If an overhead line could be employed no difficulty + would be experienced in working a distance of over 1000 miles, but a line + of this length is impossible—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.</p> + + <p>If we take for an example the London-Paris telephone line with a + length of 311 miles and a capacity of 10.62 microfarads, we find that + about half this capacity, or 5.9 microfarads,<a name="NtA1" + href="#Nt1"><sup>[1]</sup></a> is contributed by the 23 miles of cable + connecting England with France.</p> + + <p>In practice the reduction of speed due to capacity has, to a great + extent, been overcome by means of apparatus known as a line-balancer, + which hastens the slow discharge of the line and <!-- Page 4 --><span + class="pagenum"><a name="page4"></a>{4}</span>allows each current sent + out from the transmitter—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 + <sup>1</sup>/<sub>200000</sub>th part of an ampere, and this necessitates + the use of apparatus of a very sensitive character. One system of + photo-telegraphy in use at the present time, employs what is known as an + electrolytic receiver (see Chapter III.) which can record signals over a + length of line in which the capacity effects are very slight, with the + marvellous speed of 12,000 a minute, but this speed rapidly decreases + with an increase of distance between the <!-- Page 5 --><span + class="pagenum"><a name="page5"></a>{5}</span><span class="figright" + style="width:38%;"><a href="images/illo-fig01.png"><img + style="width:100%" src="images/illo-fig01.png" alt="Fig. 1" title="Fig. 1" + /></a><span class="sc">Fig.</span> 1.</span> two stations. The effect of + capacity upon an intermittent current is clearly shown in Fig. 1. If we + were to send twenty brief currents in rapid succession over a line of + moderate capacity in a given time, we should find that instead of being + recorded separately and distinctly as at <i>a</i>, each mark would be + pointed at both ends and joined together as shown at <i>b</i>, while only + perhaps fifteen could be recorded. If the capacity be still farther + increased as at <i>c</i>, only perhaps half the original number of + currents could be recorded in the same time, owing to the fact that with + an increase of resistance, capacity, and inductance of the line a longer + time is required for it to charge up and discharge, thereby materially + lessening the rate at which it will allow separate signals to pass; the + number of signals that can therefore be recorded in a given time is + greatly diminished. If we were to attempt to send the same number of + signals over a line of great capacity, as could be sent, and recorded + separately and distinctly over a line of small capacity—the time + limit being of course the same in both instances—we should find + that the <!-- Page 6 --><span class="pagenum"><a + name="page6"></a>{6}</span>signals would be recorded practically as a + continuous line. The two latter cases <i>b</i>, and <i>c</i>, Fig. 1, + clearly shows the retardation that takes place at the commencement of a + current and the prolongation that takes place at the finish. If the + photo-telegraphic system previously mentioned could be rendered sensitive + enough to work on the Atlantic cables, we should find that only about + 1200 signals a minute could be recorded, and this would mean that a + photograph which could be transmitted over ordinary land lines in about + ten minutes would take at least fifty minutes over the cable. This would + be both costly and impracticable, and time alone will show whether, for + long-distance work, transmission by wireless will be both cheaper and + more rapid than any other method. At present wireless telegraphy has not + superseded the ordinary methods of communicating over land, but there can + be no doubt that wireless telegraphy, if free from Government + restrictions, would in certain circumstances very quickly supersede + land-line telegraphy, while it has proved a formidable commercial + competitor to the cable as a means of connecting this country with + America. Likewise we cannot say that no system of radio-photography will + ever come into general use, but where there is any great distance to be + bridged, especially over water, wireless transmission is really the only + practical solution. From the <!-- Page 7 --><span class="pagenum"><a + name="page7"></a>{7}</span>foregoing remarks, it is evident that a + reliable system of radio-photography would secure a great victory in the + matter of time and cost alone, besides which, the photo-telegraphic + apparatus would be merely an accessory to the already existing wireless + installation.</p> + + <div class="figcenter" style="width:40%;"> + <a href="images/illo-fig02.png"><img style="width:100%" src="images/illo-fig02.png" + alt="Fig. 2." title="Fig. 2." /></a> + <span class="sc">Fig.</span> 2. + </div> + + <p>There have been numerous suggestions put forward for the wireless + transmission of photographs, but they are all more or less impracticable. + One of the earliest systems was devised by de' Bernochi of Turin, but his + system can only be regarded interesting from an historical point of view, + and as in all probability it could only have been made to work over a + distance of a few hundred yards it is of no practical value. Fig. 2 will + help to explain the apparatus. A glass cylinder A' is fastened at one end + to a threaded steel shaft, which runs in two bearings, one bearing having + an internal thread corresponding with that on the <!-- Page 8 --><span + class="pagenum"><a name="page8"></a>{8}</span>shaft. Round the cylinder + is wrapped a transparent film upon which a photograph has been taken and + developed. Light from a powerful electric lamp L, is focussed by means of + the lens, N, to a point upon the photographic film. As the cylinder is + revolved by means of a suitable motor, it travels upwards simultaneously + by reason of the threaded shaft and bearing, so that the spot of light + traces a complete spiral over the surface of the film. The light, on + passing through the film (the transmission of which varies in intensity + according to the density of that portion of the photograph through which + it is passing), is refracted by the prism P on to the selenium cell S + which is in series with a battery B and the primary X of a form of + induction coil. As light of different intensities falls upon the selenium + cell,<a name="NtA2" href="#Nt2"><sup>[2]</sup></a> the resistance of + which alters in proportion, current is induced in the secondary Y of the + coil and influences the light of an arc lamp of whose circuit it is + shunted. This arc lamp T is placed at the focus of a parabolic reflector + R, from which the light is reflected in a parallel beam to the receiving + station.</p> + + <p>The receiver consists of a similar reflector R' with a selenium cell E + placed at its focus, whose resistance is altered by the varying light + falling upon it from the reflector R. The selenium cell <!-- Page 9 + --><span class="pagenum"><a name="page9"></a>{9}</span>E is in series + with a battery F and the mirror galvanometer H. Light falls from a lamp D + and is reflected by the mirror of the galvanometer on to a graduated + aperture J and focussed by means of the aplanatic lens U upon the + receiving drum A<sup>2</sup>, which carries a sensitised photographic + film. The two cylinders must be revolved synchronously. The above + apparatus is very clever, but cannot be made to work over a distance of + more than 200 yards.</p> + + <p>A system based on more practical lines was that invented and + demonstrated by Mr. Hans Knudsen, but the apparatus which he employed for + receiving has been discarded in wireless work, as it is not suitable for + working with the highly-tuned systems in use at the present time.</p> + + <p>Knudsen's transmitter, a diagrammatic representation of which is given + in Fig. 3, consists of a flat table to which a horizontal to-and-fro + motion is given by means of a clockwork motor. Upon this table is + fastened a photographic plate which has been prepared in the following + manner. The plate upon which the photograph is to be taken has the + gelatine film from three to four times thicker than that commonly used in + photography. In the camera, between the lens and this plate, a single + line screen is interposed, which has the effect of breaking the picture + up into parallel lines. Upon the plate being developed and before it is + <!-- Page 10 --><span class="pagenum"><a + name="page10"></a>{10}</span><span class="figright" style="width:43%;"><a + href="images/illo-fig03.png"><img style="width:100%" + src="images/illo-fig03.png" alt="Fig.3." title="Fig.3." /></a><span + class="sc">Fig.</span> 3.</span> completely dry, it is sprinkled over + with fine iron dust. With this type of plate the transparent parts dry + much quicker than the shaded or dark parts, and on the iron dust being + sprinkled over the plate it adheres to the darker portions of the film to + a greater extent than it does to the lighter portions; a picture partly + composed of iron dust is thus obtained. A steel point attached to a flat + spring rests upon this plate and is made to travel at right angles to the + motion of the table. As the picture is partly composed of iron dust, and + as the steel needle is fastened to a delicate spring it is evident that + as the plate passes to and fro under the needle, both the spring and + needle are set in a state of vibration. This vibrating spring makes <!-- + Page 11 --><span class="pagenum"><a name="page11"></a>{11}</span>and + breaks the battery circuit of a spark coil, which in turn sets up + sparking in the spark-gap of the wireless apparatus.</p> + + <p>The receiver consists of a similar table to that used for + transmitting, and carries a glass plate that has been smoked upon one + side. A similar spring and needle is placed over this plate, but is + actuated by means of a small electro-magnet in circuit with a battery and + a sensitive coherer. As the coherer makes and breaks the battery circuit + by means of the intermittent waves sent out from the transmitting aerial, + the needle is made to vibrate upon the smoked glass plate in unison with + the needle at the transmitting end. Scratches are made upon the smoked + plate, and these reproduce the picture on the original plate. A print can + be taken from this scratched plate in a similar manner to an ordinary + photographic negative.</p> + + <p>The two tables are synchronised in the following manner. Every time + the transmitting table is about to start its forward stroke a powerful + spark is produced at the spark-gap. The waves set up by this spark + operate an ordinary metal filings coherer at the receiving end which + completes the circuit of an electro-magnet. The armature of this magnet + on being attracted immediately releases the motor used for driving, + allowing it to operate the table. The time taken to transmit a + photograph, quarter-plate size, is about fifteen minutes. <!-- Page 12 + --><span class="pagenum"><a name="page12"></a>{12}</span>Although very + ingenious this system would not be practicable, as besides speed the + quality of the received pictures is a great factor, especially where they + are required for reproduction purposes. The results from the above + apparatus are said to be very crude, as with the method used to prepare + the photographs no very small detail could be transmitted.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 13 --><span class="pagenum"><a name="page13"></a>{13}</span></p> + +<h3>CHAPTER II</h3> + +<p class="cenhead">TRANSMITTING APPARATUS</p> + + <p>Let us now consider the requirements necessary for transmitting + photographs by means of the wireless apparatus in use at the present + time.</p> + + <div class="figright" style="width:24%;"> + <a href="images/illo-fig04.png"><img style="width:100%" src="images/illo-fig04.png" + alt="Fig. 4." title="Fig. 4." /></a> + <span class="sc">Fig.</span> 4. + </div> + + <p>The connections for an experimental syntonic wireless transmitting + station are shown in the diagram Fig. 4. A is the aerial; T, the + inductance; E, earth; L, hot-wire ammeter. The closed oscillatory circuit + consists of an inductance F, spark-gap G, and a block condenser C. H is a + spark-coil for supplying the energy, the secondary J being connected to + the spark-gap. A <!-- Page 14 --><span class="pagenum"><a + name="page14"></a>{14}</span>mercury break N and a battery B are placed + in the primary circuit of the coil. The Morse key K is for completing the + battery circuit for signalling purposes. When the key K is depressed, the + battery circuit is completed, and a spark passes between the balls of the + spark-gap G producing oscillations in the closed circuit, which are + transposed to the aerial circuit by induction. For signalling purposes it + is only necessary for the operator by means of the key K to send out a + long or short train of waves in some pre-arranged order, to enable the + operator at the receiving station to understand the message that is being + transmitted.</p> + + <p>If a photograph could be prepared in such a manner that it would serve + the purpose of the key K, and could so arrange matters that a minute + portion of the photograph could be transmitted separately but in + succession, and that each portion of the photograph having the same + density could be given the same signal, then it would only be necessary + to have apparatus at the receiving station capable of arranging the + signals in proper sequence (each signal recorded being the same size and + having the same density as the transmitted portion of the photograph) in + order to receive a facsimile of the picture transmitted.</p> + + <p>The following method of preparing the photograph<a name="NtA3" + href="#Nt3"><sup>[3]</sup></a> is one that has been adopted in several + <!-- Page 15 --><span class="pagenum"><a + name="page15"></a>{15}</span>systems of photo-telegraphy, and is the only + one at all suitable for wireless transmission. The photograph or picture + which is to be transmitted is fastened out perfectly flat upon a + copying-board. A strong light is placed on either side of this copying + board, and is concentrated upon the picture by means of reflectors. The + camera which is used for copying has a single line screen interposed + between the lens and sensitised plate, and the effect of this screen is + to break the picture up into parallel lines. Thus a white portion of the + photograph would consist of very narrow lines wide apart, while the dark + portion would be made up of wide lines close together; a black part would + appear solid and show no lines at all. From this line negative it will be + necessary to take off a print upon a specially prepared sheet of metal. + This consists of a sheet of thick lead- or tinfoil, coated upon one side + with a thin film of glue to which bichromate of potash has been added; + the bichromate possessing the property of rendering the glue waterproof + when acted upon by light. The print can be taken off by artificial light + (arc lamps being generally used), but the exact time to allow for + printing can only be found by experiment, as it varies considerably + according to the thickness of the film. The printing finished, the metal + print is washed under running water, when all those parts not acted upon + by light, <i>i.e.</i> the parts between the lines, are <!-- Page 16 + --><span class="pagenum"><a name="page16"></a>{16}</span>washed away, + leaving the bare metal. We have now an image composed of numerous bands + of insulating material (each band varying in width according to the + density of the photograph at any point from which it is prepared) + attached to a metal base, so that each band of insulating material is + separated by a band of conducting material. It is, of course, obvious + that the lines on the print cannot be wider apart, centre to centre, than + the lines of the screen used in preparing it. A good screen to use is one + having 50 lines to the inch, but one is perhaps more suitable for + experimental work a little coarser, say 35 lines to the inch. To use a + screen having 50 or more lines to the inch, the transmitting apparatus, + as will be evident later on, will require to be very nearly perfect.</p> + + <div class="figright" style="width:23%;"> + <a href="images/illo-fig05.png"><img style="width:100%" src="images/illo-fig05.png" + alt="Fig. 5." title="Fig. 5." /></a> + <span class="sc">Fig.</span> 5. + </div> + + <p>Before proceeding further it will perhaps be as well to make an + experiment. If we take one of the metal prints or, more simple, draw a + sketch in insulating ink upon a sheet of metal A, Fig. 5, and connect a + battery B and the galvanometer D as shown, we shall find on drawing the + free end of the wire across the metal plate that all the time the wire is + in contact with the lines of insulating material the needle of the + galvanometer will remain <!-- Page 17 --><span class="pagenum"><a + name="page17"></a>{17}</span>at zero, but where it is in contact with the + metal plate the needle is deflected.</p> + + <p>From this experiment it will be seen that we have in our metal line + print, which consists of alternate lines of insulating and conducting + material, a method by which an electric circuit can be very easily made + and broken. It is, of course, necessary to have some arrangement whereby + the whole of the surface of the metal print is utilised for this purpose + to the best advantage. One type of transmitting machine used for this + purpose is represented by the diagram, Fig. 6. The cylinder A is fastened + to the steel shaft B, which runs in the two bearings D and D', the + bearing D' having an internal thread corresponding to that on the shaft. + The stylus in this class of machine is a fixture, the cylinder being + given a lateral as well as a revolving movement. As it is impossible to + use a rigid drive, a flexible coupling F is employed between the shaft B + and the motor.</p> + + <div class="figcenter" style="width:31%;"> + <a href="images/illo-fig06.png"><img style="width:100%" src="images/illo-fig06.png" + alt="Fig. 6." title="Fig. 6." /></a> + <span class="sc">Fig.</span> 6. + </div> + + <p>Another type of machine is shown in Fig. 7. The drum in this case is + stationary, the table T moving laterally by reason of the screwed shaft + <!-- Page 18 --><span class="pagenum"><a + name="page18"></a>{18}</span><span class="figright" style="width:22%;"><a + href="images/illo-fig07.png"><img style="width:100%" + src="images/illo-fig07.png" alt="Fig. 7." title="Fig. 7." /></a><span + class="sc">Fig.</span> 7.</span> and half nut F. The table, shown + separate in Fig. 8, carries a stiff brass spring A, to which is attached + a holder B made to take a hardened steel point. The holder is provided + with a set screw P for securing the steel point Z. The spring and needle + are insulated from the rest of the machine, as shown in the drawing. In + working, the metal print is wrapped tightly round the cylinder of the + machine, the glue image being, of course, uppermost. To fasten the print + a little seccotine should be applied to one edge, and the joint carefully + smoothed down with the fingers. <span class="figleft" + style="width:27%;"><a href="images/illo-fig08.png"><img + style="width:100%" src="images/illo-fig08.png" alt="Fig. 8." title="Fig. 8." + /></a><span class="sc">Fig.</span> 8.</span> If there is any tendency on + the part of the print to slip round on the drum, a couple of small spring + clips placed over the ends of the drum will act as a preventive. It is + necessary to place the print upon the drum in such a manner that the + stylus draws away from the edge of the lap and not towards it, and the + metal prints should be of such a size that when placed round the drum of + the <!-- Page 19 --><span class="pagenum"><a + name="page19"></a>{19}</span>machine a lap of about + <sup>3</sup>/<sub>16</sub>ths of an inch is allowed.</p> + + <div class="figright" style="width:24%;"> + <a href="images/illo-fig09.png"><img style="width:100%" src="images/illo-fig09.png" + alt="Fig. 9." title="Fig. 9." /></a> + <span class="sc">Fig.</span> 9. + </div> + + <p>The steel point Z (ordinary gramophone needles may be used and will be + found to answer the purpose admirably) is made to press lightly upon the + metal print, and while the pressure should be sufficient to make good + electrical contact, it should not be sufficient to cause the needle to + scratch the surface of the foil. The pressure is regulated by means of + the milled nut H. The electrical connections are given in Fig. 9. One + wire from the battery M is taken to the terminal T, and the other wires + from M and F lead to the relay R. The current flows from the battery M + through the spring Y, through the drum and metal print, the stylus Z, + spring A, down to the relay R, and from R back to the battery M. As the + drum carrying the single line half-tone print is revolved, the stylus, by + reason of the lateral movement given to the table or cylinder as the case + may be, will trace a spiral path over the entire surface of the print. As + the stylus traces over a conducting strip the circuit is completed, and + the tongue of the relay R is attracted, making contact with the stop S. + <!-- Page 20 --><span class="pagenum"><a name="page20"></a>{20}</span>On + passing over a strip of insulation the circuit is broken and the tongue + of the relay R returns to its normal position.</p> + + <p>As already stated, the conducting and insulating bands on the print + vary in width according to the density of the photograph from which it is + prepared, so that the length of time that the tongue of the relay R is + held against the stop S, is in proportion to the width of the conducting + strip which is passing under the stylus at any instant. The function of + the transmitter is therefore to send to the relay R an intermittent + current of varying duration.</p> + + <p>The two photographs Figs. 10 and 10<i>a</i> are of a machine designed + and used by the writer in his experiments. In this machine the drum is + 3.5 inches long and 1.5 inches in diameter. The lead screw has 30 threads + to the inch, and the reduction between it and the drum is 3:1, so that + the table has a movement of <sup>1</sup>/<sub>90</sub>th inch per + revolution of the drum.</p> + + <p>From the brief description of the various types of machines that have + been given it will be apparent that in the design of the machine proper + there is nothing very complicated, although the addition of the driving + and synchronising apparatus complicates matters rather considerably. The + questions of driving and synchronising the machines at the two stations + is fully dealt with in Chapter IV.</p> + + <div class="figcenter" style="width:30%;"> + <a href="images/illo-fig10a.png"><img style="width:100%" src="images/illo-fig10a.png" + alt="Fig. 10a." title="Fig. 10a." /></a> + <span class="sc">Fig.</span> 10a. + </div> + + <div class="figcenter" style="width:38%;"> + <a href="images/illo-fig10b.png"><img style="width:100%" src="images/illo-fig10b.png" + alt="Fig. 10b." title="Fig. 10b." /></a> + <span class="sc">Fig.</span> 10b. Enlarged view of an image broken up + by a cross screen. + </div> + +<p><!-- Page 21 --><span class="pagenum"><a name="page21"></a>{21}</span></p> + + <p>Although the design of the machines is rather simple great attention + must be paid both to accuracy of construction and accuracy of working, + and this applies, not only to the machines (whether for transmitting or + receiving) but for all the various pieces of apparatus that are used. Too + much care cannot be bestowed upon this point, as in the wireless + transmission of photographs there is a large number of instruments all + requiring careful adjustment, and which have to work together in perfect + unison at a high speed.</p> + + <p>The machine shown in Figs. 10 and 10<i>a</i> was designed and used by + the writer solely for experimental work. It will be noticed in the + description given in the appendix of the method of preparing the metal + prints that a 5" × 4" camera is recommended, while the machine, Fig. 10, + is designed to take a print procured from a quarter-plate negative. This + size of drum was adopted for several reasons, and although it will be + found quite large enough for general experimental work the writer has + come to the conclusion that for practical commercial work a drum to take + a print 5" × 4" will give better results.</p> + + <p>In making a negative of a picture that is required for reproduction + purposes, the line screen in the camera is replaced by a "cross screen," + <i>i.e.</i> two single line screens placed with their lines at an angle + of 90° to one another, and this breaks the <!-- Page 22 --><span + class="pagenum"><a name="page22"></a>{22}</span>image up into small + squares instead of lines. By looking at any ordinary newspaper or book + illustration through a powerful magnifying glass the effects of a cross + screen will readily be seen. With a cross screen a certain amount of + detail is necessarily lost, but with a single line screen the amount lost + is much greater. If there is any very small detail in the picture most of + this would be lost in a coarse screen, hence the necessity of employing + as fine a line screen as practicable in order to get as much detail in as + possible. It is mainly on this account that a 5" × 4" print is + recommended, as, if fairly bold subjects are used for copying, the small + detail (this is, of course, a very vague and indefinable term) will not + be too fine, and the time required for transmitting reasonable. For + obvious reasons it is a great advantage to put the print under pressure + to cause the glue image to sink into the soft metal base and leave a + perfectly flat and smooth surface. It is essential that the bands on the + print lie along the axis of the cylinder, so that the stylus traces its + path across them, and not with them.</p> + + <p>We have now an arrangement that is capable of taking the place of the + key K, Fig. 4, and the diagram, Fig. 11, gives the connections for the + complete transmitter. A is the aerial, E earth, T inductance, L ammeter. + The closed oscillatory circuit consists of a spark-gap G, inductance F, + <!-- Page 23 --><span class="pagenum"><a + name="page23"></a>{23}</span><span class="figright" style="width:38%;"><a + href="images/illo-fig11.png"><img style="width:100%" + src="images/illo-fig11.png" alt="Fig. 11." title="Fig. 11." /></a><span + class="sc">Fig.</span> 11.</span> and a condenser C. The secondary J of + the coil H is connected to the spark-gap, and the primary P is in circuit + with the mercury break N, the battery B, and the local contacts of the + relay R. The action is as follows. When contact is made between the + stylus Z and the drum V by means of the conducting bands on the line + print, the circuit of the relay R and the battery M is completed. The + closing of the local circuit of the relay R actuates the second relay R', + allowing the primary circuit of the coil H to be closed. As soon as the + primary circuit of the coil is completed sparks pass between the + electrodes of the spark-gap G, causing waves to radiate from the aerial. + The duration of the wave-trains radiated depends upon the duration of + contact made by the relays <!-- Page 24 --><span class="pagenum"><a + name="page24"></a>{24}</span>R and R', and this in turn depends upon the + width of the conducting strip that is passing under the stylus. The + battery M should be about 4 volts, and the battery D about 2 volts. The + two-way switch X is connected up so that the relay R' can be thrown out + and the key K switched in for ordinary signalling purposes. If any + sparking takes place at the point of the stylus, a small condenser C' + (about 1 microfarad capacity) should be connected as shown. In the + present instance the condenser should be used more as a preventive than + as a cure, as in all probability the voltage from M will not be + sufficient to cause destructive (if any) sparking; but, as most wireless + workers know, anything in the nature of a spark occurring in the + neighbourhood of a detector (this, of course, only applies when the + receiving apparatus is placed in close proximity to the transmitter) is + liable to destroy the adjustment.</p> + + <p>In transmitting over ordinary conductors where the initial voltage is + fairly high and the self-induction of the circuit very great, the use of + the condenser will be found to be absolutely essential. It has also been + noted that the angle which the stylus presents to the drum has a marked + effect upon the sparking, an angle of about 60° being found to give very + good results.</p> + + <p>If the size of the single line print used is 5 inches by 4 inches, and + a screen having 50 lines <!-- Page 25 --><span class="pagenum"><a + name="page25"></a>{25}</span>to the inch is used for preparing it, then + the stylus will have to make 250 contacts during one revolution of the + drum. Assuming the drum to make one revolution in three seconds, then the + time taken to transmit the complete photograph can be found from the + equation T = <i>w</i> × <i>t</i> × <i>s</i>, where <i>w</i> is the width + of the print, <i>t</i> the travel of the stylus during one revolution of + the drum, and <i>s</i> the time required for one revolution of the drum. + In the present instance this will be T = 4 × 90 × 3 = 1080 seconds = 18 + minutes. The number of contacts made by the stylus per minute is 5000, + and in working at this speed the first difficulty is encountered in the + use of the two relays. The relay R is lightly built, and capable of + working at a fairly high speed, but R' is a heavier pattern, and + consequently works at a slightly lower rate. This relay must necessarily + be heavier, as more substantial contacts are needed in order to pass the + heavy current taken by the spark-coil.</p> + + <p>Relays sensitive and accurate enough to work at this speed will in all + probability be beyond the reach of the majority of workers, but there are + several types of relays on the market very reasonable in price that will + answer very well for experimental work, although the speed of working + will no doubt be slower.</p> + + <p>For the best results the duration of the wave-trains sent out should + be of the same duration as <!-- Page 26 --><span class="pagenum"><a + name="page26"></a>{26}</span>the contact made by R, and therefore equal + to the time taken by the stylus to trace over a conducting strip; but if + the duration of the contact made by R is <i>t</i>, then that made by R' + and consequently the duration of the groups of wave-trains would be + <i>t</i> - <i>v</i> where <i>v</i> equals the extra time required by R' + to complete its local circuit. The difference in time made by the two + relays, although very slight, will be found to affect very considerably + the quality of the received pictures. Renewing the platinum contacts is + also a great expense, as they are soon burnt out where a heavy current is + passed. If the distance experimented over is short so that the power + required to operate the spark-coil is not very heavy, one relay will be + sufficient providing the contacts are massive enough to carry the current + safely. It is useless to expect any of the ordinary relays in general use + to work satisfactorily at such a high speed, and in order to compensate + for this we must either increase the time of transmitting, or, as already + suggested, make use of a coarser line screen in preparing the + photographs.</p> + + <p>For reasons already explained, all points of make and break should be + shunted by a condenser. The effective working speed of an ordinary type + of relay may be anything from 1000 to 2500 dots a minute, depending upon + accuracy of design and construction.</p> + + <p>In the wireless transmission of photographs it <!-- Page 27 --><span + class="pagenum"><a name="page27"></a>{27}</span>is absolutely essential + to use some form of rotary spark-gap, as where sparks are passed in rapid + succession the ordinary type of gap is worse than useless. When a spark + passes between the electrodes of an ordinary spark-gap, Fig. 12, we find + that for a fraction of a second after the first spark has passed, the + normally high resistance of the gap has been lowered to less than one + ohm. If the column of hot gas which constitutes the spark is not + instantly dispersed, but remains between the electrodes, it will provide + an easy path for any further discharges, and if sparks are passed at all + rapidly, what was at first a disruptive and oscillatory discharge will + degenerate into a hot, non-oscillatory arc.<a name="NtA4" + href="#Nt4"><sup>[4]</sup></a></p> + + <div class="figcenter" style="width:18%;"> + <a href="images/illo-fig12.png"><img style="width:100%" src="images/illo-fig12.png" + alt="Fig. 12." title="Fig. 12." /></a> + <span class="sc">Fig.</span> 12. + </div> + + <p>Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are + known as "synchronous" and "non-synchronous" gaps respectively. In the + synchronous gap the cog-wheel is mounted on the shaft of the alternator, + and a cog comes opposite the fixed electrode when the maximum of + potential is reached in the condenser, thus ensuring a discharge at every + alternation of current. With this type of gap a spark of pure tone is + obtained which <!-- Page 28 --><span class="pagenum"><a + name="page28"></a>{28}</span><span class="figleft" style="width:31%;"><a + href="images/illo-fig13.png"><img style="width:100%" + src="images/illo-fig13.png" alt="Fig. 13." title="Fig. 13." /></a><span + class="sc">Fig.</span> 13.</span> <span class="figright" + style="width:32%;"><a href="images/illo-fig14.png"><img + style="width:100%" src="images/illo-fig14.png" alt="Fig. 14." title="Fig. 14." + /></a><span class="sc">Fig.</span> 14.</span> is of great value where the + signals are received by means of a telephone, but where the signals are + to be mechanically recorded the tone of the spark is of little + consequence. In a non-synchronous gap a separate motor is used for + driving the toothed wheel, and can either be mounted on the motor shaft + or driven by means of a band, there being no regard given to synchronism + with the alternator. The fixed electrode is best made long enough to + cover about two of the teeth, as this ensures regular sparking and a + uniform sparking distance; the <!-- Page 29 --><span class="pagenum"><a + name="page29"></a>{29}</span>spark length is double the length of the + spark-gap. The toothed wheel should revolve at a high speed, anything + from 5000 to 8000 revolutions per minute, or even more being required. + The shaft of the toothed wheel is preferably mounted in + ball-bearings.</p> + + <p>Owing to the large number of sparks that are required per minute in + order to transmit a photograph at even an ordinary speed, it is necessary + that the contact breaker be capable of working at a very high speed + indeed. The best break to use is what is known as a "mercury jet" + interrupter, the frequency of the interruptions being in some cases as + high as 70,000 per second. No description of these breaks will be given, + as the working of them is generally well understood.</p> + + <p>In some cases an alternator is used in place of the battery B, Fig. 4, + and when this is done the break M can be dispensed with. In larger + stations the coil H is replaced with a special transformer.</p> + + <p>The writer has designed an improved relay which will respond to + currents lasting only <sup>1</sup>/<sub>100</sub>th part of a second, and + capable of dealing with rather large currents in the local circuit.<a + name="NtA5" href="#Nt5"><sup>[5]</sup></a> This relay has not yet been + tried, but if it is successful the two relays R and R' can be dispensed + with, and the result will be more accurate and effective + transmission.</p> + +<p><!-- Page 30 --><span class="pagenum"><a name="page30"></a>{30}</span></p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig15.png"><img style="width:100%" src="images/illo-fig15.png" + alt="Fig. 15." title="Fig. 15." /></a> + <span class="sc">Fig.</span> 15. + </div> + + <p>The connections for a complete experimental station, transmitting and + receiving apparatus combined, are given in Fig. 15. The terminals W, W + are for connecting to the photo-telegraphic receiving apparatus Q, being + a double pole two-way switch for throwing either the transmitting or + receiving apparatus in circuit. There is another system of transmitting + devised by Professor Korn, which employs an entirely different method + from the foregoing. By using the apparatus just described, the waves + generated are what are known as "damped waves," and by using these damped + waves, tuning, which is so essential to good commercial working, can be + made to reach a fairly high degree of efficiency. <!-- Page 31 --><span + class="pagenum"><a name="page31"></a>{31}</span></p> + + <p>The question of damped <i>versus</i> undamped waves is a somewhat + burning one, and no attempt will be made here to deal with the merits or + demerits of the claims made for the respective systems. A series of + articles describing the production of undamped waves and their efficiency + in working compared with damped waves will be found in the <i>Wireless + World</i>, Nos. 3 and 4, 1913, and are well worth reading by any one + interested in the subject.</p> + + <div class="figcenter" style="width:40%;"> + <a href="images/illo-fig16.png"><img style="width:100%" src="images/illo-fig16.png" + alt="Fig. 16." title="Fig. 16." /></a> + <span class="sc">Fig.</span> 16. + </div> + + <p>A diagrammatic representation of the apparatus as arranged by + Professor Korn is given in Fig. 16. The undamped or "continuous" waves + are generated by means of a high-frequency alternator or Poulsen arc. In + Fig. 16, X is the generator, F inductance, C condenser; the aerial + inductance T is connected by the aerial A and earth E. By this means the + waves are tuned to a certain period. <!-- Page 32 --><span + class="pagenum"><a name="page32"></a>{32}</span>A metal print, similar to + what has already been described, is wrapped round the drum D of the + machine, and when the stylus Z traces over an insulating strip the waves + generated are in tune with the receiving station, but when it traces over + a conducting strip, a portion of the inductance T is short-circuited, the + period of the oscillations is altered, and the two stations are thrown + out of tune.</p> + + <p>The receiving station is provided with an aperiodic circuit, which + consists of an inductance F', condenser C', and a thermodetector N. A + string galvanometer H (described in Chapter III.), and the self-induction + coils B, B' are connected as shown, the coils B, B' preventing the + high-frequency currents, which change their direction, from flowing + through the galvanometer. The manner in which the string galvanometer is + arranged to reproduce a transmitted picture is shown in Fig. 24.</p> + + <p>The connections adopted by the Poulsen Company for photographically + recording wireless messages are given in Fig. 17, a string galvanometer + of the Einthoven type being used. The two self-induction coils S and S' + are in circuit with the detector D and the galvanometer G. The condenser + C' prevents the continuous current produced by the detector from flowing + through the high frequency circuit; P is the primary of the aerial <!-- + Page 33 --><span class="pagenum"><a + name="page33"></a>{33}</span>inductance and F the secondary. The method + of transmitting adopted by Professor Korn appears to be a simple and + reliable arrangement, provided that an equally reliable method of + producing the undamped waves can be found. Owing to the absence of + mechanical inertia it should be capable of working at a good speed, while + the absence of a number of pieces of delicate apparatus all requiring + careful adjustment add greatly to its reliability.</p> + + <div class="figcenter" style="width:28%;"> + <a href="images/illo-fig17.png"><img style="width:100%" src="images/illo-fig17.png" + alt="Fig. 17." title="Fig. 17." /></a> + <span class="sc">Fig.</span> 17. + </div> + + <p>In any spark system with a properly designed aerial a coil taking ten + amperes is capable of transmitting signals over a distance of thirty to + fifty miles, but where the number of interruptions of the break required + per second is very high, as in radio-photography, it must be remembered + that a much higher voltage is needed to drive the requisite amount of + current through the primary winding of the coil than would be the case if + the interruptions were slower. It is possible to use platinum <!-- Page + 34 --><span class="pagenum"><a name="page34"></a>{34}</span>contacts for + the relays, for currents up to ten amperes, but for heavier currents than + this some arrangement where contact is made with mercury will be found to + be more economical and reliable.</p> + + <p>In the transmitter already described and given in Fig. 11, the best + results would be obtained by finding the speed at which the relay R' + works best, and regulating the number of contacts made by the stylus + accordingly.</p> + + <p>The method employed by De' Bernochi (see Chapter I.) of varying the + intensity of a beam of light by passing it through a photographic film, + which in turn alters the resistance of a selenium cell, has been very + successfully employed in at least one system of photo-telegraphy. Its + application has also been suggested for wireless transmission, and + although with any system using continuous waves this would not be very + difficult, it could hardly be adapted to work with the ordinary spark + system. The apparatus for receiving from this type of transmitter would, + on the other hand, necessarily be more elaborate than the methods that + are described in the next chapter, and as far as the writer's experience + goes, experiments along these lines would not prove very profitable, as + simplicity is the keynote of success in any radio-photographic + system.</p> + + <p>It has been suggested that in order to decrease the time of + transmission a cylinder capable of <!-- Page 35 --><span + class="pagenum"><a name="page35"></a>{35}</span>taking a print 7 inches + by 5 inches be employed, the print being prepared from rather a coarse + line screen—say 35 to the inch—and a traverse of about + <sup>1</sup>/<sub>50</sub> inch given to the stylus, thus reducing the + time of transmission to about twelve minutes. It is questionable, + however, whether the increase in speed would compensate for the loss of + detail, as only very bold subjects could be transmitted. As already + pointed out, wireless transmission would only be employed for fairly long + distances, and the extra time and expense required to receive a fairly + good detailed picture is negligible when compared with the enormous time + it would take to receive the original photograph by any ordinary means of + transit.</p> + + <p>The public much prefer to have passable pictorial illustrations of + current events than wait several days for a more perfect + picture—the original, and the advantage of any newspaper being able + to publish photographs several days before its rivals is obvious. There + can also be no doubt but that a system of radio-photography, if fairly + reliable and capable of working over a distance of say thirty miles, + would be of great military use for transmitting maps and written matter + with a great saving of time and even life. Written matter could be + transmitted with even greater safety than messages which are sent in the + ordinary way in Morse Code, as the signals received in the receiver <!-- + Page 36 --><span class="pagenum"><a name="page36"></a>{36}</span>of an + hostile installation would be but a meaningless jumble of sounds, and + even were they possessed of radio-photographic apparatus the received + message would be unintelligible, unless they knew the exact speed at + which the machines were running and could synchronise accurately.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 37 --><span class="pagenum"><a name="page37"></a>{37}</span></p> + +<h3>CHAPTER III</h3> + +<p class="cenhead">RECEIVING APPARATUS</p> + + <p>There are only two methods available at present for receiving the + photographs, and both have been used in ordinary photo-telegraphic work + with great success. They have disadvantages when applied to wireless + work, however, but these will no doubt be overcome with future + improvements. The two methods are (1) by means of an ordinary + photographic process, and (2) by means of an electrolytic receiver.</p> + + <p>In several photo-telegraphic systems the machine used for transmitting + has the cylinder twice the size of the receiving cylinder, thus making + the area of the received picture one-quarter the area of the picture + transmitted. The extra quality of the received picture does not + compensate for the disadvantage of having to provide two machines at each + station, and in the writer's opinion results, quite good enough for all + practical purposes, can be obtained by using a moderate size cylinder so + that one machine answers for both transmitting <!-- Page 38 --><span + class="pagenum"><a name="page38"></a>{38}</span>and receiving, and using + as fine a line screen as possible for preparing the photographs.</p> + + <div class="figcenter" style="width:30%;"> + <a href="images/illo-fig18.png"><img style="width:100%" src="images/illo-fig18.png" + alt="Fig. 18." title="Fig. 18." /></a> + <span class="sc">Fig.</span> 18. + </div> + + <p>The writer, when first experimenting in photo-telegraphy, endeavoured + to make the receiving apparatus "self-contained," and one idea which was + worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and + is placed just within the focus of a lens which has a focal length of + <sup>3</sup>/<sub>4</sub> inch. When a source of light is placed at some + point between a lens and its principal focus, the light rays are not + converged, but are transmitted in a parallel beam the same size as the + lens. It has been found that this arrangement gives a sharper line on the + drum than would be the case were the light focussed direct upon the hole + in the cone A. An enlarged drawing of the cone is given in Fig. 19. The + hole in the tip of the cone A is a bare <sup>1</sup>/<sub>90</sub> inch + in diameter—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 <!-- Page 39 --><span + class="pagenum"><a name="page39"></a>{39}</span>drum without being in + actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and + the armature is made as light as possible. The spring to which the + armature is attached should be of such a length that its natural period + of vibration is equal to the number of contacts made by the transmitting + stylus. The spring must be stiff enough to bring the armature back with a + fairly crisp movement. The spring and armature is shown separate in Fig. + 20.</p> + + <div class="figcenter" style="width:23%;"> + <a href="images/illo-fig19.png"><img style="width:100%" src="images/illo-fig19.png" + alt="Fig. 19." title="Fig. 19." /></a> + <span class="sc">Fig.</span> 19. + </div> + + <div class="figright" style="width:15%;"> + <a href="images/illo-fig20.png"><img style="width:100%" src="images/illo-fig20.png" + alt="Fig. 20." title="Fig. 20." /></a> + <span class="sc">Fig.</span> 20. + </div> + + <p>The shutter C is about <sup>1</sup>/<sub>4</sub> inch square and made + from thin aluminium. The hole in the centre is <sup>1</sup>/<sub>16</sub> + × <sup>1</sup>/<sub>8</sub> inch, and the movement of the armature is + limited to about <sup>3</sup>/<sub>32</sub> inch. In all arrangements of + this kind there is a tendency for the armature spring to vibrate, as it + were, sinusoidally, if the coil is magnetised and demagnetised at a + higher rate than the natural period of vibration of the spring. <!-- Page + 40 --><span class="pagenum"><a name="page40"></a>{40}</span>This causes + an irregularity in the rate of the vibrations which affects the received + image very considerably. A photographic film is wrapped round the drum of + the machine, being fastened by means of a little celluloid cement smeared + along one edge.</p> + + <p>This device, although it will work well over artificial conductors, is + not suitable for wireless work, as it is too coarse in its action; it can + be made sensitive enough to work at a speed of 1000 to 1500 contacts per + minute, with a current of .5 milliampere. It is impossible to obtain a + current of this magnitude from the majority of the detectors in use, so + that if any attempt is made to use this device for radio-photography it + will be necessary to employ a Marconi coherer (filings), as this is + practically the only coherer from which so large a current can be + obtained.</p> + + <p>There have been many attempts made to receive with an ordinary filings + coherer, but as was pointed out in Chapter I. these have now been + discarded in serious wireless work, being only used in small amateur + stations or experimental sets. As the reasons for this are well known to + the majority of wireless workers there is no need to enumerate them + here.</p> + + <p>A method whereby a filings coherer can be decohered, the act of + decohering closing a local circuit which contains the photographic <!-- + Page 41 --><span class="pagenum"><a + name="page41"></a>{41}</span>receiving apparatus, is given in the diagram + Fig. 21.</p> + + <div class="figcenter" style="width:36%;"> + <a href="images/illo-fig21.png"><img style="width:100%" src="images/illo-fig21.png" + alt="Fig. 21." title="Fig. 21." /></a> + <span class="sc">Fig.</span> 21. + </div> + + <p>In the figure, the coherer C is fixed in rigid supports, one support + being provided with a platinum pin F. To the coherer is connected the + sensitive electro-magnet M, which becomes magnetised as soon as the + incoming waves act upon the coherer. To the armature B is attached a + light aluminium arm S, pivoted at K, and carrying at the other end the + striker G, which is fitted with a platinum contact. When the armature B + is attracted the coherer is decohered by the force of the impact between + the contacts F and G. To prevent damage to the coherer the force of the + blow is taken off by the ability of the striker to work back through a + hole in the arm S, the spring <!-- Page 42 --><span class="pagenum"><a + name="page42"></a>{42}</span>N keeping it normally in a fixed position. T + and P are adjusting screws, and the terminals J are for connecting to the + receiving apparatus. With this arrangement a very short wave-train causes + only one tap of the contacts, so that only one mark is registered on the + receiving drum for every contact made on the transmitter.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig22.png"><img style="width:100%" src="images/illo-fig22.png" + alt="Fig. 22." title="Fig. 22." /></a> + <span class="sc">Fig.</span> 22. + </div> + + <p>The drawing, Fig. 22, gives a diagrammatic representation of apparatus + arranged for another photographic method of receiving. The machine shown + in Fig. 6 is used in this case. A is the aerial, E earth, P primary of + oscillation-transformer, S secondary of transformer, C variable + condenser, C' block condenser, D detector, X two-way switch, T + telephone.</p> + + <p>A De' Arsonval galvanometer H is also connected to the switch X, so + that either the telephone or the galvanometer can be switched in. The + <!-- Page 43 --><span class="pagenum"><a + name="page43"></a>{43}</span>galvanometer can be made sensitive enough to + work with a current as small as 10<sup>-7</sup> of an ampere, with a + period of about <sup>1</sup>/<sub>150</sub>th of a second. The screen J + has a small hole about <sup>1</sup>/<sub>8</sub> inch diameter drilled in + the centre. Under the influence of the brief currents which pass through + the detector every time a group of waves is received, the mirror of the + galvanometer swings to-and-fro in front of the screen J, and allows the + light reflected from the source of light M to pass through the aperture + in the screen, on to the lens N.</p> + + <p>Round the drum V of the machine is wrapped a sensitive photographic + film, and this records the movements of the mirror which correspond to + the contacts on the half-tone print used in transmitting. Every time + current passes through the galvanometer, the light that is received from + M,<a name="NtA6" href="#Nt6"><sup>[6]</sup></a> passes through the + aperture in the screen J, and is focussed by the lens N to a point upon + the revolving film. As soon as the current ceases, the mirror swings back + to its original position, and the film is again in darkness. Upon being + developed a photograph, similar to the negative used for preparing the + metal print is obtained. If desired the apparatus can be so arranged that + the received picture is a positive instead of a negative.</p> + +<p><!-- Page 44 --><span class="pagenum"><a name="page44"></a>{44}</span></p> + + <p>The detector used should be a Lodge wheel-coherer or a Marconi + valve-receiver, as these are the only detectors that can be used with a + recording instrument. If the swing of the galvanometer mirror is too + great, a small battery with a regulating resistance can be inserted in + order to limit the movement of the mirror to a very short range; the + current of course flowing in an opposite direction to the current flowing + through the coherer.</p> + + <p>In this, as in all other methods of receiving, the results obtained + depend upon the fineness of the line screen used in preparing the metal + prints; and as already shown the fineness of the screen that can be used + is dependent upon the mechanical efficiency of the entire apparatus.</p> + + <p>Another system, and one that has been tried as a possible means of + recording wireless messages, is as follows. The wireless arrangements + consist of apparatus similar to that shown in Fig. 22, but instead of a + Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is + substituted for the reflecting galvanometer. The Einthoven galvanometer + consists of a very powerful electro-magnet, the pole pieces of which + converge almost to points. A very fine silvered quartz thread is + stretched between the pole pieces, as shown in Fig. 23, the tension being + adjustable. The period of swing is about <sup>1</sup>/<sub>250</sub>th of + a second. A hole is bored through the poles, and one of them is fitted + <!-- Page 45 --><span class="pagenum"><a + name="page45"></a>{45}</span><span class="figright" style="width:30%;"><a + href="images/illo-fig23.png"><img style="width:100%" + src="images/illo-fig23.png" alt="Fig. 23." title="Fig. 23." /></a><span + class="sc">Fig.</span> 23.</span> with a sliding tube which carries a + short focus lens N. The light from M passes through the magnets, and a + magnified image of the quartz thread is thrown upon the ebonite screen J. + This screen is provided with a fine slit, and when the galvanometer is at + rest the shadow of the thread just covers the slit in the screen and + prevents any light from M reaching the photographic film. Upon signals + being received the shadow of the thread moves to one side for a long or + short period, uncovering the slit, and allowing light to pass through. + The lens R concentrates the collected light to a point upon the revolving + film. The connections for the complete receiver are given in Fig. 24.</p> + + <p>The modified form of the Einthoven galvanometer, as arranged by + Professor Korn for use with his selenium machines for photo-telegraphy + over ordinary land lines, consists of two fine silver wires which are + displaced in a lateral direction between the pole pieces when traversed + by a current; the current passing through both wires in the same <!-- + Page 46 --><span class="pagenum"><a + name="page46"></a>{46}</span>direction. A small shutter of aluminium foil + is attached to the wires at the optical centre. The silver wires used are + <sup>1</sup>/<sub>1000</sub> inch in diameter, with a natural period of + about <sup>1</sup>/<sub>120</sub>th of a second; the length of wires free + to swing being usually about 5 cm.</p> + + <div class="figcenter" style="width:40%;"> + <a href="images/illo-fig24.png"><img style="width:100%" src="images/illo-fig24.png" + alt="Fig. 24." title="Fig. 24." /></a> + <span class="sc">Fig.</span> 24. + </div> + + <p>The period of the wires depends to a great extent upon their length + and diameter, and also upon their tension. By using short fine wires the + period can be made much smaller, but a greater current is required to + produce a similar displacement. Where the current available, as in + wireless telegraphy, is very small, and a definite displacement of the + wires is required, it is at once apparent that with wires of a given + diameter there is a limit to their length and therefore to the period. + Finer wires can be used, but here again there is a practical limit to + their fineness, although galvanometers have been constructed with a + single silvered quartz thread <sup>1</sup>/<sub>12000</sub>th of an inch + diameter, which, when placed in a powerful field, will give a good + displacement with a current as small as 10<sup>-8</sup> ampere. <!-- Page + 47 --><span class="pagenum"><a name="page47"></a>{47}</span></p> + + <p>With the apparatus arranged by the Poulsen Company, given in the + diagram, Fig. 17, for photographically recording wireless signals, the + current required to operate the galvanometer for signals transmitted at + the rate of 1500 a minute is 1 × 10<sup>-6</sup> ampere, while for + signals up to 2500 a minute a current about 5 × 10<sup>-6</sup> ampere is + necessary.</p> + + <p>Another very sensitive instrument, employed by M. Belin, and known as + Blondel's oscillograph, consists of two fine wires stretched between the + poles of a powerful electro-magnet, a small and very light mirror being + attached to the centre of the wires. The current passes down one wire and + up the other, and the wires, together with the mirror, are twisted to a + degree depending upon the strength of the received current. In order to + render the instrument dead-beat the moving parts are arranged to work in + oil. The light reflected from the mirror is made use of in a manner + similar to that shown in Fig. 22.</p> + + <p>In all photographic methods of receiving, the apparatus must be + enclosed in some way to prevent any extraneous light from reaching the + film, or better still placed in a room lighted only by means of a ruby + light.</p> + + <p>The following method is given more as a suggestion than anything else, + as I do not think it has been tried for wireless receiving, although it + is stated to have given some good results over <!-- Page 48 --><span + class="pagenum"><a name="page48"></a>{48}</span>ordinary land lines. It + is the invention of Charbonelle, a French engineer, and is quite an + original idea. His method consists of placing a sheet of carbon paper + between two sheets of thin white paper, and wrapping the whole tightly + round the drum of the machine. A hardened steel point is fastened to the + diaphragm of a telephone receiver, and this receiver is placed so that + the steel point presses against the sheets of paper. As the diaphragm and + steel point vibrates under the influence of the received currents marks + are made by the carbon sheet on the bottom paper.</p> + + <p>Over a line where a fair amount of current is available at the + receiver, the diaphragm would have sufficient movement to mark the paper, + but the movement would be very small with the current received from a + detector. This difficulty could no doubt be overcome to a certain extent + by making a special telephone receiver, with a large and very flexible + diaphragm, and wound for a very high resistance. The movement of an + ordinary telephone diaphragm for a barely audible sound is, measured at + the centre, about 10<sup>-6</sup> of a c.m. With a unit current the + movement at the centre is about <sup>1</sup>/<sub>700</sub>th of an inch. + Greater movement of the diaphragm could be obtained by connecting a + <i>Telephone relay</i> to the detector, and using the magnified current + from the relay to operate the telephone. <!-- Page 49 --><span + class="pagenum"><a name="page49"></a>{49}</span></p> + + <div class="figcenter" style="width:31%;"> + <a href="images/illo-fig25.png"><img style="width:100%" src="images/illo-fig25.png" + alt="Fig. 25." title="Fig. 25." /></a> + <span class="sc">Fig.</span> 25. + </div> + + <p>The telephone relay consists of a microphone C, Fig. 25, formed of the + two pieces of osmium iridium alloy. The contact is separated to a minute + degree partly by the action of the local current from F, which flows + through it and also through the winding W of the two magnet coils. The + local current from F assists in forming the microphone by rendering the + space between the contacts conductive. The vibrating reed P is fastened + to the metal frame (not shown) which carries a micrometer screw by which + the distance between the contacts can be accurately regulated. It will be + seen from Fig. 25 that the local circuit consists of a battery F (about + 1.5 volts), the microphone contacts C, the windings W, milliampere meter + B, and the terminals T, for connecting to the galvanometer or telephone, + all in <!-- Page 50 --><span class="pagenum"><a + name="page50"></a>{50}</span>series. On the top of the magnet cores N, S + is a smaller magnet D, wound with fine wire for a resistance of about + 4935 ohms, the free ends of the coils being connected to the detector + terminals. The working is as follows. Supposing the current from the + detector flows through D in such a way that its magnetism is increased, + the reed P will be attracted, the contacts opened, and their resistance + increased. It will be seen that the current from F is passed through the + coils W, in such a way as to increase the magnetism of the permanent + magnet, so that any opening of the microphone contact increases their + resistance, causes the current to fall, and weakens the magnets to such + an extent that the reed P can spring back to its normal position. On the + other hand, if the detector current flows through D in such a direction + as to decrease the magnetism in the permanent magnets, the reed P will + rise and make better contact owing to the removal of the force opposing + the stiffness of the reed. Owing to the decrease in the resistance of the + microphone, the strength of the local current will be increased, the + magnets strengthened, and the reed P will be pulled back to its original + position. This relay gives a greatly magnified current when properly + adjusted, the current being easily increased from 10<sup>-4</sup> to + 10<sup>-2</sup> amperes. It is also very sensitive, but needs careful + adjustment in order that the best results may <!-- Page 51 --><span + class="pagenum"><a name="page51"></a>{51}</span>be obtained. A greater + range of magnification can be obtained by placing two or more relays in + series.</p> + + <div class="figcenter" style="width:23%;"> + <a href="images/illo-fig26.png"><img style="width:100%" src="images/illo-fig26.png" + alt="Fig. 26." title="Fig. 26." /></a> + <span class="sc">Fig.</span> 26. + </div> + + <p>A very sensitive receiver designed by the writer is given in the + figures 26 and 27. To the centre of a telephone diaphragm is fastened a + light steel point P, and the movement of this point is communicated to + the aluminium arm D, which is pivoted at C. As will be seen the telephone + receiver is of special construction, it containing only one coil and + therefore only one core; by this means the movement of the diaphragm is + centralised. The coil is wound for a resistance of about 200 ohms, and + the diaphragm should be fairly thin but very resillient.</p> + + <div class="figcenter" style="width:23%;"> + <a href="images/illo-fig27.png"><img style="width:100%" src="images/illo-fig27.png" + alt="Fig. 27." title="Fig. 27." /></a> + <span class="sc">Fig.</span> 27. + </div> + + <p>To the free end of D is fastened the mirror T, made from thin + diaphragm glass about 1<sup>1</sup>/<sub>2</sub> centimetres diameter, + and having a focal length of 40 inches. Light from the lamp L is + transmitted by the lens N in a parallel beam to the mirror which <!-- + Page 52 --><span class="pagenum"><a + name="page52"></a>{52}</span>concentrates it to a point upon a hole + <sup>1</sup>/<sub>100</sub>th of an inch in diameter in the screen J. As + the telephone diaphragm vibrates under the influence of the received + signals the arm, and consequently the mirror, vibrates also, and the hole + in the screen J is constantly being covered and uncovered by the spot of + light. It will be seen from Fig. 27 that the ratio between the centre of + the mirror and the pivot C, and C and the steel point P is 10:1, so that + if a movement of <sup>1</sup>/<sub>20000</sub>th of an inch is obtained + at the centre of the diaphragm the mirror will move + <sup>1</sup>/<sub>2000</sub>th of an inch; and as the focal length of the + mirror is 40 inches a movement of <sup>1</sup>/<sub>50</sub>th inch is + given to the spot of light.</p> + + <p>This receiver is capable of working at a fairly high speed, as the + inertia of the moving parts is practically negligible; the weight of the + arm and mirror being less than 20 grains. The hole in the screen is made + slightly less in diameter than the traverse of the revolving cylinder, + the slight distance between the cylinder and the screen allowing the + light to disperse sufficiently to produce a line on the film of about the + right thickness.</p> + + <p>There are two other possible means of photographically receiving the + picture that upon investigation may yield some results; but it is + doubtful whether the current available, even that obtained from a + telephone relay, will be sufficient to produce the desired magnetic + effect, and the <!-- Page 53 --><span class="pagenum"><a + name="page53"></a>{53}</span>insertion of a second relay would detract + greatly from the efficiency by decreasing the speed of working. If rays + of monochromatic light from a lamp L, Fig. 28, pass through a Nicol prism + P (polarising prism), then through a tube containing CS<sub>2</sub> + (carbon bisulphide), afterwards passing through the second prism P' + (analysing prism), and if the two Nicol prisms are set at the polarising + angle, no light from L would reach the photographic film wrapped round + the drum V of the machine. Upon the tube being subjected to a field + produced by a current passing through the coil C, the refractive index of + the liquid will be changed, and light from L will reach the photographic + film.<a name="NtA7" href="#Nt7"><sup>[7]</sup></a></p> + + <div class="figcenter" style="width:31%;"> + <a href="images/illo-fig28.png"><img style="width:100%" src="images/illo-fig28.png" + alt="Fig. 28." title="Fig. 28." /></a> + <span class="sc">Fig.</span> 28. + </div> + + <p>The second method is rather more complicated, and is based upon the + fact that the kathode rays in a Crookes' tube can be deflected from their + course by means of a magnet. In Fig. 29 the kathode K of the X-ray tube + sends a kathode ray discharge through an aperture in the anode A, through + a small aperture in the ebonite screen J <!-- Page 54 --><span + class="pagenum"><a name="page54"></a>{54}</span>on to the drum V of the + machine, round which is wrapped a photographic film; A and K being + connected to suitable electrical apparatus. Upon the coil M being + energised, the kathode-ray is deflected from its straight-line course, + and the drum V is left in darkness.</p> + + <div class="figcenter" style="width:31%;"> + <a href="images/illo-fig29.png"><img style="width:100%" src="images/illo-fig29.png" + alt="Fig. 29." title="Fig. 29." /></a> + <span class="sc">Fig.</span> 29. + </div> + + <p>The method which is now going to be described is very ingenious, as it + makes use of what is known as an electrolytic receiver. This method of + receiving has proved to be the most practical and simple of all the + photo-telegraphic systems that have been devised.</p> + + <p>The application of this system to wireless reception is as follows. + The aerial A, and the earth E, are joined to the primary P of a + transformer, the secondary S being connected to a Marconi valve receiver + C. The valve receiver is connected to the battery B and silvered quartz + thread K of an Einthoven galvanometer (already described). The thread is + <sup>1</sup>/<sub>12000</sub>th of an inch in diameter, and will respond + to currents as small as 10<sup>-8</sup> of <!-- Page 55 --><span + class="pagenum"><a name="page55"></a>{55}</span>an ampere. The light from + M throws an enlarged shadow of the thread over a slit in the screen J, + and as the thread moves to one side under the influence of a current, the + slit in J is uncovered, and the light from M is thrown upon a small + selenium cell R. In the dark the selenium cell has a very high + resistance, and therefore no current can flow from the battery D to the + relay F. When the string of the galvanometer moves to one side and + uncovers the slit in the screen J, a certain amount of light is thrown + upon the selenium cell lowering its resistance, allowing sufficient + current to pass through to operate the relay.</p> + + <p>Round the drum of the machine (shown in Fig. 7) is wrapped a sheet of + paper that has been soaked in certain chemicals that are decomposed on + the passage of an electric current through them. As soon as the local + circuit of the relay is closed, the current from the battery Z (about 12 + volts) flows through the paper and produces a coloured mark. The picture, + therefore, is composed of long or short marks which correspond to the + varying strips of conducting material on the single line print. In order + to render the marks short and crisp, a small battery Y, and regulating + resistance L, is placed across the drum and stylus. The diagram, Fig. 30, + gives the connections for the complete receiver. <!-- Page 56 --><span + class="pagenum"><a name="page56"></a>{56}</span></p> + + <p>The paper used is soaked in a solution consisting of</p> + +<table class="nob" summary="Soaking solution." title="Soaking solution."> +<tr><td class="nspacsingle"> Ferrocyanide of potassium </td><td class="nspacsingle" style="text-align:right;"> <sup>1</sup>/<sub>4</sub> oz.</td></tr> +<tr><td class="nspacsingle"> Ammoniac Nitrate </td><td class="nspacsingle" style="text-align:right;"> <sup>1</sup>/<sub>2</sub> oz.</td></tr> +<tr><td class="nspacsingle"> Distilled water<a name="NtA8" href="#Nt8"><sup>[8]</sup></a> </td><td class="nspacsingle" style="text-align:right;"> 4 oz.</td></tr> +</table> + + <div class="figcenter" style="width:40%;"> + <a href="images/illo-fig30.png"><img style="width:100%" src="images/illo-fig30.png" + alt="Fig. 30." title="Fig. 30." /></a> + <span class="sc">Fig.</span> 30. + </div> + + <p>The paper has to be very carefully chosen, as besides being absorbent + enough to remain moist during the whole of the receiving, the surface + must also remain fairly smooth, as with a rough paper the grain shows + very distinctly, and if there is an excess of solution the electrolytic + marks are inclined to spread and so cause a blurred image. The writer + tried numerous specimens of paper before one could be found that gave + really satisfactory results. It was also found that when working in a + warm room the paper became nearly <!-- Page 57 --><span + class="pagenum"><a name="page57"></a>{57}</span>dry before the receiving + was finished, and the resistance of the paper being greatly increased + (this may be anything up to 1000 ohms), the marking became very faint. A + sponge moistened with the solution and applied to the undecomposed + portion of the paper, while still revolving, was found to help matters + considerably.</p> + + <p>Another experience which happened during the writer's early + experiments, the cause of which I am still unable to explain, occurred in + connection with the stylus. The stylus used consisted of a sharply + pointed steel needle, and after working for about three minutes it was + noticed that the lines were becoming gradually wider, finally running + into each other. Upon examination it was found that the point of the + needle had worn away considerably, becoming in fact, almost a chisel + point. Almost every needle tried acted in a similar manner, and to + overcome this difficulty the stylus shown in Fig. 31 was devised.</p> + + <p>It will be seen that it consists of a holder A, somewhat resembling a + drill chuck, fastened to the flat spring B in such a manner that the + angle the stylus makes to the drum can be altered. The needle consists of + a length of 36-gauge steel wire, and as this wears away slowly the jaws + of the holder can be loosened and a fresh length pushed through. The wire + should not project beyond the face of the holder more than + <sup>1</sup>/<sub>8</sub>th inch. The gauge <!-- Page 58 --><span + class="pagenum"><a name="page58"></a>{58}</span>of wire chosen would not + suit every machine, the best gauge to use being found by trial, but in + the writer's machine the pitch of the decomposition marks is much finer + than of those made by the commercial machines, and this gauge, with the + slight but unavoidable spreading of the marks, will produce a mark of + just the right thickness. As already mentioned, no explanation of this + peculiarity on the part of the stylus can be given, as there is nothing + very corrosive in the solution used, and the pressure of the stylus upon + the paper is so slight as to be almost negligible.</p> + + <div class="figcenter" style="width:30%;"> + <a href="images/illo-fig31.png"><img style="width:100%" src="images/illo-fig31.png" + alt="Fig. 31." title="Fig. 31." /></a> + <span class="sc">Fig.</span> 31. + </div> + + <p>No special means are required for fastening the paper to the drum, the + moist paper adhering quite firmly. Care should be taken, however, to + fasten the paper—which should be long enough to allow for a lap of + about <sup>1</sup>/<sub>4</sub> inch—in such a manner that when + working the stylus draws away from the edge of the lap and not towards + it.</p> + + <p>The current required to produce electrolysis is very small, about one + milliampere being sufficient. <!-- Page 59 --><span class="pagenum"><a + name="page59"></a>{59}</span>Providing that the voltage is sufficiently + high, decomposition will take place with practically "no current," it + being possible to decompose the solution with the discharge from a small + induction coil. The quantity of an element liberated is by weight the + product of time, current, and the electro-chemical equivalent of that + element, and is given by the equation W = <i>zct</i>, where</p> + + <div class="poem"> + <div class="stanza"> + <p>W = quantity of element liberated in grammes.</p> + <p><i>z</i> = electro-chemical equivalent,</p> + <p><i>c</i> = current in amperes,</p> + <p><i>t</i> = time in seconds.</p> + </div> + </div> + + <p>The chemical action that takes place is therefore very small, as the + intermittent current sent out from the transmitter in some cases only + lasts from <sup>1</sup>/<sub>50</sub>th to <sup>1</sup>/<sub>100</sub>th + a second.</p> + + <p>The decomposed marks on the paper are blue, and, as photographers + know, blue is reproduced in a photograph as a white, so that a photograph + taken of our electrolytic picture, which will of course be a blue image + upon a white ground, will be reproduced almost like a blank sheet of + paper. If, however, a yellow contrast filter is placed in front of the + camera lens, and an orthochromatic plate used, the blue will be + reproduced in the photograph as a dead black.</p> + + <p>There is one other point that requires attention. It will be noticed + that the metal print used for <!-- Page 60 --><span class="pagenum"><a + name="page60"></a>{60}</span>transmitting is a positive, since it is + prepared from a negative. The received picture will therefore be a + negative, making the final reproduction, if it is to be used for + newspaper work, a negative also. Obviously this is no good. The final + reproduction must be a positive, therefore the received picture must be + also a positive. To overcome this difficulty matters must be so arranged + at the receiving station that in the cases of Figs. 17, 18, 22, and 24, + the film is kept permanently illuminated while the stylus on the + transmitter is tracing over an insulating strip, and in darkness when + tracing over a conducting strip. In Fig. 30 the relay F should allow a + continuous current from Z to flow through the electrolytic paper, and + only broken when the resistance of the selenium cell is sufficiently + reduced to allow the current from D to operate the relay.</p> + + <p>The author has endeavoured to make direct positives on glass of the + picture to be transmitted, so that a negative metal print could be + prepared. The results obtained were not very satisfactory, but the method + tried is given, as it may perhaps be of interest. The plate used in the + camera has to be exposed three or four times longer than is required for + an ordinary negative. The exposed plate is then placed in a solution of + protoxalate of iron (ferrous oxalate) and left until the image shows + plainly through the back of the plate. It <!-- Page 61 --><span + class="pagenum"><a name="page61"></a>{61}</span>is then washed in water + and placed in a solution consisting of</p> + +<table class="nob" summary="Washing solution." title="Washing solution."> +<tr><td class="nspacsingle"> Distilled water </td><td class="nspacsingle" style="text-align:right;"> 1000 </td><td class="nspacsingle"> cc.</td></tr> +<tr><td class="nspacsingle"> Nitric acid </td><td class="nspacsingle" style="text-align:right;"> 2 </td><td class="nspacsingle"> cc.</td></tr> +<tr><td class="nspacsingle"> Sulphuric acid </td><td class="nspacsingle" style="text-align:right;"> 3 </td><td class="nspacsingle"> cc.</td></tr> +<tr><td class="nspacsingle"> Bichromate of potash </td><td class="nspacsingle" style="text-align:right;"> 105 </td><td class="nspacsingle"> grammes.</td></tr> +<tr><td class="nspacsingle"> Alum </td><td class="nspacsingle" style="text-align:right;"> 80 </td><td class="nspacsingle"> ,,</td></tr> +</table> + + <p>After being in this bath for about fifteen minutes the plate is again + well washed in water, and developed in the ordinary way. The first two + operations should be performed in the dark room, but the remaining + operations can be performed in daylight, once the plate has been placed + in the bichromate bath. As already stated, the results obtained were not + very satisfactory, and such a method is not now worth following up, as it + is comparatively easy so to arrange matters at the receiving station that + a positive or negative image can be received at will.</p> + + <p>It is necessary to connect the stylus of the receiving machine to the + positive pole of the battery Z, otherwise the marks will be made on the + underside of the paper. The electrolytic receiver, owing to the absence + of mechanical and electro-magnetic inertia, is capable of recording + signals at a very high speed indeed.</p> + + <p>"Atmospherics," which are such a serious nuisance in long-distance + wireless telegraphy, will also prove a nuisance in wireless photography, + <!-- Page 62 --><span class="pagenum"><a name="page62"></a>{62}</span>but + their effects will not be so serious in a photographic method of + receiving as they would be in the electrolytic system. In a photographic + receiver where the film is, under normal conditions, constantly + illuminated, the received signals (both the transmitted signals and the + atmospheric disturbances) will be recorded, after development, as + transparent marks upon the film, the remainder of the film being, of + course, perfectly opaque. By careful retouching the marks due to the + disturbances can be eradicated, a print upon sensitised paper having been + first obtained to act as a guide during the process.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 63 --><span class="pagenum"><a name="page63"></a>{63}</span></p> + +<h3>CHAPTER IV</h3> + +<p class="cenhead">SYNCHRONISING AND DRIVING</p> + + <p>Clockwork and electro-motors are the source of driving power that are + most suitable for photo-telegraphic work, and each has its superior + claims depending on the type of machine that is being used. For general + experimental work, however, an electro-motor is perhaps the most + convenient, as the speed can be regulated within very wide limits. For a + constant and accurate drive a falling weight has no equal, but the + apparatus required is very cumbersome and the work of winding both + tedious and heavy. This method of driving was at one time universally + employed with the Hughes printing telegraph, but it has now been + discarded in favour of electro-motors, which are more compact, besides + being cheaper to instal in the first instance.</p> + + <p>Synchronising and isochronising the two machines are the most + difficult problems that require solving in connection with wireless + photography, and as previously mentioned, the <!-- Page 64 --><span + class="pagenum"><a name="page64"></a>{64}</span>synchronising of the two + stations must be very nearly perfect in order to obtain intelligible + results. The limit of error in synchronising must be about 1 in 500 in + order to obtain results suitable for publication.</p> + + <p>The electrolytic system is perhaps the easiest to isochronise, as the + received picture is visible. On the metal print used for transmitting, + and at the commencing edge a datum line is drawn across in insulating + ink. The reproduction of this line is carefully observed by the operator + in charge of the receiving instrument, and the speed of the motor is + regulated until this line lies close against a line drawn across the + electrolytic paper. Although this may seem an ideal method there are one + or two considerations to be taken into account. Unless the decomposition + marks are made the correct length and are properly spaced, however good + the isochronising may be, the result will be a blurred image. Any one who + has worked with a selenium cell, will know that it cannot change from its + state of high resistance to that of low resistance with infinite + rapidity, and the effects of this inertia, or "fatigue" as it has been + called, are more pronounced when working at a high speed. In working, the + effects of this inertia would be to increase the time of contact of the + relay F (Fig. 30) as the current from D would flow for a slightly longer + period through R to F than the period of <!-- Page 65 --><span + class="pagenum"><a name="page65"></a>{65}</span>illumination allowed by + K. This, of course, would mean a lengthening of the marks on the paper; + results would also differ greatly with different selenium cells. There is + a method of compensation by which the inertia of a cell can almost + entirely be overcome, but it would add greatly to the complicacy of the + receiving apparatus.</p> + + <p>In using an electro-motor with any optical method of receiving there + are two methods available. The first is an arrangement similar to that + used by Professor Korn in his early experiments with his selenium + machines. The motor used for driving has several coils in the armature + connected with slip rings, from which an alternating current may be + tapped off; the motor acting partially as a generator, besides doing good + work as a motor in driving the machine. This alternating current is + conducted to a frequency meter, which consists of a powerful + electro-magnet, over which are placed magnetised steel springs, having + different natural periods of vibration. By means of a regulating + resistance the motor is run until the spring which has the same period as + the desired armature speed vibrates freely. The speed of the motors at + both stations can thus be adjusted with a fair amount of accuracy. + Another method is to make use of a governor similar to those employed in + the Hughes printing telegraph system. A drawing of the governor is given + in Fig. 32. It consists of a <a name="page66"></a> <span class="figright" + style="width:33%;"><a href="images/illo-fig32.png"><img + style="width:100%" src="images/illo-fig32.png" alt="Fig. 32." title="Fig. 32." + /></a><span class="sc">Fig.</span> 32.</span> <!-- Page 67 --><span + class="pagenum"><a name="page67"></a>{67}</span>metal frame which + supports an upright steel bar S, whose ends turn on pivots. This bar is + rectangular in section. The gear-wheel G is fastened near the bottom of + this rod and gears with a similar wheel on the shaft of the driving motor + (not shown). Suspended from the broader sides of S are the two flexible + arms D, each carrying a brass ball T. These balls are not fastened to the + arms, but can slide up and down, being held in position by the wire + springs M, one end of each spring being fastened to the screws C. These + screws work in a slot cut in the upper part of S, and are connected to + the adjusting screw E. When E is turned the screws are raised or lowered + accordingly, and also the balls on the arms D.</p> + + <p>Fastened to the arms are two brushes of tow B, and these revolve + inside but just clearing the inner surface of the steel ring Z. Upon the + motor speed increasing above the normal the arms D, and consequently the + balls T, swing out, making a larger circle, causing the brushes B to + press against the steel ring Z, setting up friction which, however, is + reduced as soon as the motor regains its ordinary working speed. By + careful adjustment the speed of the motors can be kept perfectly + constant. The object of having the balls T adjustable on D, is to provide + a means of altering the motor speed, as the lower the balls on D the + slower the mechanism runs, and <i>vice versa</i>. <!-- Page 68 --><span + class="pagenum"><a name="page68"></a>{68}</span></p> + + <p><span class="figleft" style="width:27%;"><a + href="images/illo-fig33.png"><img style="width:100%" + src="images/illo-fig33.png" alt="Fig. 33." title="Fig. 33." /></a><span + class="sc">Fig.</span> 33.</span></p> + + <p>A simple and effective speed regulator devised by the writer is given + in drawings 33 and 34. It comprises two parts, A and B, the part A being + connected to the driving motor, and the part B working independently. The + independent portion B consists of an ordinary clock movement M, a steel + spindle J being geared to one of the slower moving wheels, so that it + makes just one revolution in two seconds. This spindle, which runs in two + coned bearings, carries at its outer end a light <span class="figleft" + style="width:36%;"><a href="images/illo-fig34.png"><img + style="width:100%" src="images/illo-fig34.png" alt="Fig. 34." title="Fig. 34." + /></a><span class="sc">Fig.</span> 34.</span> pointer D, about two inches + long, to the underside of which is fastened the thin brass contact spring + S, which presses lightly upon the ebonite ring N. <!-- Page 69 --><span + class="pagenum"><a name="page69"></a>{69}</span>The portion A comprises a + spindle, pointer, and contact spring similar to those employed in B, the + spindle J' being geared to the driving motor by means of F, so that the + pointer D' makes a little more than one revolution in two seconds. By + means of a special form of brake on the driving motor, the speed is + reduced, so that both pointers travel at the same rate, viz. one + revolution in two seconds. By careful adjustment the two pointers can be + made to revolve in synchronism,<a name="NtA9" + href="#Nt9"><sup>[9]</sup></a> and when this is obtained the contact + springs S, S', pass over the contacts C, C', completing the circuit of + the battery B and lamp L. When working properly the lamp L lights up + regularly once every second. This regulator is an excellent one to use + for experimental work, although it depends a great deal upon the skill of + the operator, but good adjustment should be obtained in about two + minutes. It is a good plan to insert a clutch of some description between + the driving motor and the machine, so that the regulator can be adjusted + prior to the act of receiving or transmitting, the machine being + prevented from revolving by means of a catch. The motor used should be + powerful enough to take up the work of driving the machine without any + reduction in speed. The clocks M can be regulated so that they only gain + or lose a few seconds in <!-- Page 70 --><span class="pagenum"><a + name="page70"></a>{70}</span>twenty-four hours, which gives an accuracy + in working sufficient for all practical purposes.</p> + + <p>Connection is made with the contact springs S, S', by means of the + springs T, T', which press against the spindles J, J'.</p> + + <p>Another important point is the correct placing of the picture upon the + receiving drum. It is necessary that the two machines besides revolving + in perfect isochronism should synchronise as well, <i>i.e.</i> begin to + transmit and record at exactly the same position on the cylinders, viz. + at the edge of the lap, so that the component parts of the received image + shall occupy the same position on the paper or film as they do on the + metal print. If the receiving cylinder had, let us suppose, completed a + quarter of a revolution before it started to reproduce, the reproduction + when removed from the machine and opened out will be found to be + incorrectly placed; the bottom portion of the picture being joined to the + top portion, or <i>vice versa</i>, and this means that perhaps an + important piece of the picture would be rendered useless even if the + whole is not spoilt. It is evident, therefore, that some arrangement must + be employed whereby synchronism, as well as isochronism of the two + instruments can be maintained.</p> + + <p>There are several methods of synchronising that are in constant use in + high-speed telegraphy, in which the limit of error is reduced to a + minimum, <!-- Page 71 --><span class="pagenum"><a + name="page71"></a>{71}</span>and some modification of these methods will + perhaps solve the problem, but it must be remembered that synchronism is + far easier to obtain where the two stations are connected by a length of + line than where the two stations are running independently.</p> + + <p>In one system of ordinary photo-telegraphy synchronism is obtained in + the following manner. The receiving cylinder travels at a speed slightly + in excess of the transmitting cylinder, and as its revolution is finished + first is prevented from revolving by a check, and when in this position + the receiving apparatus is thrown out of circuit and an electro-magnet + which operates the check is switched in. When the transmitting cylinder + has completed its revolution (about <sup>1</sup>/<sub>100</sub>th of a + second later) the transmitting apparatus, by means of a special + arrangement, is thrown out of circuit for a period, just long enough for + a powerful current to be sent through the line. This current actuates the + electro-magnet. The check is withdrawn and the receiving cylinder + commences a fresh revolution in perfect synchronism with the transmitting + cylinder. As soon as the check is withdrawn the receiving apparatus is + again placed in circuit until another revolution is completed. As the + receiver cannot stop and start abruptly at the end of each revolution a + spring clutch is inserted between the driving motor and the machine. <!-- + Page 72 --><span class="pagenum"><a name="page72"></a>{72}</span></p> + + <p>Although a method of synchronising similar to this may later on be + devised for wireless photography, the writer, from the result of his own + experiments, is led to believe that results good enough for all practical + purposes can be obtained by fitting a synchronising device whereby the + two machines are started work at the same instant, and relying upon the + perfect regulation of the speed of the motors for correct working.</p> + + <p>The method of isochronism must, however, be nearly perfect in its + action, as it is easy to see that with only a very slight difference in + the speed of either machine this error will, when multiplied by 40 or 50 + revolutions, completely destroy the received picture for practical + purposes.</p> + + <p>From what has been written in this and in the preceding chapters it + will be evident that the successful solution of transmitting photographs + by wireless methods will necessitate the use of a great many pieces of + apparatus all requiring delicate adjustment, and depending largely upon + each other for efficient working. As previously stated, there is at + present no real system of wireless photography, the whole science being + in a purely experimental stage, but already Professor Korn has succeeded + in transmitting photographs between Berlin and Paris, a distance of over + 700 miles. If such a distance could be worked over successfully, there is + no reason to doubt that before long <!-- Page 73 --><span + class="pagenum"><a name="page73"></a>{73}</span>we shall be able to + receive pictures from America with as great reliability and precision as + we now receive messages.</p> + + <p>In nearly all wireless photographic systems devised up to the present + the chief portion of the receiver consists of a very sensitive + galvanometer, and although very good results have been obtained by their + use they are more or less a nuisance, as the extreme delicacy of their + construction renders them liable to a lot of unnecessary movement caused + by external disturbances. A galvanometer of the De' Arsonval pattern, + used by the writer, was constantly being disturbed by merely walking + about the room, although placed upon a fairly substantial table; and for + the same reason it was impossible to attempt to place the driving motor + of the machine on the same table as the galvanometer. For ship-board work + it will be evident that the use of such a sensitive instrument presents a + great difficulty to successful working, and a good opening exists for + some piece of apparatus—to take the place of the + galvanometer—that will be as sensitive in its action but more + robust in its construction.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 74 --><span class="pagenum"><a name="page74"></a>{74}</span></p> + +<h3>CHAPTER V</h3> + +<p class="cenhead">THE "TELEPHOGRAPH"</p> + + <p>In the present chapter it is proposed to give a brief description of a + system of radio-photography devised by the author, and which includes a + greatly improved method of transmitting and receiving, as well as an + ingenious arrangement for synchronising the two stations; the whole being + an attempt to produce a system that would be capable of working + commercially over fairly long distances.</p> + + <p>The system about to be described, and which I have designated the + "telephograph," is the outcome of several years' original experimental + work, many difficulties that were manifest in the working of the earlier + systems having been overcome by apparatus that has been expressly + designed for the purpose.</p> + + <p>In any practical system of radio-photography the following points are + of great importance: (1) the speed of transmission; (2) the quality of + the received picture; (3) the method of synchronising <!-- Page 75 + --><span class="pagenum"><a name="page75"></a>{75}</span>the two machines + so that transmission and reception begin simultaneously; (4) the correct + regulation of the speed of the driving motors; (5) the simplicity and + reliability of the entire arrangement. Points 1 and 2 are dependent upon + several factors; the number of contacts made by the stylus per minute; + the size of the metal print used; the number of lines per inch on the + screen used in preparing the print; and the accurate and harmonious + working of the various pieces of apparatus employed.</p> + + <p>In the system under discussion the size of the metal print used is 5 + inches by 7 inches, and a screen having 50 lines to the inch is used for + preparing it. With the drum of the machine making one revolution in four + seconds, the stylus makes 87 contacts per second, or 5220 a minute, the + time for complete transmission being twenty-five minutes. By the use of + ordinary relays not more than 2000 contacts a minute can be obtained, and + in the present system it is only by means of a specially designed relay + that such a high rate of working has been made possible. Similarly, too, + with the receiving of such a large number of signals transmitted at such + a high speed, a special instrument has been devised that can record this + number of signals without any trouble, and could even record up to 8000 + signals a minute, provided that a suitable transmitter could be designed. + <!-- Page 76 --><span class="pagenum"><a + name="page76"></a>{76}</span></p> + + <p>In the present system the writer does not claim to have completely + solved the problem of the wireless transmission of photographs, but it is + a great advance on any system previously described, and the following + advantages are put forward for recognition: (1) a greatly improved method + of transmitting and receiving; (2) a simple method of regulating the + speed of the driving motors and maintaining isochronism with a limit of + error of less than 1 in 800; (3) an arrangement for synchronising the two + machines whereby transmitting and receiving begin simultaneously; (4) the + use of one machine only at each station.</p> + +<p class="cenhead"><span class="sc">Transmitting Apparatus</span></p> + + <p>A diagrammatic representation of the apparatus required for a complete + station, transmitting and receiving combined, is given in Fig. 35, the + usual wireless equipment having been omitted from the diagram to avoid + confusion.</p> + + <p><i>The Machine.</i>—This, as will be seen from Fig. 36, consists + of a base-plate M, to which are attached the two bearings B and B'. The + bearing B' is fitted with an internal thread to correspond with the + threaded portion of the shaft D. The drum V is a brass casting, being + fastened to the shaft by set screws. The shaft is threaded 75 to the + inch. The bearings are preferably of the concentric type. The circuit + breaker C is so arranged that when <!-- Page 77 --><span + class="pagenum"><a name="page77"></a>{77}</span>the drum has traversed + the required distance, the end of the shaft pushes back the spring M, + breaking the circuit of the driving gear and stopping the machine. The + machine is connected to the driving gear by the flexible coupling A.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig35.png"><img style="width:100%" src="images/illo-fig35.png" + alt="Fig. 35" title="Fig. 35" /></a> + <span class="sc">Fig.</span> 35. + + <p class="poem">M, motor; Y, isochroniser; F, clutch; A, machine; R, + stylus; S, relay; X, gearing; O, circuit breaker; T, receiver; C, + condenser; U, telephone relay; K, polarised relay; L, contact breaker; + D, D<sup>1</sup>, D<sup>2</sup>, D<sup>3</sup>, batteries; P, friction + brake; B, B<sup>1</sup>, double-pole two-way switches; N, + N<sup>1</sup>, N<sup>2</sup>, single switches; W, key; E, electric + clock; J, telephones.</p> + </div> + + <p>The drum measures 5 inches long by 2<sup>1</sup>/<sub>8</sub> inches + diameter, and this takes a metal print 5 inches by 7 inches, which allows + for a lap of about <sup>1</sup>/<sub>4</sub> inch. In working, the print + is wrapped tightly round the drum, being secured by means of a little + seccotine smeared along one edge. Care must be taken that the edge of the + lap draws away from the point of <!-- Page 78 --><span class="pagenum"><a + name="page78"></a>{78}</span>the stylus and not towards it. A margin of + bare foil, about <sup>1</sup>/<sub>8</sub> inch wide, should be left on + the print at the commencing edge, the purpose of which will be explained + later.</p> + + <div class="figcenter" style="width:36%;"> + <a href="images/illo-fig36.png"><img style="width:100%" src="images/illo-fig36.png" + alt="Fig. 36" title="Fig. 36" /></a> + <span class="sc">Fig. 36.</span> + </div> + + <p><i>The Stylus.</i>—As the drum of the machine travels laterally, + by reason of the threaded shaft and bearing, the stylus must necessarily + be a fixture. It consists of a holder B, drilled to take a hardened steel + point S, attached to the spring M. The spring is arranged to work in the + guide F, which is provided with an adjusting screw W for regulating the + pressure of the stylus upon the print; the pressure being sufficient to + enable good contact to be made, but must not be heavy enough to scratch + the soft foil. The needle should present an angle of about 60° to the + surface of the print, as this angle has been found to give the best + results in working.</p> + + <p>To eliminate any sparking that may take place at the point of make and + break, due to the self-induction of the relay coils, a condenser C, about + 1 microfarad capacity, should be connected across <!-- Page 79 --><span + class="pagenum"><a name="page79"></a>{79}</span>the drum and stylus. The + complete stylus is given in the drawings, Figs. 37, 37<i>a</i>, and also + in the diagrams Figs. 8 and 9.</p> + + <div class="figcenter" style="width:28%;"> + <a href="images/illo-fig37.png"><img style="width:100%" src="images/illo-fig37.png" + alt="Fig. 37" title="Fig. 37" /></a> + <span class="sc">Fig. 37.</span> + + <p class="poem">Showing the arrangement for sliding the stylus to or + from the machine.</p> + </div> + + <div class="figright" style="width:25%;"> + <a href="images/illo-fig37a.png"><img style="width:100%" src="images/illo-fig37a.png" + alt="Fig. 37a" title="Fig. 37a" /></a> + <span class="sc">Fig. 37</span><i>a</i>. + </div> + + <p><i>The Relay.</i>—As will be seen from the diagram, Fig. 38, + this consists of two electro-magnets having very soft iron cores, the + magnet M being wound in the usual manner, while the magnet N is wound + differentially. The armature A is made as light as possible, and is + pivoted at P, and when there is no current flowing through any of the + coils, is held midway between the magnet cores by the two spiral springs + S and T, which are under slight but equal tension. The connections are as + follows. The wires from the winding on M are connected directly to the + relay terminals F and H, as are also the wires from one winding on N. The + other winding on N is connected in series with the battery C, ammeter B, + and regulating resistance R. <!-- Page 80 --><span class="pagenum"><a + name="page80"></a>{80}</span></p> + + <div class="figcenter" style="width:36%;"> + <a href="images/illo-fig38.png"><img style="width:100%" src="images/illo-fig38.png" + alt="Fig. 38" title="Fig. 38" /></a> + <span class="sc">Fig. 38.</span> + </div> + + <p>When the circuit of the battery C is completed, the coil of N, to + which it is connected, is energised, and the armature A is attracted + against the stop V. When in this position the tension of the spring S is + released, while the tension of the spring T is increased. As soon as the + circuit of the battery D is completed by means of the metal line print on + the transmitting machine, the current divides at the terminals F and H, a + portion flowing through the magnet coil M, and a portion through the + remaining winding on N. The current which flows through the winding on N + produces a magnetising effect equal to that caused by the other winding + on N, but since the two windings are of equal length and resistance, and + since the current flowing through the two windings is of equal strength + but in opposite directions, the result is to neutralise <!-- Page 81 + --><span class="pagenum"><a name="page81"></a>{81}</span>the magnetising + effects produced by each winding, and consequently no magnetism is + produced in the cores.</p> + + <p>The other portion of the current from D flows through the coil M, and + it becomes magnetised at the same time that the coil N becomes + demagnetised. The armature A is attracted by M against the stop X, and + this attraction is assisted by the spring T, which was under increased + tension. The conditions of the springs are now reversed, the spring S + being under increased tension, while the tension of the spring T is + released.</p> + + <p>As soon as the current from D is broken, the magnetism disappears from + M, the neutralising current in N ceases, and N once more becomes + magnetised, owing to the current which still flows through one winding + from C; the armature is therefore again attracted by N, assisted by the + spring S. The current flowing through the two windings of N must be + perfectly equal, and the regulating resistance R, and ammeters B and B', + are inserted for purposes of adjustment. The current from C must flow in + a direction opposite to that which flows from D.</p> + + <div class="figright" style="width:24%;"> + <a href="images/illo-fig39.png"><img style="width:100%" src="images/illo-fig39.png" + alt="Fig. 39" title="Fig. 39" /></a> + <span class="sc">Fig. 39.</span> + + <p class="poem">H, H', containers; M, mercury; E, paraffin oil; T, T', + terminals; C, suspending rod; D, base; F, F', dipping rods.</p> + </div> + + <p>The local circuit of the relay is completed by means of a copper + dipper in mercury, somewhat resembling an ordinary mercury break, but + modified to suit the present requirements. The arrangement will be seen + from Fig. 39. The whole of the <!-- Page 82 --><span class="pagenum"><a + name="page82"></a>{82}</span>moving parts are made as light as possible, + and for this reason the rod C and the dippers F, F' should be made as + short as convenient. The containers H, H' are separate, of cast iron, and + rectangular in shape. The dipper is of very thin copper tube—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 + <sup>1</sup>/<sub>4</sub> inch, when the armature of the relay is in its + normal position. To prevent undue churning of the mercury, which would + necessarily take place if the dipper entered and left the mercury at each + movement of the armature, a pointed ebonite plug is inserted in the end + of the tube. This will be found to give good results at a high speed, the + mercury being practically undisturbed, and the production of "sludge" + reduced to a minimum. To prevent oxidation of the mercury, and to prevent + arcing, the surface is covered with paraffin oil. If this is not + sufficient to prevent arcing a condenser should be shunted across the + <!-- Page 83 --><span class="pagenum"><a + name="page83"></a>{83}</span>containers. The volume of mercury, and the + area of the dippers, should be sufficient to carry the current used for a + considerable period without heating up to any extent. An adjustable + weight J is provided in order to balance the armature and dipping + rod.</p> + + <p>The remaining transmitting apparatus consists of the battery + D<sup>2</sup> and the usual wireless apparatus. The double-pole two-way + switch B' is to enable the photo-telegraphic set to be switched out and + the hand key W switched in for ordinary signalling purposes. The battery + D<sup>2</sup> should be about 12 volts.</p> + +<p class="cenhead"><span class="sc">Receiving Apparatus</span></p> + + <p>The wireless portion of the receiver is similar to that given in Fig. + 22, is of the usual syntonic type, and comprises an oscillation + transformer, S being the secondary, and P the primary; C' is a block + condenser, and C a variable condenser. The detector D is of the + carborundum crystal or electrolytic pattern. A two-way switch B is + provided so that the relay U can be switched out and the telephones J + switched in for ordinary receiving purposes. The relay U is a Brown's + telephone relay.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig40.png"><img style="width:100%" src="images/illo-fig40.png" + alt="Fig. 40." title="Fig. 40." /></a> + <span class="sc">Fig.</span> 40. + </div> + + <p><i>The Receiver.</i>—The magnified current from the relay U is + taken to a special telephone receiver, the construction of which is given + in Fig. 40. The diaphragm F is about 2<sup>1</sup>/<sub>2</sub> inches + diameter, and should be fairly thin but very resilient. Only one <!-- + Page 84 --><span class="pagenum"><a name="page84"></a>{84}</span><span + class="figleft" style="width:13%;"><a href="images/illo-fig41.png"><img + style="width:100%" src="images/illo-fig41.png" alt="Fig. 41." title="Fig. 41." + /></a><span class="sc">Fig.</span> 41.</span> <span class="figright" + style="width:26%;"><a href="images/illo-fig41a.png"><img + style="width:100%" src="images/illo-fig41a.png" alt="Fig. 41a." + title="Fig. 41a." /></a><span class="sc">Fig.</span> 41a.</span> coil is + provided, and this should be wound with No. 47 S.S.C. copper wire for a + resistance of about 2000 ohms. By using only one coil and therefore only + one core, the movement of the diaphragm is centralised. To the centre of + the diaphragm a light steel point is fastened, about + <sup>1</sup>/<sub>2</sub> inch long, and provided with a projecting hook + H. An enlarged view of this pin is given in Fig. 41. The movement of the + diaphragm and consequently of the steel point P is communicated to a + pivoted rod R, which is of special construction. A piece of aluminium + tube 3<sup>3</sup>/<sub>4</sub> inches long, and of the section given at + B, is bushed at one end with a piece of brass of the shape shown in Fig. + 41<i>a</i>. A stiff steel wire T about 1 inch long (20 gauge) is screwed + into the end of Z, and carries a counterbalance weight C. A hardened <!-- + Page 85 --><span class="pagenum"><a name="page85"></a>{85}</span>steel + spindle, pointed at both ends, is fastened at D, and runs between two + coned bearings, one of which is adjustable. The underside of Z is + flattened, and a small coned depression is made for the reception of the + pointed end of the pin. By means of the spring J the two pieces, Z and P, + are held firmly together, at the same time allowing perfect freedom of + movement. The bridge G is made from a piece of sheet aluminium placed in + a slot cut in the tube R, the end of the tube being pressed tight upon G, + and secured by means of a small rivet.</p> + + <p>The optical arrangements are as follows. By means of the Nernst lamp + L, and the lenses B and B', Figs. 42 and 43, a magnified shadow of G is + thrown upon the screen J. When the shutter G is in its normal position + (<i>i.e.</i> at rest), its shadow is just above the small hole in J, and + light from L reaches the photographic film wrapped round the drum V of + the machine.</p> + + <div class="figright" style="width:23%;"> + <a href="images/illo-fig42.png"><img style="width:100%" src="images/illo-fig42.png" + alt="Fig. 42." title="Fig. 42." /></a> + <span class="sc">Fig.</span> 42. + + <p class="poem">J, screen; L, Nernst lamp; G, shutter; B, condensing + lens; B<sub>1</sub>, focussing lens.</p> + </div> + + <p>When, however, signals are sent out from the transmitting apparatus, + the magnified current from the relay U energises the coil of the special + telephone S, attracting the diaphragm F, and consequently giving movement + to the pivoted rod R. As by means of the optical arrangements a <!-- Page + 86 --><span class="pagenum"><a name="page86"></a>{86}</span>magnified + movement as well as a magnified image of G is thrown upon the screen J, + the shadow of G will, when the telephone S is actuated, cover the hole in + the screen, and prevent any light from reaching the film on V, until + current from the relay U ceases to flow. Therefore, when the stylus of + the transmitter traces over a conducting strip on the metal print, no + light reaches the film on V, but when tracing over an insulating strip + the shadow of G on the screen J rises, and the light from L reaches the + film. By this means a positive picture is received, which is a great + advantage where the photographs are required for reproduction. + Atmospherics would be represented by irregular transparent marks on the + film after development, and these can be easily eradicated by + retouching.</p> + + <div class="figcenter" style="width:26%;"> + <a href="images/illo-fig43.png"><img style="width:100%" src="images/illo-fig43.png" + alt="Fig. 43." title="Fig. 43." /></a> + <span class="sc">Fig.</span> 43. E, ebonite screen; F, focussing lens; + G, shutter; O, condensing lens; L, Nernst lamp. + </div> + + <p>The drum of the machine moves laterally <sup>1</sup>/<sub>75</sub>th + of an inch per revolution, and the hole in the screen is + <sup>1</sup>/<sub>90</sub>th of an inch in diameter. As the screen J is + not in direct contact with the film, the slight diffusion of the light + that takes place will produce <!-- Page 87 --><span class="pagenum"><a + name="page87"></a>{87}</span>a mark of about the right thickness. With a + movement of the diaphragm of only <sup>1</sup>/<sub>40000</sub>th of an + inch, the actual movement of G will be <sup>1</sup>/<sub>4000</sub>th of + an inch. If the optical arrangements have a magnifying power of 100, then + the movement of the shadow upon the screen will be + <sup>1</sup>/<sub>40</sub>th of an inch, which will be ample to cover the + aperture.</p> + + <p>The aluminium rod R, minus the counter-weight, can be made to weigh + not more than 12 grains. It is necessary to enclose the optical parts in + a light tight box, indicated by the dotted lines in Fig. 43, in order to + prevent any extraneous light from reaching the film.</p> + + <p><i>The Contact Breaker.</i>—The contact breaker (L, Fig. 35), as + will be seen from Fig. 44, consists of an electro-magnet N, the windings + of which are connected with the battery B and the polarised relay K. The + armature which is supported by the spring G carries a contact arm A, + which in its normal position makes permanent contact with the contact + screw T, and completes the circuit between the relay K and the telephone + relay U (Fig. 35). As soon as the transmitter sends out the first signal, + the magnified current from the telephone relay actuates the relay K, + which in turn completes the circuit of the contact breaker. Directly the + armature M has been attracted, the contact with T is broken, and A makes + fresh contact with the screw H, by means of the spring Z <!-- Page 88 + --><span class="pagenum"><a name="page88"></a>{88}</span>fastened to the + underside of A. The armature, once it has been attracted, is held in + permanent contact with H by the catch S, independent of the magnets N. As + soon as contact is made with H, the clutch (F, Fig. 35) circuit is + completed, and the circuit of the relay K is broken. When the circuit of + the clutch F is broken by means of the circuit breaker C on the machine + (Fig. 36), the stop S is pulled back by hand, allowing the contact arm A + to rise, and again make fresh contact with the contact screw T.</p> + + <div class="figcenter" style="width:38%;"> + <a href="images/illo-fig44.png"><img style="width:100%" src="images/illo-fig44.png" + alt="Fig. 44." title="Fig. 44." /></a> + <span class="sc">Fig.</span> 44. + </div> + +<p class="cenhead"><span class="sc">Driving Apparatus</span></p> + + <p><i>The Friction Brake.</i>—This consists of a steel disc A, Fig. + 45, about 2<sup>1</sup>/<sub>2</sub> inches diameter and + <sup>3</sup>/<sub>8</sub> inch or <sup>1</sup>/<sub>2</sub> inch wide on + the face, secured to the main shaft of the driving motor. The arm H, + pivoted at C, carries at one end the curved block B, which is faced with + a pad of tow F. The other extremity is pivoted to the steel rod P, which + slides <!-- Page 89 --><span class="pagenum"><a + name="page89"></a>{89}</span><span class="figright" style="width:23%;"><a + href="images/illo-fig45.png"><img style="width:100%" + src="images/illo-fig45.png" alt="Fig. 45." title="Fig. 45." /></a><span + class="sc">Fig.</span> 45.</span> in holes bored in the standards J. One + end of the rod P is screwed with a fine thread, about 75 to the inch, and + is fitted with a regulating wheel T, by means of which the block B can be + made to press upon the disc A with any required degree of pressure. A + fairly stiff steel spring R is placed upon the rod P, between one + standard J and the collar N. As the speed of the driving motor is + slightly in excess of that required by the machine, the block B, by means + of the wheel, is made to press upon the disc A, setting up friction which + reduces the motor speed until the isochroniser indicates that the correct + working speed has been attained.</p> + + <p><i>The Clutch</i>.—The details of this will be seen from Figs. + 46 and 47. It consists of a steel shaft coned at both ends running + between two countersunk bearings, one of which is adjustable. This shaft + carries the two portions of the clutch A and B, the portion A being a + fixture on the shaft, and the portion B running free upon it. The portion + B is a gun-metal casting bored to run accurately upon the steel shaft. A + soft iron annular ring is fastened to the face.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig46.png"><img style="width:100%" src="images/illo-fig46.png" + alt="Fig. 46." title="Fig. 46." /></a> + <span class="sc">Fig. 46.</span> + + <p class="poem">E, spindle; R, bobbins; P, iron cores; D, copper rings; + T, brushes; N, back plate; V, front plate; J, gearing; S, spring; H, + collar; Z, iron ring; F, fixed bearing; C, insulating bush.</p> + </div> + + <p>The portion A consists of a gun-metal casting <!-- Page 90 --><span + class="pagenum"><a name="page90"></a>{90}</span><span class="figright" + style="width:38%;"><a href="images/illo-fig47.png"><img + style="width:100%" src="images/illo-fig47.png" alt="Fig. 47." title="Fig. 47." + /></a><span class="sc">Fig.</span> 47.</span> bored a tight fit for the + shaft E, secured by means of a set screw. The two magnet cores P are + screwed into the front plate V, which is also of gun-metal, and after the + bobbins R have been slipped on, the shanks of the cores are passed + through holes drilled in the flange N of the main casting and held in + place with nuts. The faces of both A and B must be turned perfectly + square with the shaft, so that they run accurately together. The portion + B is <!-- Page 91 --><span class="pagenum"><a + name="page91"></a>{91}</span>kept in contact with A by means of a spring + S, the pressure being regulated by the collar H. Current is taken to the + magnets by means of the two insulated copper rings D mounted upon the + body of A. The gear-wheels on both portions have teeth of very fine + pitch, the number of teeth on each being regulated by the speed of the + driving motor and the required machine speed. Connection with the circuit + breaker L and the battery B<sup>2</sup> is made with the collecting rings + D by the brushes T. The complete connections are given in the diagram + Fig. 51.</p> + + <p><i>The Isochroniser.</i>—This is a device for ensuring the + correct speed regulation of the driving motors, and is shown in detail in + Fig. 48. It comprises two portions, one portion being rotated at a + definite speed by electrical means, and the other portion rotated by the + driving motor.</p> + + <p>The main portion consists of a metal tube N, bushed at both ends, the + bottom end of the tube being arranged to work on ball-bearings. An + ebonite bush C carries three copper rings T, T<sup>1</sup>, + T<sup>2</sup>, and the brushes R, R<sup>1</sup>, R<sup>2</sup> are in + electrical contact with them. The ebonite plate J, + 3<sup>1</sup>/<sub>2</sub> inches diameter, is secured to the top end of + N, and carries a contact piece Q, shown separate at E. As will be seen + this is a block of ebonite with three contacts arranged on the top + surface. The middle contact P is <sup>1</sup>/<sub>64</sub>th of an inch + wide, and the contacts P<sup>1</sup> <!-- Page 92 --><span + class="pagenum"><a name="page92"></a>{92}</span>and P<sup>2</sup> are + placed on either side at a distance of <sup>1</sup>/<sub>16</sub> inch; + the contact strips P<sup>1</sup>, P<sup>2</sup> carry the brass pins D, + which are about <sup>1</sup>/<sub>16</sub> inch diameter, and spaced + <sup>3</sup>/<sub>8</sub> inch apart. A connecting wire is carried from + the contact P to the copper ring T, another from P<sup>1</sup> to + T<sup>1</sup>, and one from P<sup>2</sup> to T<sup>2</sup>.</p> + + <div class="figcenter" style="width:38%;"> + <a href="images/illo-fig48.png"><img style="width:100%" src="images/illo-fig48.png" + alt="Fig. 48." title="Fig. 48." /></a> + <span class="sc">Fig. 48.</span> + + <p class="poem">N, brass tube; S, bushes; G, ball-bearing; H, + gear-wheel; T, T<sup>1</sup>, T<sup>2</sup>, copper rings; C, + insulating block; R, R<sup>1</sup>, R<sup>2</sup>, brushes; J, ebonite + disc; Q, contact block; D, metal pins; O, pulley, P, P<sup>1</sup>, + P<sup>2</sup>, contact plates; K, needle; Z, spring; W, steel rod; E, + countersunk bearing.</p> + </div> + + <p>The bushes S are bored a running fit for the steel rod W (shown + separate at A), which is coned at both ends, and runs between two + countersunk bearings, the bottom bearing E being fixed while <!-- Page 93 + --><span class="pagenum"><a name="page93"></a>{93}</span>the top bearing + (not shown) is adjustable. A needle K is fastened near the end of the rod + W, and attached to this needle is the spring Z, which presses lightly but + firmly upon the contact block Q. To provide a level surface for Z to work + over, the spaces between the contact pieces are filled in with an + insulating material, and the whole surface finished off perfectly smooth. + The spring Z is <sup>1</sup>/<sub>8</sub> inch wide for portion of its + length, but at the point where it presses upon Q it is reduced in width + to <sup>1</sup>/<sub>64</sub>th of an inch (see Fig. 48). The driving + arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a + grooved pulley, is run in bearings parallel with the shaft W, and is + connected by suitable gearing to the shaft of the driving motor, so that + the needle K makes one revolution in about 2<sup>1</sup>/<sub>2</sub> + seconds. A belt passing over the pulleys connects the two shafts, and the + tension of the belt is regulated by means of an adjustable jockey + pulley.</p> + + <p>The tube N, carrying the disc J, must be rotated at a fixed speed, and + this is accomplished in the following manner. An ordinary electric clock + impulse dial, actuated from a master clock, is connected by suitable + gearing H, so that the tube N makes exactly one revolution in 2 seconds; + it being possible to adjust an electric clock of the "Synchronome" type, + so that it only gains or loses about 1 second in 24 hours, and this + provides <!-- Page 94 --><span class="pagenum"><a + name="page94"></a>{94}</span>an accuracy sufficient for all practical + purposes. The connections are given in Fig. 49, and the face of the + instrument in Fig. 50. It will be seen that a connecting wire is run from + the steel spindle W to one terminal each of the lamps L, L<sup>1</sup>, + L<sup>2</sup>, and from the other terminal of the lamps to one terminal + of the batteries J, the battery comprising a set of three 4-volt + accumulators. The other terminals of the batteries are joined one to each + of the brushes R, R<sup>1</sup>, R<sup>2</sup>.</p> + + <div class="figleft" style="width:17%;"> + <a href="images/illo-fig49.png"><img style="width:100%" src="images/illo-fig49.png" + alt="Fig. 49." title="Fig. 49." /></a> + <span class="sc">Fig. 49.</span> + </div> + + <div class="figright" style="width:20%;"> + <a href="images/illo-fig50.png"><img style="width:100%" src="images/illo-fig50.png" + alt="Fig. 50." title="Fig. 50." /></a> + <span class="sc">Fig. 50.</span> + + <p class="poem">M, terminals for connecting to electric clock; L, white + lamp; L<sup>1</sup>, blue lamp; L<sup>2</sup>, red lamp.</p> + </div> + + <p>The lamps are coloured, the lamp L being white, and the lamps + L<sup>1</sup> and L<sup>2</sup> blue and red respectively, and care must + be taken in connecting up that when the needle K makes contact with the + stud P the white lamp L is in circuit. When the machines are working, the + operator, by means of the brake (already described), reduces the speed of + the driving motor until the needle K travels in unison with the disc J, + making permanent contact with P on the contact <!-- Page 95 --><span + class="pagenum"><a name="page95"></a>{95}</span>block Q, which is + evidenced by the lamp L remaining alight. If, however, the needle travels + faster than the disc J, contact with P is broken and fresh contact is + made with P<sup>2</sup>, the lamp L is extinguished and the red lamp + L<sup>2</sup> lights up, and remains alight until the operator reduces + the speed. Similarly, too, if the needle travels slower than J, contact + is made with P<sup>1</sup>, and the circuit of the blue lamp + L<sup>1</sup> is completed. When the speed is either above or below the + normal, the needle K engages with one or the other of the pins D, and as + the tension of the driving belt is only such as is required to drive the + needle, the belt slips on the pulleys until the normal speed is + regained.</p> + +<p class="cenhead"><span class="sc">Method of Working</span></p> + + <p>The clockwork motor M, Fig. 51, should be capable of running for + several hours with one winding, and powerful enough to take up the work + of driving the machine without any appreciable effort. The main spindle + of the motor is so arranged that it makes one revolution in two minutes, + and the reduction in speed between the motor shaft and the shaft to which + the coupling A is attached is 30:1. The metal line print having been + wrapped round the drum of the machine, the stylus is put into position, + at the edge of the lap, and with the needle resting about half-way on + <!-- Page 96 --><span class="pagenum"><a name="page96"></a>{96}</span>the + margin of the bare foil left at the commencing edge of the print. Now, + when the two stations are in perfect readiness for work, the motors are + started and the speed adjusted; the speed of the machine being just under + one revolution in four seconds.</p> + + <div class="figcenter" style="width:40%;"> + <a href="images/illo-fig51.png"><img style="width:100%" src="images/illo-fig51.png" + alt="Fig. 51." title="Fig. 51." /></a> + <span class="sc">Fig. 51.</span> + + <p class="poem">M, clockwork motor; S, isochroniser; E, friction break; + T, brushes; F, electric clutch; X, gearing; D, D<sup>1</sup>, switches; + A, flexible coupling; K, polarised relay; L, circuit breaker; + B<sub>1</sub>, B<sub>2</sub>, B<sub>3</sub>, batteries; P, electric + clock; W, terminals for connection to telephone relay; H, terminals for + connection to terminals J, on transmitting machine.</p> + </div> + + <p>The switch D is then closed, and the arm of the switch D<sup>1</sup> + placed on the contact stud (1), at the transmitting station only. As soon + as the switches are closed the clutch F comes into action, and the + transmitting machine begins to revolve. When the whole of the line print + wrapped round the drum of the machine has passed under the stylus, the + end of the shaft D, Fig. 36, engages <!-- Page 97 --><span + class="pagenum"><a name="page97"></a>{97}</span>with the spring <i>m</i>, + breaking the clutch circuit and allowing the motor to run free. As soon + as the machine stops, the switch D is opened and the machine run back to + its starting position by hand.</p> + + <p>At the receiving station the switch D is also closed, and the arm of + the switch D<sup>1</sup> placed on the contact stud (2). The closing of + these switches does not bring the clutch F into operation until current + from the telephone relay U connected to the wireless receiving apparatus + works the sensitive polarised relay K, which in turn completes the + circuit of the circuit-breaker L. When the armature of L is attracted, + the circuit of the relay K is broken, the circuit of the clutch F is + completed, and the machine starts revolving.</p> + + <div class="figright" style="width:13%;"> + <a href="images/illo-fig52.png"><img style="width:100%" src="images/illo-fig52.png" + alt="Fig. 52." title="Fig. 52." /></a> + <span class="sc">Fig. 52.</span> + </div> + + <p>The current from the relay U, due to the transmitting stylus passing + over <i>one</i> contact strip on the metal print, is too brief to actuate + the heavier mechanism of the relay K, hence the need of the margin of + bare foil at the commencing edge of the metal print, so that a + practically continuous current will flow to the relay K until the + armature is attracted. As, however, the relay is not actuated at the + receipt of the first signal, and as it is necessary for the machine to + start recording at a certain point on the film, viz. <!-- Page 98 + --><span class="pagenum"><a name="page98"></a>{98}</span>at the edge of + the lap—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.</p> + + <p>It is, of course, obvious that a somewhat similar adjustment must be + made with regard to the position of the stylus on the metal print at the + transmitting machine.</p> + + <p>In the present system, as in almost every photographic method of + receiving that has been described, the Nernst lamp is invariably + mentioned as the source of illumination. Since the advent of the + high-voltage metal-filament lamps the Nernst lamp has fallen somewhat + into disuse for commercial purposes, but it possesses certain + characteristics that render it eminently suitable for the purpose under + discussion.</p> + + <p>The main principle of this type of lamp depends upon the discovery + made by Professor Nernst in 1898, after whom the lamp is named, that + filaments of certain earthy bodies when raised to a red heat became + conductive sufficiently well to pass a current which raised it to a white + heat, and furthermore that the glowing filament emitted a brighter light + for a given amount of current than carbon filaments.</p> + + <div class="figright" style="width:19%;"> + <a href="images/illo-fig52a.png"><img style="width:100%" src="images/illo-fig52a.png" + alt="Fig. 52a." title="Fig. 52a." /></a> + <span class="sc">Fig.</span> 52<i>a</i>. + </div> + + <p>Nernst lamps are made in two sizes, the larger <!-- Page 99 --><span + class="pagenum"><a name="page99"></a>{99}</span>being intended for the + same work as usually done by arc lamps, and the smaller to replace + incandescent lamps; the smaller type being made to fit into the ordinary + bayonet lampholders. The principal parts of a Nernst lamp consist of the + filament, the heater, the automatic cut-out, and the resistance, and + their arrangement in the smaller type of lamp is given in the diagram, + Fig. 52<i>a</i>. The current enters at the positive terminal, passes + through the heater M, and out through the negative terminal. The filament + B, which consists of a short length of an infusible earth made of the + oxides of several rare minerals, of which zirconia is one, is a + non-conductor at first, but becomes a conductor upon being raised to a + high temperature by means of the heater M. As soon as the filament + becomes conductive the current then passes through the automatic cut-out + H, and the armature D is attracted, thus breaking the heater circuit. The + current then flows from the positive terminal <!-- Page 100 --><span + class="pagenum"><a name="page100"></a>{100}</span><span class="figleft" + style="width:34%;"><a href="images/illo-fig52b.png"><img + style="width:100%" src="images/illo-fig52b.png" alt="Fig. 52b." + title="Fig. 52b." /></a><span class="sc">Fig.</span> 52<i>b</i>.</span> + through the cut-out H, resistance J, and filament B, and from thence out + of the lamp. Since the resistance of the filament decreases the hotter it + gets, it is necessary to insert a ballasting resistance in series with it + which has the opposite property of increasing its resistance as it gets + hotter, to prevent the filament taking too much current and destroying + itself. Such a resistance, J, consists of a filament of fine iron wire, + which, to prevent oxidation from exposure to the air, is enclosed in a + glass bulb filled with hydrogen gas. Fig. 52<i>b</i> shows the form of + ballast resistance used in the small and large type of lamp + respectively.</p> + + <p>Either direct or alternating current can be used with these lamps, and + with direct current the polarity must be strictly observed, and that the + positive wire is connected to the positive and the <!-- Page 101 --><span + class="pagenum"><a name="page101"></a>{101}</span>negative wire to the + negative terminal. With the smaller type of lamp once it has been + correctly placed in its holder it is essential that it should not be + turned, as a change in the direction of the current will rapidly destroy + the filament.</p> + + <div class="figright" style="width:19%;"> + <a href="images/illo-fig52c.png"><img style="width:100%" src="images/illo-fig52c.png" + alt="Fig. 52c." title="Fig. 52c." /></a> + <span class="sc">Fig.</span> 52<i>c</i>. + </div> + + <p>The arrangement of the larger type of Nernst lamp can be readily seen + from the drawing, Fig. 52<i>c</i>.</p> + + <p>Care must be taken to see that the voltage required by the burner and + resistance equals the voltage of the supply circuit, and that only parts + of the same amperage are used together on the same lamp. No advantage is + obtained by over-running a Nernst lamp, this only shortening its life + without increasing the light. Under normal conditions the average life of + the burner is about 700 hours.</p> + + <p>The efficiency of the Nernst lamp is fairly high, being only 1.45 to + 1.75 watts per c.p. The light given is remarkably steady, and the lamps + are adaptable for all voltages from 100 to 300. In one of the large type + of lamps for use on a 235-volt <!-- Page 102 --><span class="pagenum"><a + name="page102"></a>{102}</span>circuit the burner takes 0.5 ampere at 215 + volts, and the resistance 0.5 ampere at 20 volts, while one of the + smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts + and 0.25 ampere at 20 volts for the burner and resistance respectively. + The burner and heater are very fragile, and should never be handled + except by the porcelain plate to which they are attached. The lamps burn + in air and emit a brilliant white light of high actinic power, the + intrinsic brilliancy (c.p./square inch) varying from 1000 to 2500, as + compared with 1000 to 1200 for ordinary metal filament lamps, and 300 to + 500 for carbon filament lamps.</p> + + <p>The chief advantage of the Nernst lamp from a photographic point of + view lies in the fact that it produces abundantly the blue and violet + rays which have the greatest chemical effect upon a photographic plate or + film. These rays are known as chemical or actinic rays, and are only + slightly produced in some types of incandescent electric lamps. + Carbon-filament lamps are very poor in this respect.</p> + + <p>Because a light is visually brilliant it must by no means be assumed + that it is the best to use for purposes of photography, and this is a + point over which many photographers stumble when using artificial light. + Many sources of light, while excellent for illumination, have very low + actinic powers, while others may have low illuminating but high <!-- Page + 103 --><span class="pagenum"><a name="page103"></a>{103}</span>actinic + powers. A lamp giving a light yellowish in colour has usually low actinic + power, while all those lamps giving a soft white light are generally + found to be highly actinic.</p> + + <p>In addition to the actinic value of the source of illumination, the + photographic film used must be very carefully chosen, as the chemical + inertia of the sensitised film plays an important part in the successful + reproduction of the picture, and also, to a certain extent, affects the + speed of transmission. The length of exposure, the amount of light + admitted to the film, and the characteristics of the film itself, are all + factors which have a decided bearing upon the quality of the results + obtained, and the film found to be most suitable in one case will perhaps + give very unsatisfactory results in another.</p> + + <p>In photo-telegraphy the length of exposure is determined by the time + taken by the transmitting stylus to trace over a conducting strip on the + metal print, and this time, of course, varies with the density of the + image and also with the speed of transmission.</p> + + <p>The film in ordinary photography is chosen with regard to the subject + and the existing light conditions, and the amount of light admitted to + the film and the length of exposure are regulated accordingly. No such + latitude is, however, possible in photo-telegraphy. With each set of + apparatus <!-- Page 104 --><span class="pagenum"><a + name="page104"></a>{104}</span>the various factors, such as the light + value, the amount of light admitted to the film, and the length of + exposure, will be practically fixed quantities, and the film that will + give the most satisfactory results under these fixed conditions can only + be found by the rough-and-ready method of "trial and error."</p> + + <p>The films in common use are manufactured in four qualities, namely, + ordinary, studio, rapid, and extra rapid. These terms should really + relate to the light sensitiveness of the film (or, as it is technically + termed, the speed), but at the best they are a rough and very + unsatisfactory guide, for the reason that some unscrupulous makers, + purely for business purposes, do not hesitate to label their films and + plates as slow, rapid, etc., without troubling to make any tests for + correct classification.</p> + + <p>The speed of photographic films and plates is generally indicated by a + number, and the system of standardisation adopted by the majority of + makers in this country is that originated by Messrs. Hurter & + 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.</p> + + <p>Besides the Hurter & Driffield method of <!-- Page 105 --><span + class="pagenum"><a name="page105"></a>{105}</span>obtaining the speed + numbers of plates and films adopted by a large number of makers in this + country, there are also two standard English systems known as the W.P. + No. (Watkin's power number) and Wynne F. No., both of which are used to a + fair extent.</p> + + <p>The "Actinograph" number or speed number of a plate in the H. & 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 ÷ + <sup>1</sup>/<sub>5</sub> = 170, and the plate is H. & D. 170. The + W.P. No. is found by dividing 50 by the Inertia. Thus 50 ÷ + <sup>1</sup>/<sub>5</sub> = 250, and the plate is W.P. 250, but for all + practical purposes the W.P. No. can be taken as one and a half times H. + & D. The Wynne F. numbers may be found by multiplying the square root + of the Watkins number by 6.4. Thus</p> + + <div class="poem"> + <div class="stanza"> + <p>√250 = 15.81, and 15.81 × 6.4 = W.F. 101.</p> + </div> + </div> + + <p>For those photographers who are in the habit of using an actinometer + giving the plate speeds in H. & D. numbers, the following table, + taken from the <i>Photographer's Daily Companion</i>, is given, <!-- Page + 106 --><span class="pagenum"><a name="page106"></a>{106}</span>which + shows at a glance the relative speed numbers for the various systems. The + Watkins and Wynne numbers only hold good, however, when the inertia has + been found by the H. & D. method.</p> + +<p class="cenhead"><span class="sc">Table of Comparative Speed Numbers for Plates and Films</span></p> + +<table class="allbctr" summary="Speed Numbers." title="Speed Numbers."> +<tr><td class="allb" style="text-align:center"> H. & D.</td><td class="allb" style="text-align:center"> W.P. No.</td><td class="allb" style="text-align:center"> W.F. No.</td><td class="muspac"> </td><td class="allb" style="text-align:center"> H. & D.</td><td class="allb" style="text-align:center"> W.P. No.</td><td class="allb" style="text-align:center"> W.F. No.</td></tr> +<tr><td class="vertb" style="text-align:center"> 10 </td><td class="vertb" style="text-align:center"> 15 </td><td class="vertb" style="text-align:center"> 24 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 220 </td><td class="vertb" style="text-align:center"> 323 </td><td class="vertb" style="text-align:center"> 114 </td></tr> +<tr><td class="vertb" style="text-align:center"> 20 </td><td class="vertb" style="text-align:center"> 30 </td><td class="vertb" style="text-align:center"> 28 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 240 </td><td class="vertb" style="text-align:center"> 352 </td><td class="vertb" style="text-align:center"> 120 </td></tr> +<tr><td class="vertb" style="text-align:center"> 40 </td><td class="vertb" style="text-align:center"> 60 </td><td class="vertb" style="text-align:center"> 49 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 260 </td><td class="vertb" style="text-align:center"> 382 </td><td class="vertb" style="text-align:center"> 124 </td></tr> +<tr><td class="vertb" style="text-align:center"> 80 </td><td class="vertb" style="text-align:center"> 120 </td><td class="vertb" style="text-align:center"> 69 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 280 </td><td class="vertb" style="text-align:center"> 412 </td><td class="vertb" style="text-align:center"> 129 </td></tr> +<tr><td class="vertb" style="text-align:center"> 100 </td><td class="vertb" style="text-align:center"> 147 </td><td class="vertb" style="text-align:center"> 77 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 300 </td><td class="vertb" style="text-align:center"> 441 </td><td class="vertb" style="text-align:center"> 134 </td></tr> +<tr><td class="vertb" style="text-align:center"> 120 </td><td class="vertb" style="text-align:center"> 176 </td><td class="vertb" style="text-align:center"> 84 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 320 </td><td class="vertb" style="text-align:center"> 470 </td><td class="vertb" style="text-align:center"> 138 </td></tr> +<tr><td class="vertb" style="text-align:center"> 140 </td><td class="vertb" style="text-align:center"> 206 </td><td class="vertb" style="text-align:center"> 91 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 340 </td><td class="vertb" style="text-align:center"> 500 </td><td class="vertb" style="text-align:center"> 142 </td></tr> +<tr><td class="vertb" style="text-align:center"> 160 </td><td class="vertb" style="text-align:center"> 235 </td><td class="vertb" style="text-align:center"> 103 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 380 </td><td class="vertb" style="text-align:center"> 558 </td><td class="vertb" style="text-align:center"> 150 </td></tr> +<tr><td class="vertb" style="text-align:center"> 200 </td><td class="vertb" style="text-align:center"> 294 </td><td class="vertb" style="text-align:center"> 109 </td><td class="muspac"> </td><td class="vertb" style="text-align:center"> 400 </td><td class="vertb" style="text-align:center"> 588 </td><td class="vertb" style="text-align:center"> 154 </td></tr> +</table> + + <p>Although theoretically the higher the speed of the film the less the + duration of exposure required, there is a practical limit, as besides the + intensity and actinic value of the light admitted to the film a definite + time is necessary for it to overcome the chemical inertia of the + sensitised coating and produce a useful effect. With every make of film + it is possible to give so short an exposure that although light does fall + upon the film it does no work at all—in other words, we can say + that for every film there is a minimum amount of light action, and + anything below this is of no use. The exposure that enables the smallest + amount of light action to take place is termed the limit of the smallest + useful exposure. <!-- Page 107 --><span class="pagenum"><a + name="page107"></a>{107}</span></p> + + <p>There is also a maximum exposure in which the light affects + practically all the silver in the film, and any increased light action + has no increased effect. This is the limit of the greatest useful + exposure.</p> + + <p>In photo-telegraphy the duration of exposure, as already pointed out, + is determined by certain conditions connected with the transmitting + apparatus, and with conditions similar to those mentioned on page 75 the + length of exposure will vary roughly from 1-50th to 1-150th of a + second.</p> + + <p>The most suitable film to use for purposes of photo-telegraphy is one + having a fairly slow speed in which the range of exposure required comes + well within the limits of the film. There is no advantage in using a film + having a speed of, say, H. & 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.</p> + + <p>Another point, often puzzling to the beginner, and which increases the + difficulty of choosing a suitable make of film, is that, although one + make of film marked H. & D. 100 will give good results, another make, + also marked H. & D. 100, will give <!-- Page 108 --><span + class="pagenum"><a name="page108"></a>{108}</span>very poor results. This + is owing, not to a poor quality film, as many suppose, but to the almost + insurmountable difficulty of makers being able to employ exactly the same + standard of light for testing purposes, so that although various makes + may all be standardised by the H. & D. method, films bearing the same + speed numbers may vary in their actual speed by as much as 30 to 50 per + cent.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 109 --><span class="pagenum"><a name="page109"></a>{109}</span></p> + +<h3>APPENDIX A</h3> + +<p class="cenhead">SELENIUM CELLS</p> + + <p>Selenium is a non-metallic element, and was first discovered by + Berzelius in 1817, in the deposit from sulphuric acid chambers, which + still continues the source from which it is obtained for commercial + purposes, although it is found to a small extent in native sulphur. Its + at. wt. is 79.2, and its sp. gr. 4.8. Symbol, Se.</p> + + <p>In its natural state selenium is practically a non-conductor of + electricity, its resistance being forty thousand million times greater + than copper. Its practical value lies in the property which it possesses, + that when in a prepared condition it is capable of varying its electrical + resistance according to the amount of light to which it is exposed, the + resistance decreasing as the light increases.</p> + + <p>Selenium is prepared by heating it to a temperature of 120° C., + keeping it there for some hours, and allowing it to cool slowly, when it + assumes a crystalline form and changes from a bluish grey to a dull slate + colour. A selenium cell in its simplest form consists merely of some + prepared selenium placed between two or more metal electrodes, the + selenium acting as a high resistance conductor between them. The form + given by Bell and Tainter to the cells used in their experiments is given + in Figs. 53 and 53<i>a</i>. It consists of a number of rectangular brass + plates P, P', separated by very thin sheets of mica M, the mica sheets + being slightly narrower than the brass plates, the whole being clamped + together in the frame F by the two bolts B. <!-- Page 110 --><span + class="pagenum"><a name="page110"></a>{110}</span>By means of a sand-bath + the cell is raised to the desired temperature, and selenium is rubbed + over the surface, which melts and fills the small spaces between the + brass plates. All the plates P are connected together to form one + terminal, and the plates P' to form the other. By using very thin mica + sheets, and a large number of elements, a very narrow transverse section + of selenium, together with a large active surface, can be obtained.</p> + + <p>The cell used for commercial purposes is usually constructed as + follows. A small rectangular piece of porcelain, slate, mica, or other + insulator, is wound with many turns of fine platinum wire. The wire is + wound double, as shown in Fig. 54, the spaces between the turns being + filled with prepared selenium. A thin glass cover is sometimes placed + over the cell to protect the surface from injury.</p> + + <div class="figcenter" style="width:17%;"> + <a href="images/illo-fig53.png"><img style="width:100%" src="images/illo-fig53.png" + alt="Fig. 53." title="Fig. 53." /></a> + <span class="sc">Fig.</span> 53. + + <p class="poem">P, P', plates; M, mica; S, selenium.</p> + </div> + + <div class="figright" style="width:18%;"> + <a href="images/illo-fig53a.png"><img style="width:100%" src="images/illo-fig53a.png" + alt="Fig. 53a." title="Fig. 53a." /></a> + <span class="sc">Fig.</span> 53<i>a</i>. + </div> + + <p>A strong light falling upon a cell lowers its resistance, and <i>vice + versa</i>, the resistance of a cell being at its highest when unexposed + to light; the light is apparently absorbed and made to do work by varying + the electrical resistance of the selenium. Selenium cells vary very + considerably as regards their quality as well as in their electrical + resistance, it being possible to obtain cells of the same size for any + resistance between 10 and 1,000,000 ohms, and also, a cell may remain in + good working condition for several months, while another will become + useless in as many weeks.</p> + + <p>The ability of a cell to respond to very rapid changes in the + illumination to which it is exposed is determined largely upon its + inertia, it being taken as a general rule <!-- Page 111 --><span + class="pagenum"><a name="page111"></a>{111}</span>that the higher the + resistance of a cell the less the inertia, and <i>vice versa</i>, and + also, that the higher the resistance the greater the ratio of + sensitiveness. Inertia plays an important part in the working of a cell, + slightly opposing the drop in resistance when illuminated, and opposing + to a <span class="figleft" style="width:22%;"><a + href="images/illo-fig54.png"><img style="width:100%" + src="images/illo-fig54.png" alt="Fig. 54." title="Fig. 54." /></a><span + class="sc">Fig.</span> 54</span> much greater degree the return to normal + for no-illumination. The effects of inertia or "lag," as it is termed, + can readily be seen by reference to Fig. 55. It will be noticed that the + current value rapidly increases when the cell is first illuminated, but + if after a short time <i>t</i> the light is cut off, the current value, + instead of returning at once to normal for no-illumination, only + partially rises owing to the interference of the inertia, and some time + elapses before the cell returns to its normal condition; the time varying + from a few seconds to several minutes, depending upon the characteristics + of the cell and the amount of light to which it is exposed. An actual + curve is given in Fig. 55<i>a</i>. The inertia or "lag" of a cell + produces upon an intermittent current an effect similar to that produced + by the capacity <span class="figright" style="width:17%;"><a + href="images/illo-fig55.png"><img style="width:100%" + src="images/illo-fig55.png" alt="Fig. 55." title="Fig. 55." /></a><span + class="sc">Fig.</span> 55</span> of a line, as was noted in Chapter I., + preventing the incoming signals from being recorded separately, and + distinctly. To obtain the best results in photo-telegraphy, the + resistance of a cell should only be decreased to an extent sufficient to + pass the current required to operate the recording apparatus, and the + illumination should be regulated so that this condition of the cell takes + place.</p> + + <p>The comparative slowness of selenium in responding to <!-- Page 112 + --><span class="pagenum"><a name="page112"></a>{112}</span>any great + changes in the illumination offers a serious difficulty to its use in + photo-telegraphy, but various methods have been devised whereby the + effects of inertia can be counteracted. In the system of De' Bernochi + (see Chapter I.) the changes in the illumination are neither very rapid + nor very great, and the inertia effects would therefore be very slight; + but in any photo-telegraphic system in which a metal line print is used + for transmitting, where the source of illumination is constant and the + resistance of the cell is required to drop to a definite value and return + to normal instantly, many times in succession, the inertia effects are + very pronounced. The most successful method of counteracting the inertia + is that adopted by Professor Korn of always keeping the cell sufficiently + illuminated to overcome it, so that any additional light acts very + rapidly. Another method worked out and patented by Professor Korn, and + known as the "compensating cell" method, gives a practically dead beat + action, the resistance returning to its normal condition as soon as the + illumination ceases. The arrangement is given in the diagram Fig. 56.</p> + + <div class="figcenter" style="width:38%;"> + <a href="images/illo-fig55a.png"><img style="width:100%" src="images/illo-fig55a.png" + alt="Fig. 55a." title="Fig. 55a." /></a> + <span class="sc">Fig.</span> 55<i>a</i>. + </div> + + <p>Light from the transmitting or receiving apparatus, as the case may + be, falls upon the selenium cell S<sup>1</sup>, which is <!-- Page 113 + --><span class="pagenum"><a name="page113"></a>{113}</span>placed on one + arm of a Wheatstone bridge, a second cell S<sup>2</sup> being placed on + the opposite arm. The selenium cell S<sup>1</sup> should have great + sensitiveness and small inertia, the compensating cell S<sup>2</sup> + having proportionally small sensitiveness and large inertia. Two + batteries B, B', of about 100 volts, are connected as shown, B being + provided with a compensating variable resistance W; W' is also a + regulating resistance. When no light is falling upon the cell + S<sup>1</sup>, light from L is prevented from reaching the second cell + S<sup>2</sup> by a small shutter which is fastened to the strings of the + Einthoven galvanometer (described in Chapter III.), and the piece of + apparatus C—relay or galvanometer as the case may be—remains + in a normal condition. When, however, light falls upon the cell + S<sup>1</sup>, the balance of the bridge is upset, and light from L falls + a fraction of a second later upon the second cell S<sup>2</sup>, and the + current flowing through C completes the circuit. Needless to say it is + necessary that the two cells be well matched, as it is very easy to have + over-compensation, in which case the current is brought below zero.</p> + + <div class="figcenter" style="width:20%;"> + <a href="images/illo-fig56.png"><img style="width:100%" src="images/illo-fig56.png" + alt="Fig. 56." title="Fig. 56." /></a> + <span class="sc">Fig.</span> 56. + </div> + + <p>It is also stated that by enclosing the cells in exhausted glass + tubes, their inertia can be greatly reduced and their life considerably + prolonged. The sensitiveness of a cell is the ratio between its + resistance in the dark and its resistance when illuminated. The majority + of cells have a ratio between 2:1 and 3:1, but Professor Korn has shown + mathematically that by conforming to certain conditions regarding the + construction the ratio of sensitiveness may be between 4:1 and 5:1. Thus + a cell of R = 250,000 ohms can be reduced to 60,000 ohms from the light + of a 16 c.p. lamp placed only a short distance away; the resistance may + be still <!-- Page 114 --><span class="pagenum"><a + name="page114"></a>{114}</span>further decreased by continuing the + illumination, but this produces a permanent defect in the cells termed + "fatigue," the cells becoming very sluggish in their action and their + sensitiveness gradually becoming less, the ratio between their resistance + in the dark and their resistance when illuminated being reduced by as + much as 30 per cent.</p> + + <p>Excessive illumination will also produce similar results. The inertia + of a cell is practically unaffected by the wavelength of the light used, + but the maximum sensitiveness of a cell is towards the yellow-orange + portion of the spectrum.</p> + + <p>In addition to light, heat has also been found to vary the electrical + resistance of selenium in a very remarkable manner. At 80° C. selenium is + a non-conductor, but up to 210° C. the conductivity gradually increases, + after which it again diminishes.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 115 --><span class="pagenum"><a name="page115"></a>{115}</span></p> + +<h3>APPENDIX B</h3> + +<p class="cenhead">PREPARING THE METAL PRINTS</p> + + <p>Electricians who desire to experiment in photo-telegraphy, but who + have no knowledge of photography, may perhaps find the following detailed + description of preparing the metal prints of some value. The would-be + experimenter may feel somewhat alarmed at the amount of work entailed, + but once the various operations are thoroughly grasped, and with a little + patience and practice, no very great difficulty should be experienced. + The simpler photographic operations, such as developing, fixing, etc., + cannot be described here, and the beginner is advised to study a good + text-book on the subject.</p> + + <p>The method to be given of preparing the photographs is practically the + only one available for wireless transmission, and although the manner + given of preparing is perhaps not strictly professional, having been + modified in order to meet the requirements of the ordinary amateur + experimenter, the results obtained will be found perfectly + satisfactory.</p> + + <p>As will have been gathered from Chapter II., the camera used for + copying has to have a single line screen placed a certain distance in + front of the photographic plate, and the object of this screen is to + break the image up into parallel bands, each band varying in width + according to the density of the photograph from which it has been + prepared. Thus a white portion of the photograph would consist of very + narrow lines wide apart, while a dark portion would be made up of wide + lines close together; a black part would appear solid and show no lines + at all. It is, of course, obvious <!-- Page 116 --><span + class="pagenum"><a name="page116"></a>{116}</span>that the lines on the + negative cannot be wider apart, centre to centre, than the lines of the + screen. A good screen distance has been found to be 1 to 64, <i>i.e.</i> + the diameter of the stop is <sup>1</sup>/<sub>64</sub>th of the camera + extension, and the distance of the screen lines from the photographic + plate is 64 times the size of the screen opening. The following table + shows what this distance is for the screen most likely to be used. The + line screens used consist of glass plates upon which a number of lines + are accurately ruled, the width of the lines and the spaces between being + equal; the lines are filled in with an opaque substance. These ruled + screens are very expensive, and are only made to order,<a name="NtA10" + href="#Nt10"><sup>[10]</sup></a> a screen half-plate size costing from + 21s. to 27s. 6d. An efficient substitute for a ruled screen can be made + by taking a rather large sheet of Bristol board and ruling lines across + in pure black drawing ink, the width of the lines and the spaces between + being <sup>1</sup>/<sub>12</sub>th of an inch respectively. A photograph + must be taken of this card, the reduction in size determining the number + of lines to the inch. A card 20 × 15 inches, with 12 lines to the inch, + would, if reduced to 5 × 4 inches, make a screen having 48 lines to the + inch. Preparing the board is rather a tedious operation, but the line + negative produced will be found to give results almost as good as those + obtained from a purchased screen.</p> + +<p class="cenhead"><span class="sc">Diameter of Stop used <sup>1</sup>/<sub>64</sub>th of Camera Extension.</span></p> + +<table class="allbctr" summary="Screen line spacing." title="Screen line spacing."> +<tr><td class="allb" style="text-align:center; vertical-align:top;"> Screen ruling<br /> +lines per inch. + +</td><td class="allb" style="text-align:center; vertical-align:top;"> Actual space<br /> +in inches. + +</td><td class="allb" style="text-align:center; vertical-align:top;"> Distance of<br /> +screen ruling<br /> +in inches. + +</td><td class="allb" style="text-align:center; vertical-align:top;"> In <sup>1</sup>/<sub>32</sub><br /> +inches + +</td><td class="allb" style="text-align:center; vertical-align:top;"> In milli-<br /> +metres</td></tr> +<tr><td class="vertb" style="text-align:center"> 35 </td><td class="vertb" style="text-align:center"> <sup>1</sup>/<sub>70</sub> </td><td class="vertb" style="text-align:center"> .91 </td><td class="vertb" style="text-align:center"> 28.8 </td><td class="vertb" style="text-align:center"> 21.8 </td></tr> +<tr><td class="vertb" style="text-align:center"> 50 </td><td class="vertb" style="text-align:center"> <sup>1</sup>/<sub>100</sub> </td><td class="vertb" style="text-align:center"> .64 </td><td class="vertb" style="text-align:center"> 20.5 </td><td class="vertb" style="text-align:center"> 16.2 </td></tr> +<tr><td class="vertb" style="text-align:center"> 65 </td><td class="vertb" style="text-align:center"> <sup>1</sup>/<sub>130</sub> </td><td class="vertb" style="text-align:center"> .49 </td><td class="vertb" style="text-align:center"> 15.7 </td><td class="vertb" style="text-align:center"> 12.4 </td></tr> +</table> + + <p>As it is impossible for many to have the use of professional apparatus + designed for this particular kind of work, <!-- Page 117 --><span + class="pagenum"><a name="page117"></a>{117}</span>the fixing of the + screen into an ordinary camera must be left to the ingenuity of the + worker. A half-plate back focussing camera will be found suitable for + general experimental work, but if this is not available, a large box + camera can be pressed into service.</p> + + <div class="figleft" style="width:24%;"> + <a href="images/illo-fig57.png"><img style="width:100%" src="images/illo-fig57.png" + alt="Fig. 57." title="Fig. 57." /></a> + <span class="sc">Fig.</span> 57. + </div> + + <p>The writer has never seen a half-plate box camera, but one taking a 5 + × 4 inch plate can be obtained second-hand very cheaply. It is a + comparatively simple matter to fix the line screen into a camera of this + description, the drawings Figs. 57 and 58 showing the method adopted by + the writer. The two clips D, made from fairly stout brass about + <sup>1</sup>/<sub>2</sub> inch wide, are bent to the shape shown (an + enlarged section is given at C) and soldered at the top and bottom of one + of the metal sheaths provided for holding the plates. The distance + between the front of the photographic plate (the film side) and the back + of the line screen (also the film side), indicated by the arrow at A, is + determined by the number of lines on the screen. As will be seen from the + table given, the distance for a screen having 50 lines to the inch will + be <sup>41</sup>/<sub>64</sub>ths of an inch.</p> + + <div class="figright" style="width:24%;"> + <a href="images/illo-fig58.png"><img style="width:100%" src="images/illo-fig58.png" + alt="Fig. 58." title="Fig. 58." /></a> + <span class="sc">Fig.</span> 58. + + <p class="poem">M, sheath; P, photographic plate; D, clips; S, line + screen.</p> + </div> + + <p>In all probability there will be enough clearance between the top of + the sheath and the top of the camera to allow for the thickness of the + clip, but if not, a shallow groove a little wider than the clip should be + carefully cut in the top of the camera, so that it will slide in easily. + The screen should be placed between the clips, the film side on the <!-- + Page 118 --><span class="pagenum"><a + name="page118"></a>{118}</span>inside, <i>i.e.</i> facing the + photographic plate. As with a box camera the extension is a fixture, the + size of stop to be used is a fixture also. The extension of a camera + (this term really applies to a bellows camera) is measured from the front + of the photographic plate to the diaphragm, and if this distance in our + camera is 8 inches, then the diameter of the stop to give the best + results would be <sup>1</sup>/<sub>64</sub>th of this, or + <sup>1</sup>/<sub>8</sub>th inch. Although for all ordinary experimental + work the lens fitted to the camera will be suitable, the best type of + lens for process work of all kinds is the "Anastigmat."</p> + + <p>The picture or photograph from which it is desired to make a print + should be fastened out perfectly flat upon a board with drawing pins, and + if a copying stand is not available it must be placed upright in some + convenient position. The diagram Fig. 59 gives the disposition of the + apparatus required for copying. A simple and inexpensive copying stand is + shown in Fig. 60. The blackboard A should be about 30 inches square, and + must be fastened perfectly upright upon the base-board B. The stand C + should be made so that it slides without any side play between the guides + D, and should be of such a height that the lens of the camera comes + exactly opposite the <!-- Page 119 --><span class="pagenum"><a + name="page119"></a>{119}</span><span class="figleft" + style="width:32%;"><a href="images/illo-fig59.png"><img + style="width:100%" src="images/illo-fig59.png" alt="Fig. 59." title="Fig. 59." + /></a><span class="sc">Fig.</span> 59. L, L, lamps; A, board with + picture; S, line screen; P, photographic plate.</span> <span + class="figright" style="width:41%;"><a href="images/illo-fig60.png"><img + style="width:100%" src="images/illo-fig60.png" alt="Fig. 60." title="Fig. 60." + /></a><span class="sc">Fig.</span> 60.</span> centre of the board A. The + camera, if of the box type, can be secured to the stand by means of a + screw and wingnut, the screw being passed from the inside as shown. The + beginner is advised to photograph only very bold and simple subjects, + such as black and white drawings or enlargements. It is not safe to trust + to the view-finders as to whether the whole of the picture is included on + the plate, a piece of ground glass the same size as the plate sheaths, + and used as a focussing screen, being much more reliable. It is a good + plan to focus the camera for a number of different-sized pictures, + marking the board A, and the <!-- Page 120 --><span class="pagenum"><a + name="page120"></a>{120}</span>guides D, so that adjustment is afterwards + a very simple matter.</p> + + <p>The make of plate used is also a great factor in getting a good + negative, and Wratten Process Plates will be found excellent. As already + mentioned, such subjects as the exposure and the development of the plate + cannot be dealt with here, these subjects having been exhaustively + treated in several text-books on photography. With an arc lamp the + exposure is about twice as long as in daylight, but the exposure varies + with the amount of light admitted to the plate, character of the source + of light, and the sensitiveness of the plate used, etc. The writer has + used acetylene gas lamps for this purpose with great success. The + beginner is advised to use artificial light, as this can be kept + perfectly even. With daylight, however, the light is constantly + fluctuating, and this renders the use of an actinometer a necessity for + correct exposure. After development, if the plate is required for + immediate use, it can be quickly dried by soaking for a few minutes in + methylated spirit.</p> + + <p>Having obtained a good negative, the next operation is to prepare what + is known as a metal print. For this we shall require some stout tin-foil + or lead-foil, about 12 or 15 square feet to the pound, and this should be + cut into pieces of such a size that it allows a lap of + <sup>3</sup>/<sub>16</sub> inch when wrapped round the drum of the + transmitting machine. Obtain some good fish-glue and add a saturated + solution of bichromate of potash in the proportion of 4 parts of potash + to 40 or 50 parts of glue. Pour a little of this glue into a shallow + dish, lay a sheet of foil upon a flat board, and with a fairly stiff + brush (a flat hog's-hair as wide as possible) proceed to coat the sheet + of foil with a thin but perfectly even coating of glue. The thickness of + the coating can only be found by trial, for if the coating is too thick a + longer time will be required for printing; but it must not be thin enough + to show interference colours. After the coating has been laid on, a soft + brush, such as photographers use for dusting dry <!-- Page 121 --><span + class="pagenum"><a name="page121"></a>{121}</span>plates, should be + passed up and down, and across and across, with light, even strokes to + remove any unevenness. A glue solution used by professional + photo-engravers is as follows:</p> + +<table class="nob" summary="Glue solution." title="Glue solution."> +<tr><td class="spacsingle"> Fish-glue </td><td class="spacsingle"> 12 oz.</td></tr> +<tr><td class="spacsingle"> Bichromate of Ammonia </td><td class="spacsingle"> <sup>3</sup>/<sub>4</sub> oz.</td></tr> +<tr><td class="spacsingle"> Water </td><td class="spacsingle"> 18 to 24 oz.</td></tr> +<tr><td class="spacsingle"> Ammonia .880 </td><td class="spacsingle"> 30 minims.</td></tr> +</table> + + <p>The bichromate should be dissolved in the water, and, when added to + the glue, stir very thoroughly in order that complete mixing may take + place. The coating may be done in a good light, not bright sunlight, but + <i>it must be dried in the dark</i>, because, although insensitive while + in a moist condition, it becomes sensitive immediately on desiccation. If + allowed to dry in the light the whole coating will become insoluble, and + for this reason the brushes used should be washed out as soon as they are + finished with. The sheets will take about 15 minutes to dry in a + perfectly dry room, but it is not advisable to prepare many sheets at + once, as they will not keep for more than two or three days.</p> + + <p>The prepared negative must now be placed in an ordinary printing + frame, and a print taken off upon one of the metal sheets in the same way + as a print is taken off upon ordinary sensitised paper. In daylight the + exposure varies from 5 to 20 minutes, but in artificial light various + trials will have to be made in order to get the best results, the + exposure varying with the amount of bichromate in the coating; the + proportion of the bichromate to the glue should remain about 6 per cent. + Light from a 25 ampere arc lamp for 2 to 5 minutes, at a distance of 18 + inches, will generally suffice to "print" the impression on the metal + sheets. The printing finished, the metal print should be laid upon a + sheet of glass and held under a running stream of water. The washing is + complete as soon as the unexposed parts of the glue coating have been + entirely washed away leaving the bare metal, and this will take anything + from 3 to 7 <!-- Page 122 --><span class="pagenum"><a + name="page122"></a>{122}</span>minutes, depending upon the thickness of + the film. As soon as it is dry the print is ready for use.</p> + + <p>As already mentioned, the negative from which the metal print is made + requires that the lines be perfectly sharp and opaque, and the spaces + between perfectly transparent. Ordinary dry plates are too rapid, a + rather slow plate being required. Wratten Process Plates give excellent + results, and the following is a good developer to use with them:</p> + +<table class="nob" summary="Developer." title="Developer."> +<tr><td class="spacsingle"> Glycin </td><td class="spacsingle"> 15 grammes </td><td class="spacsingle"> 1 oz.</td></tr> +<tr><td class="spacsingle"> Sulphite of Soda </td><td class="spacsingle"> 40 ,, </td><td class="spacsingle"> 2½ ,,</td></tr> +<tr><td class="spacsingle"> Carbonate of Potash </td><td class="spacsingle"> 80 ,, </td><td class="spacsingle"> 5 ,,</td></tr> +<tr><td class="spacsingle"> Water </td><td class="spacsingle"> 1000 c.c. </td><td class="spacsingle"> 60 ,,</td></tr> +</table> + + <p>This developer should be used for 6 minutes at a temperature of 50° + F., 3<sup>1</sup>/<sub>2</sub> minutes at 65°, and + 1<sup>3</sup>/<sub>4</sub> minutes at 80°. It is best only used once. If + an intensifier is required, the following formula will be found to give + satisfactory results:</p> + +<table class="nob" summary="Intensifier." title="Intensifier."> +<tr><td class="spacsingle"> Bichloride of Mercury </td><td class="spacsingle"> 1 oz. </td><td class="spacsingle"> 60 grammes.</td></tr> +<tr><td class="spacsingle"> Hot Water </td><td class="spacsingle"> 16 ,, </td><td class="spacsingle"> 1000 c.c.</td></tr> +</table> + + <p>Allow to cool, completely pour off from any crystals, and add:</p> + +<table class="nob" summary="Acid." title="Acid."> +<tr><td class="spacsingle"> Hydrochloric Acid </td><td class="spacsingle"> 30 minims </td><td class="spacsingle"> 4 c.c.</td></tr> +</table> + + <p>Allow negative to bleach thoroughly, wash well in water, and blacken + in 10 per cent ammonia .880, or 5 per cent sodium sulphide.</p> + + <p>In preparing the negatives and metal prints the following points + should be observed:</p> + + <p>A good negative should have the lines perfectly sharp and opaque; + there should be no "fluff" between the lines even when they are close + together.</p> + + <p>A properly exposed and developed negative should not require any + reducing or intensifying.</p> + + <p>If the lamps used for illuminating the copying board are placed 2 feet + away, and the exposure required is 5 minutes, the exposure, if the lamps + are placed 4 feet away, will be <!-- Page 123 --><span class="pagenum"><a + name="page123"></a>{123}</span>20 minutes, as the amount of light which + falls upon an object decreases as the inverse square of the distance.</p> + + <p>Get the coating on the foil as thin as possible, and err on the side + of over-exposure, for if the coating is thick and has been under-exposed, + excessive washing will dissolve the whole coating; for, unless + insolubilisation has taken place right up to the metal base, the under + parts will remain in a more or less soluble condition.</p> + + <p>On no account must the unexposed sheets be placed near a fire, + otherwise they will be spoilt, the whole coating becoming insoluble; heat + acting in the same manner as light.</p> + + <p>In washing, keep the print moving so that the stream of water does not + fall continually in one place. It is best to hold the print so that the + water runs off in the direction of the lines.</p> + + <p>To dry the prints after washing they can be laid out flat in a + moderately warm oven, or before a stove, the heat of course not being + sufficient to cause the coating to peel.</p> + + <p>To render the glue image more distinct the print should be immersed + for a few seconds in an aniline dye solution, the glue taking up the + colour readily. These dyes are soluble in either water or alcohol. A dye + known as "magenta" is very good.</p> + + <p>The process of coating the metal sheets must be performed as quickly + as possible (about 10 seconds), as owing to the peculiar nature of the + bichromated glue it soon sets, and once this has taken place it is + impossible to smooth down any unevenness.</p> + + <p>See that the negative and metal sheet make good contact while + printing.</p> + + <p>If the glue solution does not adhere to the surface of the foil in a + perfectly even film, but assumes a streaky appearance, a little liquid + ammonia, or a weak solution of nitric acid, rubbed over the surface of + the foil, which is afterwards gently scoured with precipitated chalk on a + tuft of cotton <!-- Page 124 --><span class="pagenum"><a + name="page124"></a>{124}</span>wool, will remove the grease which is the + cause of the difficulty.</p> + + <p>A photograph of a picture prepared from a line negative is given in + Fig. 61. For a great many experiments, and in order to save time, + trouble, and expense, sketches drawn upon stout lead-foil in an + insulating ink will answer the purpose admirably, but if any exact work + is to be done a single line print is of course absolutely necessary. The + insulating ink can be prepared by dissolving shellac in methylated + spirit, or ordinary gum can be used. A very fine brush should be used in + place of a pen, as the gum will not flow freely from an ordinary nib + unless greater pressure than the foil will safely stand be applied. A + sketch prepared in this manner is shown in Fig. 62. A little aniline dye + should be added to the gum to render it more visible, or a mixture of gum + and liquid indian ink will be found suitable.</p> + + <div class="figcenter" style="width:22%;"> + <a href="images/illo-fig63.png"><img style="width:100%" src="images/illo-fig63.png" + alt="Fig. 63." title="Fig. 63." /></a> + <span class="sc">Fig.</span> 63. + </div> + + <p>With the copying arrangement already described it is only possible to + employ it for reducing, it being necessary to employ a bellows camera + with a back focussing attachment for purposes of enlarging, and this + constitutes the chief drawback to the use of a fixed focus camera. By + replacing the box camera with a focussing camera of the same size, we + shall have a piece of apparatus capable of reducing or enlarging, only in + this case the camera should be a fixture and the board, A, arranged to + slide backwards and forwards instead.</p> + + <div class="figcenter" style="width:30%;"> + <a href="images/illo-fig61.png"><img style="width:100%" src="images/illo-fig61.png" + alt="Fig. 61." title="Fig. 61." /></a> + <span class="sc">Fig.</span> 61. + + <p class="poem">Portions of photographs (full size) of single line + screen, and single line print. Screen 40 lines to the inch.</p> + </div> + + <div class="figcenter" style="width:34%;"> + <a href="images/illo-fig62.png"><img style="width:100%" src="images/illo-fig62.png" + alt="Fig. 62." title="Fig. 62." /></a> + <span class="sc">Fig.</span> 62. + </div> + +<p><!-- Page 125 --><span class="pagenum"><a name="page125"></a>{125}</span></p> + + <p>An extra improvement would be to rule the surface of the copying + board, A, in a manner similar to that shown in the diagram, Fig. 63. The + rulings should be marked off from the centre of the board, and should + enclose parallelograms of the various plate sizes ranging from + 3<sup>1</sup>/<sub>4</sub> × 4<sup>1</sup>/<sub>4</sub> inches up to the + full size of the board. By fastening the picture or photograph to be + copied in the space on the board corresponding in size, we can ensure + that it is in the correct position for the whole to be included on the + photographic plate, providing, of course, that the centre of lens and + board coincide.</p> + + <p>With regard to the lens required, the practice adhered to by most + photographers is to use a lens having a focal length equal to the + diagonal of the plate used. Thus for a <sup>1</sup>/<sub>4</sub>-plate + camera a 5-inch lens should be used, and for a + <sup>1</sup>/<sub>2</sub>-plate an 8-inch lens, and so on. For a 5 × 4 + inch camera a 6-inch lens will be required. The following is a simple + rule for finding the conjugate foci of a lens, and is useful in obtaining + the distance from the lens to the photographic plate and the picture to + be copied. Let us suppose that we wish to make a + 1<sup>1</sup>/<sub>2</sub> times enlarged line negative from a + 4<sup>1</sup>/<sub>4</sub> × 3<sup>1</sup>/<sub>4</sub> inch print. Add 1 + to the number of times it is required to enlarge and multiply the result + by the focal length of the lens in inches. In the present case this will + be 1<sup>1</sup>/<sub>2</sub> + 1 = 2<sup>1</sup>/<sub>2</sub>; and if a + 6-inch lens is used, 2<sup>1</sup>/<sub>2</sub> × 6 = 15 inches will be + the distance of the lens from the plate. Divide this number by the number + of times it is desired to enlarge, and the distance of the lens from the + picture to be copied is obtained; in this instance 15 ÷ + 1<sup>1</sup>/<sub>2</sub> = 10 inches. The same rule can be followed + when it is required to reduce any given number of times, only in this + case the greater number will represent the distance between the lens and + the picture to be copied, and the lesser number the distance between the + lens and the plate.</p> + + <p>In reducing, a <sup>1</sup>/<sub>4</sub>-plate lens will be found to + fully cover a 5 × 4 inch plate, providing the reduction is not greater + than three to one.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 126 --><span class="pagenum"><a name="page126"></a>{126}</span></p> + +<h3>APPENDIX C</h3> + +<p class="cenhead">LENSES</p> + + <p>In this small volume it is not desirable, neither is it intended, to + give an exhaustive treatment on the subject of lenses and their action, + but as optics plays an important part in the transmission of photographs, + both by wireless and over ordinary conductors, the following notes + relating to a few necessary principles have been included as likely to + prove of interest.</p> + + <p>Light always travels in straight lines when in a medium of uniform + density, such as water, air, glass, etc., but on passing from one medium + to another, such as from air to water, or air to glass, the direction of + the light rays is changed, or, to use the correct term, <i>refracted</i>. + This refraction of the rays of light only takes place when the incident + rays are passed obliquely; if the incident rays are perpendicular to the + surface separating the two media they are not refracted, but continue + their course in a straight line.</p> + + <p>All liquid and solid bodies that are sufficiently transparent to allow + light rays to pass through them possess the power of bending or + refracting the rays, the degree of refraction, as already explained, + depending upon the nature of the body.</p> + + <p>The law relating to refraction will perhaps be better understood by + means of the following diagram. In Fig. 64 let the line AB represent the + surface of a vessel of water. The line CD, which is perpendicular to the + surface of the <!-- Page 127 --><span class="pagenum"><a + name="page127"></a>{127}</span>water, is termed the <i>normal</i>, and a + ray of light passed in this direction will continue in a straight line to + the point E. If, however, the ray is passed in an oblique direction, such + as ND, it will be seen that the ray is bent or refracted in the direction + DM; if the ray of light is passed in any other oblique direction, such as + JD, the refracted ray will be in the direction DK. The angle NDC is + called the <i>angle of incidence</i> and MDE the <i>angle of + refraction</i>. If we measure accurately the line NC, we shall find that + it is 1<sup>1</sup>/<sub>3</sub>, or more exactly 1.336, times greater + than the line EM. If we repeat this measurement with the lines JH and PK + we shall find that the line JH also bears the proportion of 1.336 to the + line PK. The line NC is called the <i>sine of the angle of incidence</i> + NDC, and EM the <i>sine of the angle of refraction</i> MDE.</p> + + <div class="figcenter" style="width:20%;"> + <a href="images/illo-fig64.png"><img style="width:100%" src="images/illo-fig64.png" + alt="Fig. 64." title="Fig. 64." /></a> + <span class="sc">Fig.</span> 64. + </div> + + <p>Therefore in water the sine of the angle of incidence is to the sine + of the angle of refraction as 1.336 is to 1, and this is true whatever + the position of the incident ray with respect to the surface of the + water. From this we say that <i>the sines of the angles of incidence and + refraction have a constant proportion or ratio to one another</i>.</p> + + <p>The number 1.336 is termed the <i>refractive index</i>, or + <i>coefficient</i>, or the <i>refractive power</i> of water. The + refractive power varies, however, with other fluids and solids, and a + complete table will be found in any good work on optics.</p> + + <p>Glass is the substance most commonly used for refracting the rays of + light in optical work, the glass being worked up into different forms + according to the purpose for which it <!-- Page 128 --><span + class="pagenum"><a name="page128"></a>{128}</span>is intended. Solids + formed in this way are termed <i>lenses</i>. A lens can be defined as a + transparent medium which, owing to the curvature of its surfaces, is + capable of converging or diverging the rays of light passed through it. + According to its curvature it is either spherical, cylindrical, + elliptical, or parabolic. The lenses used in optics are always + exclusively spherical, the glass used in their construction being either + crown glass, which is free from lead, or flint glass, which contains lead + and is more refractive than crown glass. The refractive power of crown + glass is from 1.534 to 1.525, and of flint glass from 1.625 to 1.590. + Spherical surfaces in combination with each other or with plane surfaces + give rise to six different forms of lenses, sections of which are given + in Fig. 65.</p> + + <div class="figcenter" style="width:41%;"> + <a href="images/illo-fig65.png"><img style="width:100%" src="images/illo-fig65.png" + alt="Fig. 65." title="Fig. 65." /></a> + <span class="sc">Fig.</span> 65. + </div> + + <p>All lenses can be divided into two classes, convex or converging, or + concave or diverging. In the figure, <i>b</i>, <i>c</i>, <i>g</i> are + converging lenses, being thicker at the middle than at the borders, and + <i>d</i>, <i>e</i>, <i>f</i>, which are thinner at the middle, being + diverging lenses. The lenses <i>e</i> and <i>g</i> are also termed + meniscus lenses, and <i>a</i> represents a prism. The line XY is the axis + or <i>normal</i> of these lenses to which their plane surfaces are + perpendicular.</p> + + <p>Let us first of all notice the action of a ray of light when passed + through a prism. The prism, Fig. 66, is represented by the triangle BBB, + and the incident ray by the line TA. <!-- Page 129 --><span + class="pagenum"><a name="page129"></a>{129}</span>Where it enters the + prism at A its direction is changed and it is bent or refracted towards + the base of the prism, or towards the normal, this being always the case + when light passes from a rare medium to a dense one, and where the light + leaves the opposite face of the prism at D it is again refracted, but + away from the normal in an opposite direction to the incident ray, since + it is passing from a dense to a rare medium. The line DP is called the + <i>emergent</i> or refracted ray. If the eye is placed at T, and a bright + object at P, the object is seen not at P, but at the point H, since the + eye cannot follow the course taken by the refracted rays. In other words, + objects viewed through a prism always appear deflected towards its + summit.</p> + + <div class="figcenter" style="width:38%;"> + <a href="images/illo-fig66.png"><img style="width:100%" src="images/illo-fig66.png" + alt="Fig. 66." title="Fig. 66." /></a> + <span class="sc">Fig.</span> 66. + </div> + + <p>In considering the action of a lens we can regard any lens as being + built up of a number of prisms with curved faces in contact. Such a lens + is shown in Fig. 67, the light rays being refracted towards the base of + the prisms or towards the normal, as already explained; while the top + half of the lens will refract all the light downwards, the bottom half + will act as a series of inverted prisms and refract all the light + upwards.</p> + + <div class="figcenter" style="width:31%;"> + <a href="images/illo-fig67.png"><img style="width:100%" src="images/illo-fig67.png" + alt="Fig. 67." title="Fig. 67." /></a> + <span class="sc">Fig.</span> 67. + </div> + + <div class="figright" style="width:27%;"> + <a href="images/illo-fig68.png"><img style="width:100%" src="images/illo-fig68.png" + alt="Fig. 68." title="Fig. 68." /></a> + <span class="sc">Fig.</span> 68. + </div> + + <p>If a beam of parallel light—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 <!-- Page 130 --><span class="pagenum"><a + name="page130"></a>{130}</span>termed the principal focus of the lens, + and its distance from the lens is known as the focal length of that lens. + In a double and equally convex lens of glass the focal length is equal to + the radius of the spherical surfaces of the lens. If the lens is a + plano-convex the focal length is twice the radius of its spherical + surfaces. If the lens is unequally convex the focal length is found by + the following rule: multiply the two radii of its surfaces and divide + twice that product by the sum of the two radii, and the quotient will + <!-- Page 131 --><span class="pagenum"><a + name="page131"></a>{131}</span>be the focal length required. Conversely, + by placing a source of light at the point F the rays will be projected in + a parallel beam the same diameter as the lens. If, however, instead of + being parallel, the rays proceed from a point farther from the lens than + the principal focus, as at A, Fig. 69, they are termed divergent rays, + but they also will be brought to a focus at the other side of the lens at + the point <i>a</i>. If the source of light A is moved nearer to the + principal focus of the lens to a point A<sup>1</sup> the rays will come + to a focus at the point <i>a</i><sup>1</sup>, and similarly when the + light is at A<sup>2</sup> the rays will come to a focus at the point + <i>a</i><sup>2</sup>. It can be found by direct experiment that the + distance <i>fa</i> increases in the same proportion as AF diminishes, and + diminishes in the same proportion as AF increases. The relationship which + exists between pairs of points in this manner is termed the <i>conjugate + foci</i> of a lens, and though every lens has only one principal focus, + yet its conjugate foci are innumerable.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig69.png"><img style="width:100%" src="images/illo-fig69.png" + alt="Fig. 69." title="Fig. 69." /></a> + <span class="sc">Fig.</span> 69. + </div> + + <p>The formation of an image of some distant object in its principal + focus is one of the most useful properties of a convex lens, and it is + this property that forms the basis of several well-known optical + instruments, including the camera, telescope, microscope, etc.</p> + + <p>If we take an oblong wooden box, AA, and substitute a sheet of ground + glass, C, for one end, and drill a small pinhole, H, in the centre of the + other end opposite the <!-- Page 132 --><span class="pagenum"><a + name="page132"></a>{132}</span>glass plate, we shall find that a + tolerably good image of any object placed in front of the box will be + formed upon the glass plate. The light rays from all points of the + object, BD, Fig. 70, will pass straight through the hole H, and + illuminate the ground glass screen at points immediately opposite them, + forming a faint inverted image of the object BD. The purpose of the hole + H is to prevent the rays from any one point of the object from falling + upon any other point on the glass screen than the point immediately + opposite to it, therefore the smaller we make H, the more distinct will + be the image obtained. Reducing the size of H in order to produce a more + distinct image has the effect of causing the image to become very faint, + as the smaller the hole in H, the smaller the number of rays that can + pass through from any point of the object. By enlarging the hole H + gradually, the image will become more and more indistinct until such a + size is reached that it disappears altogether.</p> + + <div class="figcenter" style="width:38%;"> + <a href="images/illo-fig70.png"><img style="width:100%" src="images/illo-fig70.png" + alt="Fig. 70." title="Fig. 70." /></a> + <span class="sc">Fig.</span> 70. + </div> + + <p>If in this enlarged hole we place a double convex lens, LL, Fig. 71, + whose focal length suits the length of the box, the image produced will + be brighter and more distinct than that formed by the aperture, H, since + the rays which proceed from any point of the object will be brought by + the lens to a focus on the glass screen, forming a bright <!-- Page 133 + --><span class="pagenum"><a name="page133"></a>{133}</span>distinct image + of the point from which they come. The image owes its increased + distinctness to the fact that the rays from any one point of the object + cannot interfere with the rays from any other point, and its increased + brightness to the great number of rays that are collected by the lens + from each point of the object and focussed in the corresponding point of + the image. It will be evident from a study of Fig. 71 that the image + formed by a convex lens must necessarily be inverted, since it is + impossible for the rays from the end, M, of the object to be carried by + refraction to the upper end of the image at <i>n</i>. The relative + positions of the object and image when placed at different distances from + the lens are exactly the same as the conjugate foci of light rays as + shown in Fig. 69.</p> + + <div class="figcenter" style="width:33%;"> + <a href="images/illo-fig71.png"><img style="width:100%" src="images/illo-fig71.png" + alt="Fig. 71." title="Fig. 71." /></a> + <span class="sc">Fig.</span> 71. + </div> + + <p>The length of the image formed by a convex lens is to the length of + the object as the distance of the image is to the distance of the object + from the lens. For example, if a lens having a focal length of 12 inches + is placed at a distance of 1000 feet from some object, then the size of + the image will be to that of the object as 12 inches to 1000 feet, or + 1000 times smaller than the object; and if the length of the object is + 500 inches, then the length of the image will be the + <sup>1</sup>/<sub>1000</sub>th part of 500 inches, or + <sup>1</sup>/<sub>2</sub> inch. <!-- Page 134 --><span class="pagenum"><a + name="page134"></a>{134}</span></p> + + <p>The image formed by the convex lens in Fig. 71 is known as a <i>real + image</i>, but in addition convex lenses possess the property of forming + what are termed <i>virtual images</i>. The distinction can be expressed + by saying, <i>real images are those formed by the refracted rays + themselves, and virtual images those formed by their prolongations</i>. + While a real image formed by a convex lens is always inverted and smaller + than the object, the virtual image is always erect and larger than the + object. The power possessed by convex lenses of forming virtual images is + made use of in that useful but common piece of apparatus known as a + reading or magnifying glass, by which objects placed within its focus are + made larger or magnified when viewed through it; but in order to properly + understand how objects seem to be brought nearer and apparently increased + in size, we must first of all understand what is meant by the expression, + <i>the apparent magnitude of objects</i>.</p> + + <div class="figcenter" style="width:34%;"> + <a href="images/illo-fig72.png"><img style="width:100%" src="images/illo-fig72.png" + alt="Fig. 72." title="Fig. 72." /></a> + <span class="sc">Fig.</span> 72. + </div> + + <p>The apparent magnitude of an object depends upon the angle which it + subtends to the eye of the observer. The image at A, Fig. 72, presents a + smaller angle to the eye than the angle presented by the object when + moved to B, and the image therefore appears smaller. When the object is + moved to either B or C, it is viewed under a much <!-- Page 135 --><span + class="pagenum"><a name="page135"></a>{135}</span>greater angle, causing + the image to appear much larger. If we take a watch or other small + circular object and place it at A, which we will suppose is a distance of + 50 yards, we shall find that it will be only visible as a circular + object, and its apparent magnitude or the angle under which it is viewed + is then stated to be very small. If the object is now moved to the point + B, which is only 5 feet from the eye, its apparent magnitude will be + found to have increased to such an extent that we can distinguish not + only its shape, but also some of the marking. When moved to within a few + inches from the eye as at C, we see it under an angle so great that all + the detail can be distinctly seen. By having brought the object nearer + the eye, thus rendering all its parts clearly visible, we have actually + magnified it, or made it appear larger, although its actual size remains + exactly the same. When the distance between the object and the observer + is known, the apparent magnitude of the object varies inversely as the + distance from the observer.</p> + + <p>Let us suppose that we wish to produce an image of a tree situated at + a distance of 5000 feet. At this distance the light rays from the tree + will be nearly parallel, so that if a lens having a focal length of 5 + feet is fastened in any convenient manner in the wall of a darkened room + the image will be formed 5 feet behind the lens at its principal focus. + If a screen of white cardboard be placed at this point we shall find that + a small but inverted image of the tree will be focussed upon it. As the + distance of the object is 5000 feet, and as the size of the received + image is in proportion to this distance divided by the focal length of + the lens, the image will be as 5000 ÷ 5, or 1000 times smaller than the + object.</p> + + <p>If now the eye is placed six inches behind the screen and the screen + removed, so that we can view the small image distinctly in the air, we + shall see it with an apparent magnitude as much greater than if the same + small image were equally far off with the tree, as 6 inches is to 5000 + <!-- Page 136 --><span class="pagenum"><a + name="page136"></a>{136}</span>feet, that is 10,000 times. Thus we see + that although the image produced on the screen is 1000 times less than + the tree from one cause, yet on account of it being brought near to the + eye it is 10,000 times greater in apparent magnitude; therefore its + apparent magnitude is increased as 10,000 ÷ 1000, or 10 times. This means + that by means of the lens it has actually been magnified 10 times. This + magnifying power of a lens is always equal to the focal length divided by + the distance at which we see small objects most distinctly, viz. 6 + inches, and in the present instance is 60 ÷ 6, or 10 times.</p> + + <p>When the image is received upon a screen the apparatus is called a + <i>camera obscura</i>, but when the eye is used and sees the inverted + image in the air, then the apparatus is termed a <i>telescope</i>.</p> + + <p>The image formed by a convex lens can be regarded as a new object, and + if a second lens is placed behind it a second image will be formed in the + same manner as if the first image were a real object. A succession of + images can thus be formed by convex lenses, the last image being always + treated as a fresh object, and being always an inverted image of the one + before. From this it will be evident that additional magnifying power can + be given to our telescope with one lens by bringing the image nearer the + eye, and this is accomplished by placing a short focus lens between the + image and the eye. By using a lens having a focal length of 1 inch, and + such a lens will magnify 6 times, the total magnifying power of the two + lenses will be 10 × 6 = 60 times, or 10 times by the first lens and 6 + times by the second. Such an instrument is known as a <i>compound or + astronomical telescope</i>, and the first lens is called the object glass + and the second lens the magnifying glass, or eye-piece.</p> + + <p>We are now in a position to understand how virtual images are formed, + and the formation of a virtual image by means of a convex lens will be + readily followed from a <!-- Page 137 --><span class="pagenum"><a + name="page137"></a>{137}</span>study of Fig. 73. Let L represent a double + convex lens, with an object, AB, placed between it and the point F, which + is the principal focus of the lens. The rays from the object AB are + refracted on passing through the lens, and again refracted on leaving the + lens, so that an image of the object is formed at the eye, N. As it is + impossible for the eye to follow the bent rays from the object, a virtual + image is formed and is seen at A<sup>1</sup>B<sup>1</sup>, and is really + a continuation of the emergent rays. The magnifying power of such a lens + may be found by dividing 6 inches by the focal length of the lens, 6 + inches being the distance at which we see small objects most distinctly. + A lens having a focal length of <sup>1</sup>/<sub>4</sub> inch would + magnify 24 times, and one with a focal length of + <sup>1</sup>/<sub>100</sub>th of an inch 600 times, and so on. The + magnifying power is greater as the lens is more convex and the object + near to the principal focus. When a single lens is applied in this manner + it is termed a <i>single microscope</i>, but when more than one lens is + employed in order to increase the magnifying power, as in the telescope, + then the apparatus is termed a <i>compound microscope</i>.</p> + + <div class="figcenter" style="width:35%;"> + <a href="images/illo-fig73.png"><img style="width:100%" src="images/illo-fig73.png" + alt="Fig. 73." title="Fig. 73." /></a> + <span class="sc">Fig.</span> 73. + </div> + + <p>Unlike a convex lens, which can form both real and virtual images, a + concave lens can only produce a virtual image; and while the convex lens + forms an image larger <!-- Page 138 --><span class="pagenum"><a + name="page138"></a>{138}</span>than the object, the concave lens forms an + image smaller than the object. Let L, Fig. 74, represent a double concave + lens, and AB the object. The rays from AB on passing through the lens are + refracted, and they diverge in the direction RRRR, as if they proceeded + from the point F, which is the principal focus of the lens, and the + prolongations of these divergent rays produce a virtual image, erect and + smaller than the object, at A<sup>1</sup>B<sup>1</sup>. The principal + focal distance of concave lenses is found by exactly the same rule as + that given for convex lenses.</p> + + <div class="figcenter" style="width:37%;"> + <a href="images/illo-fig74.png"><img style="width:100%" src="images/illo-fig74.png" + alt="Fig. 74." title="Fig. 74." /></a> + <span class="sc">Fig.</span> 74. + </div> + + <p>Up to the present we have assumed that all the rays of light passed + through a convex lens were brought to a focus at a point common to all + the rays, but this is really only the case with a lens whose aperture + does not exceed 12°. By aperture is meant the angle obtained by joining + the edges of a lens with the principal focus. With lenses having a larger + aperture the amount of refraction is greater at the edges than at the + centre, and consequently the rays that pass through the edges of the lens + are brought to a focus nearer the lens than the rays that pass through + the centre. Since this defect arises from the spherical form of the lens + it is termed <i>spherical aberration</i>, and in lenses that <!-- Page + 139 --><span class="pagenum"><a name="page139"></a>{139}</span>are used + for photographic purposes the aberration has to be very carefully + corrected.</p> + + <p>The distortion of an image formed by a convex lens is shown by the + diagram, Fig. 75. If we receive the image upon a sheet of white cardboard + placed at A, we shall find that while the outside edges will be clear and + distinct, the inside will be blurred, the reverse being the case when the + cardboard is moved to the point B.</p> + + <div class="figcenter" style="width:39%;"> + <a href="images/illo-fig75.png"><img style="width:100%" src="images/illo-fig75.png" + alt="Fig. 75." title="Fig. 75." /></a> + <span class="sc">Fig.</span> 75. + </div> + + <div class="figleft" style="width:8%;"> + <a href="images/illo-fig76.png"><img style="width:100%" src="images/illo-fig76.png" + alt="Fig. 76." title="Fig. 76." /></a> + <span class="sc">Fig.</span> 76. + </div> + + <div class="figright" style="width:10%;"> + <a href="images/illo-fig77.png"><img style="width:100%" src="images/illo-fig77.png" + alt="Fig. 70." title="Fig. 70." /></a> + <span class="sc">Fig.</span> 77. + </div> + + <p>Aberration is to a great extent minimised by giving to the lens a + meniscus instead of a biconvex form, but as it is desirable to reduce the + aberration to below once the <!-- Page 140 --><span class="pagenum"><a + name="page140"></a>{140}</span>thickness of the lens, and as this cannot + be done by a single lens, we must have recourse to two lenses put + together. The thickness of a lens is the difference between its thickness + at the middle and at the circumference. In a double convex lens with + equal convexities the aberration is 1<sup>67</sup>/<sub>100</sub>ths of + its thickness. In a plano-convex lens with the plane side turned towards + parallel rays the aberration is 4<sup>1</sup>/<sub>2</sub> times its + thickness, but with the convex side turned towards parallel rays the + aberration is only 1<sup>17</sup>/<sub>100</sub>ths of its thickness.</p> + + <p>By making use of two plano-convex lenses placed together as at Fig. + 76, the aberration will be one-fourth of that of a single lens, but the + focal length of the lens, L<sup>1</sup>, must be half as much again as + that of L. If their focal lengths are equal the aberration will only be a + little more than half reduced. Spherical aberration, however, may be + entirely destroyed by combining a meniscus and double convex lens, as + shown in Fig. 77, the convex side being turned to the eye when used as a + lens, and to parallel rays when used as a burning glass or condenser.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<p><!-- Page 141 --><span class="pagenum"><a name="page141"></a>{141}</span></p> + +<h3>INDEX</h3> + + <div class="poem"> + <div class="stanza"> + <p>Aberration, <a href="#page139">139</a></p> + <p class="i2">spherical, <a href="#page138">138</a>, <a href="#page140">140</a></p> + <p>Accuracy of working, <a href="#page70">70</a>, <a href="#page72">72</a></p> + <p>Acetylene gas lamps, <a href="#page120">120</a></p> + <p>Actinic power, <a href="#page102">102</a></p> + <p>Actinograph, <a href="#page105">105</a></p> + <p>Actinometer, <a href="#page120">120</a></p> + <p>Alternating current, <a href="#page82">82</a>, <a href="#page100">100</a></p> + <p>Ammonia, <a href="#page123">123</a></p> + <p>Angle of stylus, <a href="#page24">24</a>, <a href="#page78">78</a></p> + <p>Aniline dye, <a href="#page123">123</a></p> + <p>Arcing, <a href="#page27">27</a>, <a href="#page82">82</a></p> + <p>Arc lamps, <a href="#page15">15</a>, <a href="#page120">120</a>, <a href="#page121">121</a></p> + <p>Atmospherics, <a href="#page61">61</a>, <a href="#page85">85</a></p> + </div> + + <div class="stanza"> + <p>Ballasting resistance, <a href="#page100">100</a></p> + <p>Belin, <a href="#page47">47</a></p> + <p>Bernochi, <a href="#page7">7</a>, <a href="#page112">112</a></p> + <p class="i2">system of, <a href="#page7">7</a>, <a href="#page34">34</a></p> + <p>Berzelius, <a href="#page109">109</a></p> + <p>Bichromate of potash, <a href="#page120">120</a></p> + <p>Blondel's oscillograph, <a href="#page47">47</a></p> + </div> + + <div class="stanza"> + <p>Camera obscura, <a href="#page136">136</a></p> + <p class="i2">extension, <a href="#page116">116</a>, <a href="#page118">118</a></p> + <p class="i2">choice of, <a href="#page117">117</a></p> + <p>Capacity of condenser, <a href="#page24">24</a>, <a href="#page78">78</a></p> + <p class="i2">electrostatic, <a href="#page3">3</a>, <a href="#page5">5</a></p> + <p class="i2">of cable, <a href="#page3">3</a></p> + <p class="i2">of London-Paris telephone line, <a href="#page3">3</a></p> + <p>Carbon bisulphide, <a href="#page53">53</a></p> + <p>Charbonelle, <a href="#page48">48</a></p> + <p class="i2">receiver of, <a href="#page48">48</a></p> + <p>Chemical solution, <a href="#page56">56</a></p> + <p>Circuit breaker, <a href="#page76">76</a></p> + <p>Clutch, details of, <a href="#page88">88</a>, <a href="#page89">89</a>, <a href="#page91">91</a></p> + <p class="i2">spring, <a href="#page71">71</a></p> + <p>Coating the metal sheets, <a href="#page120">120</a></p> + <p>Coherer, <a href="#page11">11</a>, <a href="#page40">40</a></p> + <p>Collecting rings, <a href="#page91">91</a></p> + <p>Commercial value of photo-telegraphy, <a href="#page1">1</a></p> + <p>Compensating selenium cell, <a href="#page112">112</a></p> + <p>Contact breaker, <a href="#page37">37</a></p> + <p>Copying arrangements, <a href="#page118">118</a>, <a href="#page125">125</a></p> + <p>Cross screen, <a href="#page21">21</a></p> + </div> + + <div class="stanza"> + <p>De' Arsonval galvanometer, <a href="#page47">47</a>, <a href="#page73">73</a></p> + <p>Decoherer, <a href="#page41">41</a></p> + <p>Design of machines, <a href="#page21">21</a></p> + <p>Detectors, <a href="#page83">83</a></p> + <p>Developing solutions, <a href="#page105">105</a>, <a href="#page122">122</a></p> + <p>Diaphragm, movement of, <a href="#page48">48</a>, <a href="#page52">52</a>, <a href="#page84">84</a>, <a href="#page87">87</a></p> + <p>Dipping rods, <a href="#page81">81</a>, <a href="#page83">83</a></p> + <p>Distance of transmission, <a href="#page33">33</a></p> + <p>Duration of wave-trains, <a href="#page22">22</a>, <a href="#page25">25</a></p> + </div> + + <div class="stanza"> + <p>Early experiments, <a href="#page2">2</a></p> + <p>Einthoven galvanometer, <a href="#page32">32</a>, <a href="#page44">44</a>, <a href="#page45">45</a>, <a href="#page54">54</a>, <a href="#page113">113</a></p> + <p>Electric clock, <a href="#page93">93</a></p> + <p>Electrolytic receiver, <a href="#page4">4</a>, <a href="#page37">37</a>, <a href="#page54">54</a>, <a href="#page61">61</a>, <a href="#page64">64</a></p> + <p>Enlarging arrangements, <a href="#page124">124</a>, <a href="#page125">125</a></p> + <p>Experimental machine, <a href="#page20">20</a></p> + <p>Extraneous light, <a href="#page47">47</a></p> + </div> + + <div class="stanza"> + <p>Fastening electrolytic paper, <a href="#page58">58</a></p> + <p>Fatigue of selenium cell, <a href="#page64">64</a>, <a href="#page114">114</a></p> + <p>Fish glue, <a href="#page120">120</a></p> + <p>Flexible couplings, <a href="#page77">77</a></p> + <p>Frequency meter, <a href="#page65">65</a></p> + <p>Friction brake, <a href="#page88">88</a></p> + </div> + + <div class="stanza"> +<!-- Page 142 --><span class="pagenum"><a name="page142"></a>{142}</span> + <p>High speed telegraphy, <a href="#page70">70</a></p> + <p>Hughes governor, <a href="#page65">65</a></p> + <p>Hughes printing telegraph, <a href="#page63">63</a></p> + <p>Hurter and Driffield, <a href="#page104">104</a></p> + <p>Hydrogen, <a href="#page100">100</a></p> + </div> + + <div class="stanza"> + <p>Incidence, angle of, <a href="#page127">127</a></p> + <p>Inertia, <a href="#page64">64</a>, <a href="#page65">65</a>, <a href="#page111">111</a></p> + <p class="i2">effects in photo-telegraphy, <a href="#page110">110</a></p> + <p class="i2">method of counteracting, <a href="#page103">103</a>, <a href="#page112">112</a>, <a href="#page113">113</a></p> + <p class="i2">effect of wave-length of light on, <a href="#page114">114</a></p> + <p>Intensifying solution, <a href="#page122">122</a></p> + <p>Isochroniser, <a href="#page89">89</a>, <a href="#page91">91</a></p> + <p class="i2">details of, <a href="#page91">91</a>, <a href="#page92">92</a>, <a href="#page95">95</a></p> + <p>Isochronism, <a href="#page64">64</a>, <a href="#page69">69</a>, <a href="#page70">70</a>, <a href="#page71">71</a></p> + </div> + + <div class="stanza"> + <p>Kathode rays, <a href="#page53">53</a></p> + <p>Knudsen, <a href="#page2">2</a></p> + <p class="i2">apparatus of, <a href="#page9">9</a></p> + <p>Korn, <a href="#page30">30</a>, <a href="#page33">33</a>, <a href="#page45">45</a>, <a href="#page65">65</a>, <a href="#page72">72</a></p> + <p class="i2">apparatus of, <a href="#page31">31</a></p> + </div> + + <div class="stanza"> + <p>Lamps, coloured, <a href="#page94">94</a></p> + <p>Lenses, <a href="#page85">85</a>, <a href="#page125">125</a>, <a href="#page128">128</a></p> + <p class="i2">principal focus of, <a href="#page130">130</a></p> + <p class="i2">conjugate foci of, <a href="#page131">131</a></p> + <p class="i2">action of, <a href="#page129">129</a></p> + <p class="i2">convex, <a href="#page128">128</a>, <a href="#page131">131</a>, <a href="#page136">136</a></p> + <p class="i2">concave, <a href="#page128">128</a>, <a href="#page138">138</a></p> + <p class="i2">focal length of, <a href="#page130">130</a>, <a href="#page138">138</a></p> + <p class="i2">aperture, <a href="#page138">138</a></p> + <p class="i2">meniscus, <a href="#page139">139</a></p> + <p>Light, diffusion of, <a href="#page86">86</a></p> + <p class="i2">extraneous, <a href="#page87">87</a></p> + <p>Limit of error in synchronising, <a href="#page64">64</a></p> + <p>Line balancer, <a href="#page3">3</a></p> + <p>Line screens, <a href="#page9">9</a>, <a href="#page15">15</a>, <a href="#page16">16</a>, <a href="#page116">116</a></p> + <p class="i2">making, <a href="#page116">116</a></p> + </div> + + <div class="stanza"> + <p>Magnifying power, <a href="#page136">136</a>, <a href="#page137">137</a></p> + <p>Marconi valve, <a href="#page44">44</a>, <a href="#page54">54</a></p> + <p class="i2">coherer, <a href="#page40">40</a></p> + <p>Mechanical inertia, <a href="#page33">33</a></p> + <p>Mercury break, <a href="#page81">81</a></p> + <p class="i2">churning of, <a href="#page82">82</a></p> + <p class="i2">containers, <a href="#page82">82</a></p> + <p>Mercury jet interrupter, <a href="#page29">29</a></p> + <p>Metal prints, <a href="#page15">15</a>, <a href="#page18">18</a>, <a href="#page32">32</a>, <a href="#page59">59</a>, <a href="#page64">64</a>, <a href="#page95">95</a>, <a href="#page120">120</a>, <a href="#page124">124</a></p> + <p class="i2">drying the, <a href="#page121">121</a>, <a href="#page123">123</a></p> + <p class="i2">exposure of, <a href="#page121">121</a></p> + <p class="i2">size of, <a href="#page22">22</a>, <a href="#page24">24</a>, <a href="#page75">75</a>, <a href="#page77">77</a></p> + <p class="i2">pressing the, <a href="#page22">22</a></p> + <p>Microscope, <a href="#page131">131</a>, <a href="#page137">137</a></p> + <p>Military uses, <a href="#page35">35</a></p> + <p>Mirror galvanometer, <a href="#page9">9</a>, <a href="#page42">42</a>, <a href="#page73">73</a></p> + <p>Mirror, <a href="#page47">47</a>, <a href="#page51">51</a></p> + <p>Morse code, <a href="#page35">35</a></p> + <p>Motor speed, <a href="#page89">89</a>, <a href="#page95">95</a></p> + <p class="i2">driving, <a href="#page91">91</a>, <a href="#page93">93</a>, <a href="#page95">95</a></p> + <p class="i2">clockwork, <a href="#page63">63</a></p> + <p class="i2">electric, <a href="#page63">63</a></p> + </div> + + <div class="stanza"> + <p>Nernst lamps, <a href="#page43">43</a>, <a href="#page85">85</a>, <a href="#page98">98</a></p> + <p class="i2">heater of, <a href="#page99">99</a></p> + <p class="i2">filament of, <a href="#page99">99</a></p> + <p class="i2">principle of, <a href="#page98">98</a></p> + <p class="i2">resistance of, <a href="#page100">100</a></p> + <p class="i2">efficiency of, <a href="#page101">101</a>, <a href="#page102">102</a></p> + <p class="i2">overrunning, <a href="#page101">101</a></p> + <p>Nicol prism, <a href="#page53">53</a></p> + </div> + + <div class="stanza"> + <p>Paper for electrolytic receiver, <a href="#page56">56</a></p> + <p>Parabolic reflector, <a href="#page8">8</a></p> + <p>Period of galvanometer, <a href="#page43">43</a>, <a href="#page44">44</a>, <a href="#page46">46</a></p> + <p><i>Photographic Daily Companion</i>, <a href="#page105">105</a></p> + <p>Photographic films, <a href="#page40">40</a>, <a href="#page43">43</a>, <a href="#page45">45</a>, <a href="#page53">53</a>, <a href="#page54">54</a>, <a href="#page62">62</a>, <a href="#page85">85</a>, <a href="#page86">86</a>, <a href="#page98">98</a></p> + <p class="i2">process, <a href="#page37">37</a></p> + <p class="i2">chemical inertia, <a href="#page103">103</a></p> + <p class="i2">exposure of, <a href="#page103">103</a>, <a href="#page107">107</a></p> + <p class="i2">speed of, <a href="#page104">104</a>, <a href="#page105">105</a></p> + <p class="i2">plates, orthochromatic, <a href="#page59">59</a></p> + <p class="i2">plates, <a href="#page120">120</a></p> + <p>Points to be observed in preparing metal prints, <a href="#page123">123</a></p> + <p>Poulsen Company, <a href="#page32">32</a>, <a href="#page47">47</a></p> + <p class="i2">arc, <a href="#page31">31</a></p> + <p>Preparing selenium, <a href="#page109">109</a></p> + <p class="i2">photographs for transmitting, <a href="#page15">15</a>, <a href="#page115">115</a></p> + <p class="i2">sketches on metal foil, <a href="#page124">124</a></p> + <p>Prism, <a href="#page128">128</a></p> + <p class="i2">action of, <a href="#page129">129</a></p> + <p>Process plates, <a href="#page122">122</a></p> + <p>Professor Nernst, <a href="#page98">98</a></p> + </div> + + <div class="stanza"> +<!-- Page 143 --><span class="pagenum"><a name="page143"></a>{143}</span> + <p>Radio-photography, requirements of, <a href="#page74">74</a></p> + <p>Refraction, angle of, <a href="#page127">127</a></p> + <p>Refractive power, <a href="#page127">127</a></p> + <p>Relay, <a href="#page25">25</a>, <a href="#page39">39</a>, <a href="#page49">49</a>, <a href="#page53">53</a>, <a href="#page55">55</a>, <a href="#page60">60</a>, <a href="#page75">75</a></p> + <p class="i2">differential, <a href="#page79">79</a></p> + <p class="i2">polarised, <a href="#page97">97</a></p> + <p class="i2">working speed of, <a href="#page26">26</a>, <a href="#page75">75</a></p> + <p>Reproducing for newspapers, <a href="#page60">60</a></p> + <p>Resistance of selenium, <a href="#page109">109</a></p> + <p class="i2">of selenium cells, <a href="#page110">110</a></p> + <p class="i2">regulating, <a href="#page113">113</a></p> + <p>Retardation of current, <a href="#page6">6</a></p> + <p>Retouching, <a href="#page62">62</a></p> + <p>Rotary spark-gap, <a href="#page28">28</a></p> + </div> + + <div class="stanza"> + <p>Selenium, <a href="#page99">99</a></p> + <p class="i2">cells, <a href="#page8">8</a>, <a href="#page34">34</a>, <a href="#page55">55</a>, <a href="#page60">60</a>, <a href="#page64">64</a>, <a href="#page109">109</a>, <a href="#page110">110</a></p> + <p class="i2">machines, <a href="#page45">45</a></p> + <p>Self-induction, <a href="#page24">24</a>, <a href="#page78">78</a></p> + <p>Sensitiveness of selenium cells, <a href="#page113">113</a></p> + <p class="i2">ratio of, <a href="#page113">113</a></p> + <p>Silvered quartz threads, <a href="#page44">44</a>, <a href="#page46">46</a></p> + <p>Spark-gap, <a href="#page27">27</a></p> + <p>Speed regulator, <a href="#page68">68</a></p> + <p class="i2">adjustments of, <a href="#page69">69</a></p> + <p>Spring clutch, <a href="#page71">71</a></p> + <p>Starting position of machines, <a href="#page98">98</a></p> + <p>String galvanometer, <a href="#page32">32</a></p> + <p>Stylus, <a href="#page17">17</a>, <a href="#page18">18</a>, <a href="#page57">57</a>, <a href="#page61">61</a>, <a href="#page78">78</a>, <a href="#page95">95</a>, <a href="#page103">103</a></p> + <p class="i2">sparking at, <a href="#page24">24</a></p> + <p>Stylus, angle of, <a href="#page24">24</a>, <a href="#page78">78</a></p> + <p class="i2">defects of, <a href="#page57">57</a></p> + <p>Submarine cable, <a href="#page4">4</a></p> + <p>Synchronism, <a href="#page11">11</a>, <a href="#page20">20</a>, <a href="#page36">36</a>, <a href="#page64">64</a>, <a href="#page69">69</a>, <a href="#page71">71</a></p> + </div> + + <div class="stanza"> + <p>Telephograph, <a href="#page74">74</a></p> + <p class="i2">advantages of, <a href="#page76">76</a></p> + <p class="i2">method of working, <a href="#page96">96</a></p> + <p>Telephone receiver, <a href="#page83">83</a>, <a href="#page85">85</a></p> + <p class="i2">diaphragm, <a href="#page48">48</a></p> + <p class="i2">improved, <a href="#page51">51</a></p> + <p>Telephone relay, <a href="#page48">48</a>, <a href="#page50">50</a>, <a href="#page52">52</a>, <a href="#page83">83</a>, <a href="#page85">85</a>, <a href="#page97">97</a></p> + <p>Telescope, <a href="#page131">131</a>, <a href="#page136">136</a></p> + <p>Thermodetector, <a href="#page32">32</a></p> + <p>Tow, <a href="#page88">88</a></p> + <p>Transmission, distance of, <a href="#page35">35</a>, <a href="#page72">72</a></p> + <p class="i2">speed of, <a href="#page25">25</a>, <a href="#page35">35</a>, <a href="#page75">75</a></p> + </div> + + <div class="stanza"> + <p>Vibration, natural period of, <a href="#page39">39</a></p> + </div> + + <div class="stanza"> + <p>Watkins, <a href="#page105">105</a></p> + <p class="i2">power number, <a href="#page105">105</a></p> + <p>Waves, damped, <a href="#page30">30</a></p> + <p class="i2">undamped, <a href="#page30">30</a>, <a href="#page31">31</a></p> + <p>Wheatstone bridge, <a href="#page113">113</a></p> + <p>Wireless apparatus, <a href="#page13">13</a></p> + <p><i>Wireless World</i>, <a href="#page31">31</a></p> + <p>Wynne, <a href="#page105">105</a></p> + </div> + + <div class="stanza"> + <p>Zirconia, <a href="#page99">99</a></p> + </div> + </div> + + <p> </p> + +<p class="cenhead">THE END</p> + + <p> </p> + +<p class="cenhead"><i>Printed by</i> <span class="sc">R. & R. Clark, Limited</span>, <i>Edinburgh</i>.</p> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<h3>PUBLICATIONS OF</h3> + +<h1>THE WIRELESS PRESS, <span class="sc">Ltd.</span></h1> + +<h3>12 AND 13 HENRIETTA STREET,<br /> +STRAND, LONDON, W.C.2.</h3> + + <p><b>The Year Book of Wireless Telegraphy and Telephony.</b></p> + + <p>With Map of the World, showing Wireless Stations; British, Colonial + and foreign "Wireless" Laws and Regulations. Price <b>10s. 6d.</b> net. + (<b>Post free, 11s. Inland; 11s. 4d. Abroad.</b>)</p> + + <p><b>The Wireless Telegraphists' Pocket Book of Notes, Formulæ and + Calculations.</b></p> + + <p>By Dr. <span class="sc">J. A. Fleming</span>, M.A., D.Sc., F.R.S., + M.Inst.E.E., etc. A valuable compendium for Wireless Engineers and + Operators. Price <b>9s.</b> net. (<b>Postage 5d.</b>)</p> + + <p><b>The Handbook of Technical Instruction for Wireless + Telegraphists.</b></p> + + <p>By <span class="sc">J. C. Hawkhead</span> and <span class="sc">H. M. + Dowsett</span>, M.I.E.E. Provides a complete theoretical course for the + Postmaster-General's certificate of proficiency. 310 pages. 240 Diagrams + and Illustrations. Price <b>7s.</b> net. (<b>Postage 6d.</b>)</p> + + <p><b>Manual de Instrucción Técnica para Operadores de Telegrafia sin + Hilos.</b></p> + + <p>Por <span class="sc">J. C. Hawkhead</span> y <span class="sc">H. M. + Dowsett</span>, M.I.E.E. Precio: España, <b>10</b> pesetas; Franqueo, 1 + peseta extra. América Latina, <b>$2.25</b>, oro, neto; Franqueo, 25 cents + extra. (Great Britain, <b>9s.</b>; <b>Postage 6d.</b>)</p> + + <p><b>The Elementary Principles of Wireless Telegraphy.</b></p> + + <p>By <span class="sc">R. D. Bangay</span>. In two Parts. Price + <b>3s.</b> each. (<b>Postage 4d.</b>) Or in one Volume, price <b>7s.</b> + net. (<b>Postage 6d.</b>) Used by H.M. Government for instructional + purposes.</p> + + <p><b>Principios Element ales de Telegrafia sin Hilos.</b></p> + + <p>Por <span class="sc">R. D. Bangay</span>. (Partes 1a y 2a en un + Volumen.) <b>Precio</b>: España, <b>10</b> pesetas; Franqueo, 1 peseta + extra. América Latina, <b>$2.25</b>, oro, neto; Franqueo, 25 cents extra. + (Great Britain, <b>9s.</b>; <b>Postage 6d.</b>)</p> + + <p><b>Principes Élémentaires de Télégraphie sans Fil.</b></p> + + <p>Par <span class="sc">R. D. Bangay</span>. (Great Britain, <b>9s.</b>; + <b>Postage 6d.</b>)</p> + + <p><b>Magnetism and Electricity for Home Study.</b></p> + + <p>By <span class="sc">H. E. Penrose</span>. Crown 8vo. Over 500 pages. + Price <b>5s</b>. net, (<b>Postage 6d.</b>) Contains fifty complete + lessons.</p> + + <p><b>The Calculation and Measurement of Inductance and Capacity.</b></p> + + <p>By <span class="sc">W. H. Nottage</span>, B.Sc. Invaluable to all + engaged in Telegraph Engineering. Indispensable to the Wireless Engineer, + Student and Experimenter. Price <b>3s. 6d.</b> net. (<b>Postage + 5d.</b>)</p> + + <p><b>A Short Course in Elementary Mathematics and their application to + Wireless Telegraphy.</b></p> + + <p>By <span class="sc">S. J. Willis</span>. To Students in Wireless + Telegraphy, as well as those engaged in the practical application of this + Science, this book should prove of real value. Price <b>3s. 6d.</b> net. + (<b>Postage 6d.</b>)</p> + + <p><b>The Marconi Official Gramophone Records.</b></p> + + <p>For self-tuition in receiving Morse Signals. Price <b>4s.</b> each, + double-sided. (<b>Postage 9d.</b>) Set of Six Records, <b>24s.</b> post + <b>free</b>.</p> + + <p><b>The Maintenance of Wireless Telegraph Apparatus.</b></p> + + <p>By <span class="sc">P. W. Harris</span>. An up-to-date Manual, full of + practical hints and explanations. Diagrams of all ship installations, + from ¼ kw. to 5 kw. Price <b>2s. 6d.</b> net. (<b>Postage 4d.</b>)</p> + + <p><b>Dictionary of Technical Terms used in Wireless Telegraphy.</b></p> + + <p>By <span class="sc">Harold Ward</span>. Vest Pocket Edition. 2nd + Edition, revised and enlarged. Contains over 1500 definitions. Price + <b>2s. 6d.</b> net. (<b>Postage 2d.</b>)</p> + + <p><b>Armature Model for 1½ kw. Rotary Converter.</b></p> + + <p>Shows every Winding of the Converter Armature from start to finish. + Price <b>1s.</b> net. (<b>Postage 3d.</b>)</p> + + <p><b>Morse Made Easy.</b></p> + + <p>By <span class="sc">A. L. Rye</span>. Linen backed, for rapidly + learning the Morse Code. Price <b>3d.</b> net, or post free + <b>3½d.</b></p> + + <p><b>Morse Code Card.</b></p> + + <p>Contains full alphabet, with punctuation marks, figures, abbreviations + and contractions. Price <b>2d.</b>, post free.</p> + + <p><b>Practical Wireless Telegraphy.</b></p> + + <p>By <span class="sc">E. E. Bucher</span>. 352 pages. 340 Illustrations. + Price <b>12s. 6d.</b> (<b>Postage 6d.</b>)</p> + + <p><b>Radio-Telephony.</b></p> + + <p>By <b>Alfred N. Goldsmith</b>, Ph.D. 256 pages. 226 Illustrations. + Price <b>15s.</b> net. (<b>Postage 6d.</b>)</p> + + <p><b>Standard Tables and Equations in Radio-Telegraphy.</b></p> + + <p>By <span class="sc">Bertram Hoyle</span>, M.Sc.Tech., A.M.I.E.E. 159 + pages. Price <b>9s.</b> net. (<b>Postage 6d.</b>)</p> + + <p><b>Vacuum Tubes in Wireless Communication.</b></p> + + <p>By <span class="sc">E. E. Bucher</span>. Deals with the Oscillation + Valve. 178 pages. 130 Illustrations. Price <b>12s. 6d.</b> net. + (<b>Postage 6d.</b>)</p> + + <p><b>Useful Notes on Wireless Telegraphy.</b> (Students' Library.)</p> + + <p>By <span class="sc">Harold E. Penrose</span>. Price <b>1s. 4d.</b> net + each. (<b>Postage 2d.</b>)</p> + + <div class="poem"> + <div class="stanza"> + <p>Book I. DIRECT CURRENT.</p> + <p>Book II. ALTERNATING CURRENT.</p> + <p>Book III. HIGH-FREQUENCY CURRENT AND WAVE PRODUCTION.</p> + <p>Book IV. THE 1½ KW. SHIP SET.</p> + <p>Book V. THE OSCILLATION VALVE.</p> + </div> + </div> + + <p><b>The Oscillation Valve: The Elementary Principles of its Application + to Wireless Telegraphy.</b></p> + + <p>By <span class="sc">R. D. Bangay</span>. 215 pages. Price <b>5s.</b> + (<b>Postage 3d.</b>)</p> + + <p><b>The Thermionic Valve and its Developments in Radio-Telegraphy and + Telephony.</b></p> + + <p>By Dr. <span class="sc">J. A. Fleming</span>, M.A., D.Sc., F.R.S., + M.Inst.E.E., etc. 279 pages. Price <b>15s.</b> (<b>Postage 6d.</b>)</p> + + <p><b>Alternating Current Work: An Outline for Students of Wireless + Telegraphy.</b></p> + + <p>By <span class="sc">A. Shore</span>. 163 pages. Price <b>3s. 6d.</b> + (<b>Postage 4d.</b>)</p> + + <p><b>Telephony without Wires.</b></p> + + <p>By <span class="sc">Philip R. Coursey</span>, B.Sc., A.M.I.E.E., + F.P.S.L. 414 pages. Price <b>15s.</b> (<b>Postage 6d.</b>)</p> + + <p><b>The Wireless World.</b></p> + + <p>A Monthly Magazine devoted to Wireless Telegraphy and Telephony. Price + <b>9d.</b> (<b>Postage 3d.</b>) Annual Subscription, <b>11s.</b> post + free.</p> + + <p><b>The Radio Review.</b></p> + + <p>A Monthly Record of Scientific Progress in Radio-telegraphy and + Telephony. Price <b>2s. 6d.</b> (<b>Postage 3d.</b>) Annual Subscription, + <b>30s.</b> post free.</p> + + <p><b>Conquest.</b></p> + + <p>A Popular Illustrated Monthly Magazine dealing with Science, Industry + and Invention. Price <b>1s.</b> (<b>Postage 3d.</b>) Annual Subscription, + <b>15s.</b> post free.</p> + + <p><b>Continuous Wave Wireless Telegraphy.</b> Part I.</p> + + <p>By Dr. <span class="sc">W. H. Eccles</span>, D.Sc., A.R.C.S., M.I.E.E. + [<i>In the Press.</i></p> + + <p><br style="clear:both" /></p> +<hr class="short" /> + +<h3>COMPLETE CATALOGUE POST FREE.</h3> + + <p><br style="clear:both" /></p> +<hr class="full" /> + +<h3>Notes</h3> + +<div class="note"> + <p><a name="Nt1" href="#NtA1">[1]</a> These measurements only apply to a + single line. Where a double line is employed the capacity is halved.</p> + + <p><a name="Nt2" href="#NtA2">[2]</a> See Appendix A.</p> + + <p><a name="Nt3" href="#NtA3">[3]</a> See Appendix B.</p> + + <p><a name="Nt4" href="#NtA4">[4]</a> In wireless telegraphy "arcing" is + principally caused by the continuation of the supply current in the + spark-gap after the capacity has been charged to a potential sufficient + to break down the insulation of the gap.</p> + + <p><a name="Nt5" href="#NtA5">[5]</a> See Chapter V.</p> + + <p><a name="Nt6" href="#NtA6">[6]</a> Nernst lamps are the best to use, + as they produce abundantly the blue and violet rays which have the + greatest chemical effect upon a photographic film. Carbon filament lamps + are very poor in this respect.</p> + + <p><a name="Nt7" href="#NtA7">[7]</a> A description of the apparatus + required will be found in Ganot's <i>Physics</i>.</p> + + <p><a name="Nt8" href="#NtA8">[8]</a> Great care must be exercised in + using this solution, as it is exceedingly poisonous.</p> + + <p><a name="Nt9" href="#NtA9">[9]</a> Two clocks would isochronise if + their hands travelled at precisely the same rate round the dials, but + would not synchronise unless they both registered the same time as + well.</p> + + <p><a name="Nt10" href="#NtA10">[10]</a> Line screens can be obtained + from Messrs. Penrose, 109 Farringdon Street, London; or Messrs. + Fallowfield, 146 Charing Cross Road, London.</p> + +<p> </p> +<p> </p> +<hr class="pg" /> +</div> +<p>***END OF THE PROJECT GUTENBERG EBOOK WIRELESS TRANSMISSION OF PHOTOGRAPHS***</p> +<p>******* This file should be named 34052-h.txt or 34052-h.zip *******</p> +<p>This and all associated files of various formats will be found in:<br /> +<a href="http://www.gutenberg.org/dirs/3/4/0/5/34052">http://www.gutenberg.org/3/4/0/5/34052</a></p> +<p>Updated editions will replace the previous one--the old editions +will be renamed.</p> + +<p>Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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