910] oil the Telegraphy of Photographs, Wireless and hy Wire. 841 



transmitting cylinder can be used as the receiving cylinder, and the 

 apparatus is thus reduced to the limits of simphcity. 



Towards the end of last year I designed a portable machine, two 

 of which Mr. Sanger-Shepherd has just completed, embodying in 

 them a number of improvements of his own, and these machines, 

 which have worked successfully on their trials, are shown on the 

 lecture table to-night. They are suitable for line or wireless work, 

 and will, I believe, prove of great value in naval and military 

 operations. 



The ' Daily Mirror ' inaugurated the Paris-London photographic 

 service in November 1907 with Professor Korn's selenium instru- 

 ments, which I shall briefly describe, as Korn is now making two 

 new selenium apparatus with a view to transmitting photographs 

 from New York to London. In this system use is made of the fact 

 that the electrical resistance of the metal selenium varies according to 

 the strength of illumination to which it is subjected, a beam of 

 light passed through the hght and dark parts of a photograph in 

 succession being used to vary the strength of an electric current sent 

 to the receiving apparatus. 



In Korn's selenium transmitter light is concentrated from a 

 Nernst lamp to pass through a revolving glass cylinder, round which 

 a transparent photograph (printed on celluloid) is fixed, the beam 

 traversing the film at its brightest part, where the rays come to a 

 focus. The hght which passes through the picture is reflected by a 

 prism inside the cylinder on to the selenium cell, through which the 

 current passes. Across the cu'cuit is shunted a galvanometer of the 

 Einthoven pattern, containing two fine silver strings free to move 

 laterally in a strong magnetic field. These are represented by A B, 

 the magnet poles being M M. When a bright part of the photograph 

 admits of light falling on the sensitive cell current passes through 

 A B, and it shifts aside, allowing light from a Nernst lamp Ng to 

 enter the prism P, whence it is reflected on to the second cell S S. 

 The telephone lines connecting the two instruments go direct to the 

 wires of a similar galvanometer, which is in series with the galvano- 

 meter of the transmitting instrument. If we imagine MM to be 

 the receiving galvanometer, then we remove the prism P, and the 

 light acts on a sensitive photographic film attached to the drum C, 

 which revolves synchronously with the glass cyhnder of the sending 

 instrument. 



The inertia of selenium once overcome, the metal immediately 

 becomes of great use for many purposes. Professor Korn's method 

 of compensation is to let the light fall at the same time on two cells 

 of opposite characteristics ; one has great inertia and small sensitive- 

 ness, the other low mertia and great sensitiveness. By using the 

 two cehs on opposite sides of a Wheatstone bridge, dividing the 

 battery into two parts for the other sides, the deflection in the 

 galvanometer is very rapid. You will see the effect from the two 



