PHOTOGRAPHS BY TELEGRAPHY 



335 



The two cells are connected on the two sides of a bridge, 

 as will be seen from fig. 3. Here, if u\ and zv., are the resist- 

 ances of the two cells, the displacement in the galvano- 

 meter is approximately proportional to D in the equation 



D = Ck( V where C is the current and k a constant depend- 



ing on the galvanometer. The resistance of Se t is usually 

 very high, about 150,000 ohms, while that of Se 2 is much less, 

 about 30,000 ohms, etc. The great inertia is largely overcome 

 by the use of the two cells, especially as the light only reaches 

 the compensating cell a fraction of a second after it has 

 affected the first. 



The function of the Korn transmitter is thus to send into 



10 

 k 



1 



/ LLUrt I A/ A Tl O A/ 



Fig. 4. 



the line an ever-varying current which depends always on the 

 density of the film being telegraphed. In the receiver, this 

 current is made to displace the magnesium shutter, so that 

 the Nernst lamp can illuminate the revolving sensitive film 

 more or less. The aperture in front of the film is, as already 

 stated, triangular. This is because the stronger the light 

 acting on the selenium cells, the less comparatively is 

 their resistance altered ; hence as the shutter uncovers the 

 triangular hole, moving always towards the base, more 

 and more light (comparatively) is admitted to the lens 

 behind it which concentrates it upon the film. The curve 

 representing the effect of the illumination on the resistance 

 is usually somewhat as shown in fig. 4. The form of the 



