DUPLEX TRANSMISSIONS 



79 



it be of sufficient intensity to do this, nothing is 

 obtained on development. The result is that if the 

 complete upward and backward displacement of 

 the string takes a time T, and during that time 

 the sensitive film on the revolving drum is travel- 

 ling with a velocity v, the length of the mark 

 produced on development will not be rT, but 

 will be vt, where t <; T and the distance v (Tt) 



To Earth 



To Earth 



To Earth 



To Earth 



FIG. 36. 



corresponds to the inertia of the film. Con- 

 sequently the darkest parts only of the component 

 dots of the picture are given in the developed print, 

 and there is more contrast in the picture. 



Simultaneous transmissions in two directions 

 have been recently suggested by Professor Korn, 

 which would reduce the cost of photo -telegraphy, 

 and save operators' time as well. The scheme is 

 seen at a glance in Fig. 36. The parallel lines 

 represent the telephone lines between the two 

 stations. AI is the transmitting flrum of one 

 machine, GI the string galvanometer; A 2 and G 2 



