96 PHOTO-TELEGRAPHY 



It is interesting to note that often during very 

 wet weather less " balancing " of the line is re- 

 quired in working with the telectrograph, this bear- 

 ing out the distortion expression 



R S 



2 L 2 K* 



The capacity of a single line of diameter d and 

 length / at a height H above the ground is given 

 by the following expression : 



r __ I X 2*415 X io 6 

 ~ -- ~~~' 



but this becomes effectively much less where there 

 are two lines close together. In the case of tele- 

 phone cables we can divide the capacity of one line by 

 two to get the total capacity of the closed line, since the 

 capacity of two condensers in series is equal to half that 

 of one singly. As will be seen later, the capacity of 

 a Paris London single line is 10*6 m.f., so that the 

 capacity of the two lines forming a closed circuit is 

 5 '3 m.f. The submarine cable between Sangatte 

 (France) and St. Margaret's Bay is 5*52 m.f. in 

 capacity, or more than equal to the total capacity of 

 the land lines, which measure 287 miles against 23 

 miles of the submarine, 



The preparation of the chemical paper for 

 receiving the photographs is not an easy matter, 



