36 PHOTO-TELEGRAPHY 



" strings " or wires., but as there is a practical 

 limit to their fineness, we must make the factor e 

 increase by the strings becoming shorter ; hence the 

 current necessary to displace them an equal amount 

 becomes greater, and where selenium is used its 

 resistance is so high that a limit is immediately 

 set to the length of strings practicable. The 

 magnetic field may of course be increased when the 

 galvanometer constant K in the expression 



D = CK ( ) becomes greater. The Einthoven 



\r\ r*J 



galvanometer, of which Korn's apparatus is a 

 special form, is often provided with a far greater 

 field than he employs. 



Once in each revolution of the receiving drum 

 there is a distinct kick in the galvanometer, as for 

 the synchronising of the sending and receiving 

 apparatus it is necessary, as will be seen later, to 

 cut the current out of the galvanometer circuit and 

 switch it into the synchronising gear. This kick, 

 due to a capacity discharge front the line, was to 

 some extent overcome by Korn by shunting a small 

 resistance across the galvanometer at the terminals 

 K, C. To facilitate its regulation I replaced this 

 fixed resistance by a regulating shunt resistance of 

 o to 50 ohms, but it was ultimately found that the 

 kick could be best avoided by very careful adjust- 

 ment of the position of the fleeting contact through 

 which the synchronising current passed. 



