

HISTORY AND PROGRESS. 



161 



When K* is depressed alone, the currents of b pass from z 

 (6 and 4 of K' ) to earth, and from c (1 and 2 of K') to line. 



When both K and K' are depressed, the united currents of 

 b and c pass from zinc of b (6 and 4 of K') to earth, and from 

 copper of c (1 and 2 of K) to line. 



Relay 



Fig. 88. 



By the depression of one or other or both the keys at the 

 sending station, three currents are therefore produced, whose 

 intensities are in the relation of 1, 2, 3. These currents 

 we will call s, s 19 and S 2 . 



At the receiving station all currents pass through two 

 relays, i and IT, Fig. 89. A common local battery E 1 serves 

 both these instruments ; its zinc-pole being connected with 

 the tongue of each of them, and its copper-pole with their 

 metal contacts. The relay u is furnished with outer coils, 

 which are put into circuit with another local battery E and a 

 resistance R, by means of the tongue of relay I. 



The tongue of relay i is held on its insulated contact by 

 a spiral spring, whose force is adjusted that the currents s, or 

 those of the portion c of the battery, are unable to move it ; 

 but that it is easily moved by s x and s 2 the currents of 

 section b and the whole. Relay u, on the contrary, was 

 adjusted delicately, so as to be deflected by the weaker 

 currents. 



When, therefore, the key K at the sending station is 



M 



