HISTORY AND PROGRESS. 



173 



common roller, in which two grooves are cut to receive the 

 points. 



A double transmitting key is used, each lever being of the 

 ordinary construction. 



A plan of the arrangement is shown in Fig. 93. K, K 1 are 

 the two levers of the key. The lever K is to earth, and K 1 

 connected with a circuit breaker, u, thence to end 1 of the 

 relay coils ; the other end, 2, of the coils is connected to the 

 opposite side of u, and thence to line. The front and back 

 contacts of K and K 1 are made with metal bars, 1 and 2, com- 

 mon to both levers, and between 1 and 2 is inserted the line 

 battery L B. The pole c of the local battery B goes to D, and 



Zine 



Fig. 93. 



is therefore in permanent connection with the poles of the 

 electro-magnet ; the other pole, z, goes to both the printing 

 magnets M and M 1 , and from the further ends of their coils to 

 the armatures E and F of the relay magnets. 



When the apparatus is to be used as transmitter and 

 correspond with another station on the line, the contact 

 peg is put into the hole of the circuit breaker u. The levers 

 K and K 1 of the key are then pressed down, one at a time, 

 according to arrangement of alphabet and words to be trans- 

 mitted. When the knob of the lever K is pressed down, the 

 current of L B goes from c (copper over 2 and lever of K) to 

 earth, and from z (zinc over bar 1, K 1 , u) to zinc, and through 

 the apparatus at the other station to earth. K being let go, 

 and K 1 pressed down, the current of L B goes from c (bar 2, 



