CHAP. XIL] ELECTRICITY AND MAGNETISM. ' 395 



to send currents in either direction through the circuit, 

 a " signalling-key " or " tapper " is usually employed. 

 The tapper at one end of the line works the instru- 

 ment at the other ; but for the sake of convenience it 

 is fixed to the receiving instrument. In Fig. 158 the 

 two protruding levers at the base form the tapper, and 

 by depressing the right hand one or the left hand one, 

 currents are sent in either direction at will. 



The principle of action will be made more clear by 

 reference to Fig. 159, which shows a separate signalling 

 key. The two 

 horizontal levers 

 are respectively 

 in communica- 

 tion with the 

 "line," and with 

 the return - line 

 through "earth." 

 When not in use 



they both spring Fig . I59 . 



up against a cross 

 strip of metal joined to the zinc pole of the battery. 

 Below them is another cross strip, which communicates 

 with the copper (or + ) pole of the battery. On 

 depressing the " line " key the current runs through the 

 line and back by earth, or in the positive direction. f 

 On depressing the " earth " key (the line key remaining 

 in contact with the zinc-connected strip), the current 

 runs through the earth and back by the line, or in the 

 negative direction. Telegraphists ordinarily speak of 

 these as positive and negative currents respectively. 



As it is necessary that a line should be capable of 

 being worked from either end, a battery is used at each, 

 and the wires so connected that when at either end a 

 message is being received, the battery circuit at that end 

 shall be open. Fig. 160 shows the simplest possible 

 case of such an arrangement. At one end is a battery 



