240 
THE 1ELEGRAPH. 
fixed metal pegs a and b. Let ns suppose the part in which b 
is fixed to be in connection with the line, while a communicates 
with the earth. Suppose, also, that the two battery levers are 
pressed against these pegs by the springs s s', one of the levers 
C being connected with the copper pole of the battery b and 
the other z with the zinc pole. It is evident that, in the first 
case, the negative current enters the. line, whilst in the other 
case it is the positive current. Therefore, if the disc d can 
oscillate between these positions in a regular and continuous 
manner, it will send into the line a series of alternate currents, 
which will show themselves by a series of dots in the receiving 
instrument at the other end of the line. 
If, however, the line or the earth connection be temporarily 
interrupted before the disc takes the second position, the in¬ 
verse current will not be sent, and a dash will appear at the 
transmitting apparatus instead of two dots. The use of the 
perforated paper is to regulate the movement of the transmitter. 
