214 
THE TELEGRAPH . 
In the key form of the apparatus, the currents are sent on a 
different plan. In the key shown in figs. 151 and 152, £ and c, 
with which the poles of the battery are connected, are strips 
of metal; e and l represent two stiff metal strips, which, how¬ 
ever, can be pressed down by the hand. They are both in 
contact with z when at rest. One of them is joined to the 
Fig. 148. 
line wire L, and the other, e, to the earth. If we depress L, it 
will touch c and complete the circuit, the copper current then 
passing through the line while z is put to earth. The line and 
the coils of the distant station will then be traversed by the 
copper current, which returns through the earth to the z of 
the battery. 
If, on the other hand, the key e be pressed down while L 
remains in its normal position, the direction of the current is 
