212 
THE TELEGRAPH 
small ivory studs. When a current passes through the coils, 
the needle and indicator are deflected, and the direction of that 
deflection depends entirely upon the way the current passes. 
Although the coils appear separate, they form in reality but 
one and the same circuit, as they are both attached to a metallic 
frame which allows one of the coils to act by itself should the 
other happen to be damaged by lightning. 
To make the needle deflect to the right or to the left at wiT, 
we only require an arrangement for reversing the direction of 
the current. An examination of the manipulator will show us 
how this is to be effected. The wires from the zinc and copper 
poles of the battery are attached to z and c respectively. The 
line wire enters at A, and from b proceeds a wire connecting 
with the earth (fig. 149). The arbor n f (fig. 150), which 
