1ELEGRAPHIC APPARATUS. 
231 
electro-magnet. Fig. 165 illustrates this system : r is the relay, 
and d z! the local battery that it brings into action. When the 
key m is depressed, contact is made at 0 , a current is sent into 
the line and through the key M to the electro-magnet of the 
relay r and to earth. The attraction of its armature by R puts 
N into contact with the local battery ?J c! and this traverses the 
local magnet r which brings the inking apparatus into action. 
K' may evidently be hundreds of miles away from r, and then 
1 / would be a second line, while the earth would take the place 
of that part of the circuit between z' and r'. 
The relays are so constructed that a very slight current, or a 
slight increase in the strength of a current, suffices to make 
4 
Fig. 164a. 
the contacts of the local battery. For example, the relay may 
be so adjusted that a single unit of current briugs it into action; 
or it may be adjusted so that while it does not act with 100 
units of current, it will at once determine the contact when 
the current reaches 101 units. 
Relays are also often made, so that the armature is attracted 
by the current from only one of the battery poles, and not by 
the other. In such case the soft iron of the armature is 
replaced by magnetized steel. In other arrangements that 
have been contrived, a current in the opposite direction is 
required to make the lever resume its position. 
