HISTORY AND PROGRESS. 79 



When a current is sent through the coils of the electro- 

 magnet in the direction which increases the magnetism of 

 the more distant pole, whilst it reverses the magnetism of the 

 nearer, the latter forthwith repels the armature with nearly 

 the same force with which the former pole attracts it, and 

 the armature in consequence goes over to the former pole, and 

 rests there, not only as long as the current lasts, but after- 

 wards, when no current is circulating in the line. It is 

 this which renders this indicator so sensitive for induction 

 currents which are only of momentary duration. 



At the end of the armature is a German silver fork, 

 carrying two horizontal arms thin steel springs with hooks 

 which catch into the teeth of a ratchet-wheel. The ratchet- 

 wheel has thirteen teeth, so that when the armature oscillates 

 from right to left, or from left to right, the wheel advances 

 half a tooth ; and in order that the pointer may march over 

 the whole dial, the armature must oscillate thirteen times in 

 each direction. This is provided for by exactly that number 

 of reverse currents being transmitted from the sending 

 station, when the handle of the transmitter is turned once 

 round the dial. 



Behind the hooks are two screw stops which limit their 

 motion and prevent the skipping of the wheel. 



56. The alarm is generally a separate piece of mechanism ; 

 it was, however, sometimes in the earlier forms of this 

 apparatus combined with the indicator by attaching a hammer 

 to a continuation of the armature of the electro-magnet, and 

 letting it strike on two bells placed just within its reach ; 

 but this plan has been almost entirely abandoned in favour 

 of the alarm shown in Fig. 42. The principle is the same 

 as that on which the indicator is constructed : a polarised 

 electro-magnet, m m, is supported by the upper pole of the 

 angular bent permanent magnet M on a wooden base, b. 

 From the lower pole of M springs a movable armature or 

 tongue of soft iron which takes polarity from it, and plays 

 between the projecting cores of the electro-magnet, which are 

 also polarised. A continuation of the armature forms a 

 hammer, h t and strikes upon the bells b b'. 



