HISTORY AND PROGRESS. 



73 



At an intermediate station, during the correspondence 

 between two end or distant stations, when the continued 

 interruptions of the circuit would cause the alarm to sound 

 whilst the correspondence lasts, which would become 

 annoying to the employes, an arrangement is made for so 

 weakening the currents in the coils of the electro-magnets 

 by means of a shunt, that the alarm does not sound. This 



Fig. 37. 



shunt is shown in Fig. 37, by a wire, w, w ', and resistance 

 coil, R, which has about five times the resistance of the coil 

 of the electro-magnet, allowing therefore only about five- 

 sixths of the current to pass through the legitimate route. 



This shunt circuit has another object. At the moment 

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

 m m, the induced current tends to weaken the effect of the 

 battery current. This would not be perceptible if the induced 

 current were obliged to traverse the whole line. But in 

 going round the shunt L, R, L', with little resistance, it 

 exercises its full opposing force on the magnet, and prevents 

 the armature being attracted. 



The plan, Fig. 38, shows an ingenious arrangement for 

 providing against the errors frequently arising in systems 

 based on the principle of closed circuits, from bad insu- 

 lation of the line. Should there be, as is sometimes 

 the case, a battery at each of the end stations, and the 

 line, in some points intermediate, in imperfect contact 



