420 



By the above arrangement signals are sent from station to station. 

 But the extreme simplicity of the battery, the bell, and the language 

 allows the arrangements to be so modified that signals may be made 

 on a pair of bells from any joint, intermediate between two bell- 

 stations, without the necessity of providing the signaller with any 

 telegraph or battery, or any electrical apparatus whatever. The ad- 

 dition of this property to the bells does not in any way interfere 

 with their being in perfect action and constant use for the ordinary 

 work of train-signalling, and therefore if the guards of trains and 

 the plate-layers of the permanent way are provided with a signal for 

 expressing their wants, a great advance is made in telegraphy, and 

 a large element of safety is gained for the travelling public. 



It is well known to electricians that, if two equal and opposed 

 currents are presented to the respective ends of a wire, no evidence is 

 manifested of the circulation of electric force ; the wire is in a null 

 state, as much so as if no current was presented to it. Taking ad- 

 vantage of this law, in connexion with the simple bell-system above 

 described, the circuit is made to contain the two batteries, one at 

 each station, as well as the pair of bells ; the same pole, the graphite, 

 for instance, of each battery being connected with the earth. 



When the home-station signaller desires to make a signal, he 

 depresses the spring as before ; but the connexions are such, that 

 by this act he excludes his own battery from the circuit. The circuit 

 then contains but one battery, namely that at the pass station ; the 

 current of which is now able to circulate from end to end, being no 

 longer counterbalanced by an equal and opposite current ; and con- 

 sequently the bells are sounded. This, then, is the process for ordi- 

 nary train-signalling, under this arrangement. 



By altering the contact-maker so that it inverts the battery in the 

 circuit, instead of putting it out of circuit, both batteries are made 

 available for each signal ; and consequently the power and with it the 

 cost of each may be reduced. 



But the null state of the wire is equally well and very readily 

 destroyed, by connecting it with the earth at any point intermediate 

 between the two stations ; for by this process a complete circuit is 

 made or channel opened for the discharge of both ends of both 

 batteries, each independently of the other, except that the attached 

 wire between the earth and the telegraph wire is common to both 



