TRANSMISSION OP SIGNALS. 



the wire which would connect it with Dover would be the down 

 '-ire. 



The manner in which the current arriving at any station is made 

 to ring a bell or alarum at that station, will be explained hereafter. 



In explaining the manner in which the agent at a station is 

 enabled to control a current by means of the commutator, two 

 cases are to be considered first, when he desires to transmit 

 signals ; and, secondly, when he expects to receive them. 



In the former case, he takes the current from his own battery ; in 

 the latter, he receives it on its arrival by the up or down line wire. 



"We shall first consider the case in which he desires to transmit 

 signals. 



113. To transmit a current on the up line only. Let the hand 

 A" be placed on x, A on P, and A' on u. The negative pole x of 

 the battery being then in connection with the earth E by the hand 

 A", and the positive pole r in connection with the up wire u by 

 the hands A and A', while the up wire itself at the station at 

 which it arrives is in connection with the earth, the current will 

 flow from r by A and A' along the up wire to the station at which 

 the wire goes to the earth. 



114. To transmit a current on the down line onlu. Let A" and A 

 be placed as before, and let A' be moved to D. The current will then 

 How on the down line, as may be explained in the same manner. 



115. To transmit a current along the entire line from terminus 

 to terminus. Let A' be turned upon tr, and A upon x, and let two 

 similar hands at the back of the disc be at the same time turned 

 upon P and D, the hand A" being removed from both x and r. 

 In that case, the current will flow from the positive pole P along 

 the hands at the back of the disc to D, and thence on the down 

 wire to the terminal station, where it will take the earth, by 

 which it will pass to the earth plate at the up terminal station, 

 and from thence by the up wire to r, and from u by the hands A' 

 and A to the negative pole x. 



Thus it appears that it will pass along the entire line from 

 terminus to terminus, flowing from the up station downwards. 



116. To reverse the direction of the current. To accomplish 

 this, it is obviously sufficient to reverse the connections with the 

 poles of the battery. Thus, if the current be transmitted on the 

 up line only, the hand A' will be upon r, A on P, and A" on x, 

 when, as already explained (113), the current will flow from r 

 towards the up station. If A" be removed to r, and A to x, the 

 direction will be reversed, the course of the current then being as 

 follows : From the positive pole P to E by the hand A" ; from the 

 earth E to the earth plate at the upper station ; from that to 

 the up wire ; from thence to r, and from u by A' and A to x. 



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