ALARUM BY SHORT CIRCUIT. 



made to oscillate between p and P*, by the motion of the wheel and 

 the action of the sinuous groove, the current will be alternately 

 transmitted and suspended, and will, in fine, receive a succession 

 of pulsations corresponding exactly with the sinuosities of the 

 groove. Thus, if there be sixty undulations of the groove in the 

 circumference of the wheel, the current will receive sixty pulsa- 

 tions in one revolution of the wheel, and if the wheel revolve at 

 the rate of sixty revolutions per minute, the current will have 3600 

 pulsations per minute. 



134. An expedient has been sometimes adopted in telegraphic 

 apparatus for diverting the electric current from its direction, 

 which differs in principle from the commutator, and which 

 depends on the tendency of the current to follow the shortest and 

 widest route open to it between one point and another. 



Let w, fig. 52, be the line-wire, B the bell-apparatus, and T 

 the telegraphic instrument. The line-wire is bent upwards in 

 the direction m to the 



bell B, and then down- lg ' " 



wards, and by m' and 

 "W to the telegraph T. 

 The current would, ac- 

 cording to this arrange- 

 ment, first pass by the 

 wire m to the bell #, 

 which it would ring, 

 and then by the wire 

 m' W to the telegraph T. 

 If the dispatch were 

 then transmitted, the 

 current constantly pass- ^ ^ 



ing through B during its \y 



transmission, the bell 

 would be constantly 

 ringing, which would be inconvenient as well as unnecessary. 



This is prevented, and the current transmitted directly to T, 

 without passing through B, by the following very simple expedient. 



A thick piece of metal, a &, turns on an axis c, so that when it 

 is placed in the horizontal position, the ends a and b are brought 

 into close contact with the conducting wire at e and /. The 

 current, on arriving at e divides itself into two parts, one going 

 by a b to/, and thence to T, and the other as before, by m, through 

 the bell. But as a b is much shorter and thicker than the wire 

 m m\ the greater part of the current will go by a &, and the part 

 which passes along m m' will be too inconsiderable to exercise the 

 force necessary to ring the bell. 



191 



