Electric Waves passing through Telegraph Lines. 323 



put into connexion with a contact-stud m, against which the 

 key (Z) presses. To I is connected one side of an accumulator, 



Fte. 1. 





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0f* 







B~ 





<f) 4" 





I 



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A, the other side of which is put to earth (W). A second 

 contact-stud, n, connected through a galvanometer to the earth 

 side of the accumulator, allows the latter to be discharged and 

 the potential of its contents ascertained at any moment. The 

 potential of the accumulator (whose capacity must be small 

 in comparison with the capacity of the line), whilst in contact 

 with the point c, maybe assumed to be identical with that of c; 

 so that by depressing the key I the excursion of the galvano- 

 meter-needle indicates the electrical state which c had at the 

 instant that I was separated from m. Now suppose an appre- 

 ciable time to be required for any recognizable strength of 

 wave to travel from a to c ; if the key e be depressed and 

 make contact with g at the instant I is separated from m, it is 

 evident that (no alteration having taken place in the potential 

 at c) no movement of the galvanometer will be observed. If, 

 however, the key e be held doAvn some time before I is sepa- 

 rated from m, electricity will flow through, and each point 

 along the resistance a b will assume the potential due to its 

 position, from p (at a) to zero (at b), as indicated by the dia- 

 gonal. The discharge of the accumulator will then give an 

 excursion of the needle proportional to the potential (p x ) of the 

 point c. After contact of the battery at a, every point in the 

 conductor takes a certain time before it attains the maximum 

 potential due to its position. By gradually increasing the 

 time which elapses between the contact of e with g and the 

 break between I and m, the potential which the point c from in- 

 terval to interval attains being observed, the contour of the 

 wave is found. The point c may of course be chosen in any 

 position between a and b, to suit the galvanometer or otherwise. 

 By a slight modification, the time which the neutral point 

 Y2 



