380 THE ELECTRIC TELEGRAPH. 



175), one of which is adjustable between limits, is connected the 

 beam of a contact key, K, between the point-contact of which 

 and earth a battery, E, is inserted. The farther ends of the 

 resistances r and R are connected respectively to the interiors 

 of the unit jar u, and of the cable L, and to the two ends of 

 the galvanometer coil G. The external coatings of both the jar 

 and cable are to earth. On pressing down the key, the 

 charge current flows in r and R, and into u and L, either 

 directly or partly through G, into one of them. 



The charge current or that current which flows into the 

 jars at the instant of closing the circuit is proportional to, 

 and may be taken as expressing, the quantity of electricity 

 conveyed by the currents to commence their charge, and 

 therefore to their capacities for this charge. c and D being 

 the intensities of the currents flowing into the jar and 

 cable at the moment of closing the circuit, 



C : D = Ktr : KL 



K v and ~K L being the two capacities, or 



. . . (L 



C K/7 



D KL 



By KirchhofF's law of branch circuits, 



1) . . Ar BR G^ = 0, 



2) . . A G C = 0, and 



3) . . B G D =* 0. 



When the intensity G in the galvanometer circuit = 0, 

 which is the condition upon which the method rests, and 

 which is obtained by adjusting the value of K, these equations 

 become 



A r B E = or ^- = 

 5 , r 



A C = or A = C 

 and B D = o or B = D 



from which 



.= . (II 



B r ' 



