418 THE ELECTRIC TELEGRAPH. 



connection with the battery B. The charge will therefore 

 pass through the galvanometer in the direction of the arrow, 

 and deflect the needle. 



When only one end of the cable is to be had, it may be 

 connected with the beam b, as in 

 ijljljl ~^ Q Fig. 189, while the battery circuit 

 is made to include the galvano- 

 meter G between s and s f . The 

 spring s is also connected with 

 earth, and the anvils are insulated. 

 In this position successive charge- 

 currents are measured on the gal- 

 vanometer in the battery circuit 



p- 1QQ 



cable, b, s', galvanometer, &c. and 



the discharge returns out of the cable, from b, over s, to earth. 

 The galvanometer, if a sine instrument, is turned after the 

 needle through an angle, a. The charge is then calculated 

 by the formula 



K = C sin. a 



Jil 



When a mirror galvanometer is used, the reading m of the 

 scale being directly proportional to the intensities, 



, K = -Lc. ' 



C, being the number of charge and discharge contracts in 

 a second of time, and E, the electro-motive force of the 

 battery. 



Having a jar of known capacity, K', which, substituted 

 for the cable in one of the above arrangements, gives, with 

 the same speed of oscillation and electro-motive force 

 requiring the coils of the sine-galvanometer to be turned 

 through an angle, a , or giving a deflection represented by 

 m' divisions of the scale when a mirror galvanometer is used, 

 the capacity of the measured cable is 



K = K' sin ' a 

 sin. n' 



in the one case, and 



