372 



THE ELECTRIC TELEGRAPH. 



conductor of the cable. When all is ready for the test, the 

 lever of the switch is turned upon the contact 2, closing the 

 circuit of the battery from one pole through E, t, B, joint, 

 water in Y, P, /, and plate 2 of condenser ; andfromthe other 

 pole, through s, 2, 1, I 1 , and plate 1 of the condenser. What- 

 ever leakage occurs through the joint carries with it an equi- 



valent quantity of electricity, 

 which is accumulated upon the 

 plates 1 and 2 of c. After the 

 lapse of a certain time, the lever 

 of the switch is taken from the 

 anvil, 2, and put upon 3, complet 

 ing the circuit between the plates 

 of the condenser with the galvano- 

 meter (c, 2, G, 3, s, 1, /', c, 1). 

 The discharge from the condenser 

 plates passes thereupon through 

 the galvanometer whose magnet 

 needle is deflected, the deflection 

 depending of course upon the 

 leakage of the joint, the length of 

 time during which the condensa- 

 tion continues, the degree of insulation of the condenser 

 itself, and the force of the battery. 



In the Gutta-percha Works, Mr. Smith uses a length of 

 cable instead of an ordinary condenser. In this case the 

 conductor of the condenser cable is connected with I, instead 

 of c, 2 ; and as the outside (which represents c,) is to earth, 

 the lever s t of the switch must also be put to earth. 



Mr. Yarley prefers the use of condensers made of alter- 

 nate leaves of tinfoil and paper saturated with paraffin. 



At first sight, it might seem doubtful that any satisfactory 

 result could be obtained with this method, in which the con- 

 denser conducts, perhaps, ten thousand times as well as the 

 joint which is tested, The objection, however, is easily satis- 

 fied. The proportion between the current which goes through 

 the condenser and the static quantity which is retained as 



Fig. 172. 



