finding. Faults in Insulated Wires. 417 



The electrometers first used were a modified form of Coulomb's 

 torsion instrument by Mr. Latimer Clark, which was replaced 

 by a more sensitive arrangement of Peltier's electrometer which 

 I had constructed. This in turn gave way to Sir William Thom- 

 son's quadrant electrometer, which under skilful manipulation 

 is the most certain and convenient instrument, especially for 

 demonstration, as a few yards of core will suffice for the expe- 

 riment. 



In practice an ordinary reflecting astatic galvanometer is ge- 

 nerally used, in which case, instead of being guided by the 

 deflections, we note the rush of current into the core when each 

 drum alternately is connected to earth after a few moments' 

 previous insulation. 



I will first describe the method employed when an electro- 

 meter is used. 



The length of wire to be operated on is immaterial, provided 

 that the whole or a portion of it can be coiled on an insulated 

 drum, and that between the parts coiled the surface of the core 

 for a length of 6 or 8 inches can be cleaned and dried so as to 

 prevent conduction. 



I'l'I'K 



EARTH 



In the first case (when the whole can be coiled on a drum), 

 one half is coiled off on a second drum, and the two drums A 

 and B afterwards carefully insulated. The surface of the core 

 C D between the drums is well cleaned and dried. The con- 

 ductor is attached to an electrometer, and the two drums are 

 connected to earth by an attendant at each drum, when by con- 

 necting the battery to the electrometer and core the whole will 

 become charged ; the battery is then disconnected from the elec- 

 trometer, and the earth-wires simultaneously taken off the drums. 

 It is best to leave the battery on until the earth-wires are re- 

 moved from the drums. 



The insulation of the drums A and B and the electrometer E 

 should be such that no loss can be perceived after a few minutes, 

 when, if the earth-wire be applied first to one drum and then to 

 the other, the fault will be found on that drum which causes the 

 greatest fall in the electrometer. The wire is coiled from the 

 faulty side to the other, and the test repeated as often as is 

 required. A mile of core with a small fault in it can by a little 



Phil. Mag. S. 4. Vol. 47. No. 314. June 1874. 2 E 



