THE MEASUREMENT OF CURRENT 5 



they are used. This quantity is subject to great and erratic varia- 

 tions due to the proximity of electric cars, feeders, and struc- 

 tural iron work, and as it enters as a direct factor, present-day 

 conditions have rendered these instruments practically useless. 



Any of these absolute galvanometers will be reduced to sec- 

 ondary forms, if, in the attempt to gain sensitiveness, the coils 

 are brought close to the needle. 



The Thomson or Kelvin Reflecting Galvanometer. 4 This 

 form of galvanometer is the most sensitive instrument for the 



FIG. 4. The original Thomson galvanometer used on hoard the Niagara 

 in 1858, the first instrument by which a signal was received through a 

 transatlantic cable. 



detection and measurement of direct currents. It was invented 

 by Lord Kelvin (then Professor William Thomson) for use 

 originally as a signalling device in submarine cable work. It 

 had been noticed that signals when transmitted through the short 

 submarine cables then in use (1854) lost their sharpness and in 

 some of the experiments became unintelligible. The first ade- 

 quate explanation of this was given by Prof essor Thomson in 1854. 

 He showed that it was due to the electrostatic capacity and resist- 



