434 



ELECTRICAL MEASUREMENTS 



THE VIBRATION GALVANOMETER 



In 1891 Max Wien suggested that it was possible to greatly 

 increase the sensitivity of the detectors used in alternating-cur- 

 rent measurements, where zero methods are employed, by taking 

 advantage of the principle of resonance. To do this, the moving 

 member of the detector is mechanically tuned so that its natural 

 period is the same as that of the alternating electro-magnetic 

 forces which cause its deflection. The idea was realized by 



FIG. 255. Vibration galvanometer, Leeds and Northrup Co. 



Wien in his "optical telephone." The later development of 

 this instrument into the vibration galvanometer has been due 

 more especially to Wien, Rubens, A. Campbell, Duddell and 

 Drysdale, who have utilized both the moving needle and the 

 moving coil principles. 30 



The instrument is read by the mirror and scale method and 

 the optical arrangement should be such that when no current 

 is passing, a sharply defined image may be seen on the screen. 

 When the bridge, or other apparatus to which the galvanometer 



