24 



ELECTRICAL MEASUREMENTS 



In the instrument shown in Fig. 12, a single pair of coils acting 

 principally on the consequent poles is used. 



The control magnet is at B. By it the time of vibration may be 

 varied from about 5 to 20 seconds. A clamping device by which the 

 needle may be held in place is actuated by the knob Z>. Damping 

 is accomplished by a light aluminum vane G which swings be- 

 tween two adjustable plates, manipulated by the knobs C. A 

 quartz fiber suspension is used. 



The coils are so mounted that they are readily changed and the 

 instrument thus adapted to different pieces of work. 



FIG. 12. Broca galvanometer. 

 THE MOTION OF THE SUSPENDED SYSTEM OF A 



Suppose that when the movable system of a galvanometer is at 



t, the circuit is suddenly closed so that a current flows through 



the instrument. Experience shows that in some cases the mov- 



system arrives at its final deflected position by a series of 



illations of diminishing amplitude, in other cases by a steady 



increase of the deflection without oscillations. 



It is desirable to derive the equations which will represent the 



ion of the system sufficiently well for practical purposes for 



e general considerations thus introduced are of importance in 



