CHAPTER II 

 THE BALLISTIC GALVANOMETER 



In the comparison of the electrostatic capacities of condensers 

 and cables, and in the examination of the magnetic properties of 

 iron, an instrument is necessary which will measure the quantity 

 of electricity displaced in a circuit by a transient current. For 

 this purpose the ballistic galvanometer is employed, and to avoid 

 disturbances due to local magnetic fields an instrument of the 

 D'Arsonval type is now generally used. 



For reading the instrument, a telescope and a uniformly 

 divided circular scale, having its center at the axis of the movable 

 system, should be employed; with this arrangement the deflection 

 as read from the scale is directly proportional to the angle turned 

 through by the movable system. 



Observations are made as follows: The movable system is 

 brought to rest in its proper zero position; the discharge is then 

 passed through the instrument, giving an impulse to the movable 

 system, which slowly deflects; the reading is taken at the first 

 turning point, or elongation, just as the movable system is about 

 to begin its swing back towards zero. If this maximum angular 

 deflection of the coil from its original position be called 0i and 

 Q be the quantity of electricity in the discharge, then when a 

 D'Arsonval instrument is employed, 



Q = K'e, 



For any particular instrument used in a definite manner K f is a 

 constant. As will be shown later, its value depends on the cur- 

 rent sensitivity of the instrument, on the time of vibration of the 

 movable system, on the amount of damping and on the manner 

 in which the discharge is sent through the galvanometer. 



The ballistic instrument differs .from the ordinary current 

 galvanometer in one essential particular, in that the moment of 

 inertia of the movable system is made very large compared with 



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