THE BALLISTIC GALVANOMETER 



101 



ordinary discussion of the ballistic galvanometer this is assumed 

 to be true, but in certain cases the assumption is not tenable. 



Some of the devices for obtaining a large moment of inertia 

 are indicated in Fig. 52. 



No. 1 is for the Kelvin galvanometer, while the others are for 

 the D'Arsonval type. In 1 and 2 the crossbar is of aluminum 

 with a screw-thread on it; the little non-magnetic weights are thus 

 made adjustable. In 3 the weights may be removed from the 

 pans and others substituted as desired. Instruments of very 

 long period may have the moment of inertia increased as shown in 

 Fig. 51. The rim of the disc, seen just below the movable coil, 

 is made of brass, the web of aluminum. 



FIG. 52. Suspended systems for ballistic galvanometer. 



The time of vibration which it is necessary to give the movable 

 system in order to obtain accurate results depends entirely on 

 the use to which the instrument is to be put. In comparing 

 condensers, when the resistance of the circuit is low, the discharge 

 is practically instantaneous and an instrument with a period 

 of about 20 sec. is convenient; so long a period is not necessary 

 in this case for the fulfilment of the assumption that the en- 

 tire discharge has passed before the movable system has been de- 

 flected appreciably, but it renders the reading of the instrument 

 much easier. For magnetic work with the usual small-sized 

 specimens such a period would be adequate, but for investiga- 

 tion of the behavior of massive electromagnets, an instrument 

 with so short a period would be of no value whatsoever, since 

 in this case the change of flux is very slow. When a solid iron 

 core is tested as much as 30 sec. may elapse before the change 

 in flux is practically complete; for such work a galvanometer with 

 a period as great as 600 sec. is sometimes employed. 



If the motion of the movable system results from a series of 



