102 



ELECTRICAL MEASUREMENTS 



impulses given as the coil swings from its zero position, or if it 

 be due to a prolonged discharge, the magnitude of the deflection 

 will be affected by an amount which will depend on the manner 

 in which the galvanometer current varies. 



It is frequently stated that one of the essential characteristics 

 of a ballistic galvanometer is absence of damping, or, as damping 

 must of necessity be present to a certain degree, that it must be 

 reduced to a minimum. This does not mean, however, that a 

 damped instrument cannot be used ballistically ; in fact, a critic- 

 ally damped ballistic galvanometer is frequently most convenient, 

 it being a great timesaver. The damping should be electro- 

 magnetic ; the law governing it is then definite and capable of a 

 simple mathematical expression. If air damping be present, the 

 proviso that it be small is a safe one, for its law is not exactly 



known. In the analytical 

 theory it is assumed to be the 

 same as for electro-magnetic 

 damping. 



Checking Devices. If 

 ^ nere be little damping, it is 

 necessary in order to econo- 

 mize time to have some form 

 of checking device by which 

 the moving system may be 

 For general purposes, that shown in 



FIG. 53. Checking device for ballistic 

 galvanometer. 



brought promptly to rest. 

 Fig. 53 is convenient. 



By a little practice the motion of the magnet and the manipula- 

 tion of the key may be so timed that the zero is promptly attained. 

 A key in series with a resistance placed across the galvanometer 

 terminals is frequently convenient; the resistance should be of 

 such a value that the instrument may be critically damped. 

 Thermo-electric currents are frequently present and somewhat 

 complicate the action of these devices. 



Precautions in Reading 1 . Trouble is likely to be experi- 

 enced with galvanometers of the . D'Arsonval form, due to 

 changes in direction of the very weak magnetism of the sup- 

 posedly non-magnetic coil, and also possibly to "set" in the sus- 

 pension. Both of these must be reduced to a minimum in the 

 manufacture of the instrument, the first by the use of a radial 



