38 ELECTRICAL MEASUREMENTS 



shunt is most probably of a material having zero temperature 

 coefficient. 



When the D' Arson val galvanometer is used as a potential gal- 

 vanometer or a millivoltmeter, there is an additional source of 

 variation due to the change of resistance. If the necessary series 

 resistance is made partly of copper and partly of manganin in 

 the proper proportion, its net temperature coefficient may be 

 made such that the instrument is almost exactly compensated 

 for variations of room temperature. 



It should be remembered that when the temperature varies 

 very rapidly, there will be a time lag in the change of resistance 

 and in the change of the strength of the magnet. 



Magnetic Damping. To prevent loss of time when using any 

 galvanometer, it is necessary that it be properly damped. In 

 the D' Arson val instrument the damping may be attained by 

 use of closed loops of wire attached to the movable coil or 

 by winding the coil on a very light metal bobbin, as is done 

 in direct-current ammeters and voltmeters. The current in- 

 duced in the closed circuit as the coil swings through the 

 magnetic field promptly brings the coil to rest. Similarly a 

 moving coil galvanometer, as will be seen below, will be appreci- 

 ably damped if it is used in a closed circuit of moderate resist- 

 ance and it is frequently possible, by adjusting the resistance 

 of the circuit and the constants of the galvanometer, to attain 

 critical damping. 



When it is necessary that the coil be perfectly free to move 

 from and be brought back quickly to its zero position, a short 

 circuiting key, which must be free from thermo-electromotive 

 forces, may be placed across the terminals of the galvanometer. 

 The motion of the coil is promptly checked by depressing the key. 



The damped D'Arsonval instrument is very useful as a ballistic 

 galvanometer on account of its quick return to the zero reading. 



The Critically Damped Moving-coil Galvanometer. 7 The 

 field in which the coil moves will be assumed to be radial. 



SYMBOLS USED IN THE DISCUSSION 



H - strength of field. 

 I = total length of active wire. 

 b one-half the breadth of movable coil. 



