110 ELECTRICAL MEASUREMENTS 



2. Modification of the current through the instrument by 

 the electromotive force induced in the movable coil by its motion. 



3. Air friction. 



4. Internal friction in the suspension wire. 



In the D'Arsonval type of instrument, 4 is entirely negligible 

 and 3 is small. 



As the ballistic galvanometer is ordinarily used, the time during 

 which the current flows is very short. But cases arise where the 

 displacement of electricity through the instrument is not instan- 

 taneous, and such cases must be included in the general discus- 

 sion. 3 



The ballistic galvanometer is commonly employed: 



(a) In the determination of magnetic fluxes. 



(6) In the comparison of capacities. 



In case (a) the instrument is used in a closed circuit, consisting 

 of the galvanometer and of the exploring coil wound around the 

 specimen. 



In any case where the instrument is used in a closed circuit 

 of resistance, r, the equation governing the motion is (see page 40) 



&' is the damping coefficient when the instrument is on open cir- 



C 2 . 

 3Ui "t and is the damping coefficient due to the e.m.f. set up in 



the ma i n circuit by the motion of the coil. In this discussion 

 let 



e is the ms t an taneous value of the e.m.f. impressed on the circuit ; 

 it is a function of t, and usually becomes zero in a comparatively 

 short tin ie .-L J i s tne back e.m.f. due to the inductance of 



the galvan ome ter circuit. In the following discussion it will be 

 assumed to b e negligible. 



f the co^i tions are such that the current t h rou gh the instru- 



^unodified by the e.m.f. due to the motion of the 



coil, then in fe) the termy is absent and the second member 



V 



