53 2 MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



It is sufficient to turn the inducing frame through 90 from its 

 original direction, to bring it into the meridian; to arrange at the 

 centre a magnetised needle, and to pass the same current through 

 both instruments. If the degrees of sensitiveness of the two in- 

 struments are very different, the use of a shunt, of a known value, 

 on the ballistic galvanometer will enable us to obtain deflections 

 of suitable magnitude. Let G be the galvanometric constant of the 

 frame, A and 8 the deflections produced by the common current, 

 and /A the multiplying power of the shunt, if there is one ; equation 



--tanA = tan 8 

 G g 



gives 



tan 8 i 



tan A Ta 



The ratio only of the deflections 8 and a, relative to the ballistic 

 galvanometer, affect the formula ; it is unnecessary, therefore, to 

 determine accurately the distance of the scale, and no correction 

 need be made for the needle. 



If a is the mean radius, n the number of turns, and / the length 

 of the wire, we have, to within terms of correction, 





 a 



or, if / is the total length of the wire, 



The quantities which must be exactly determined are, the length 

 /of the wire coiled on the frame, and the distance of its mirror 

 from the corresponding scale, the time T of the oscillations of the 

 ballistic galvanometer, and the angle of swing a. 



1116. When voltaic induction is used to produce the discharge, 

 the intensity of the inducing current I, is determined by the de- 

 flection ft of a tangent galvanometer, the elements of which are 

 H and G; we have then 



1 = 5 tan J3. 

 G 



