440 The Magnetic Field produced by Electric Currents [CH. xin 



32. If there be an error a in the determination of the magnetic meridian, find the 

 true strength of a current which is i as ascertained by means of a sine galvanometer. 



33. In a tangent galvanometer, the sensibility is measured by the ratio of the incre- 

 ment of deflection to the increment of current, estimated per unit current. Shew that 



the galvanometer will be most sensitive when the deflection is j , and that in measuring 



the current given by a generator whose electromotive force is E, and internal resistance 

 ft, the galvanometer will be most sensitive if there be placed across the terminals a shunt 

 of resistance 



ffftr 



where r is the resistance of the galvanometer, and H is the constant of the instrument. 

 What is the meaning of the result if the denominator vanishes or is negative ? 



34. A tangent galvanometer consists of two equal circles of radius 3 cms. placed on a 

 common axis 8 cms. apart. A steady current sent in opposite directions through the two 

 circles deflects a small needle placed on the axis midway between the two circles through 

 an angle a. Shew that if the earth's horizontal magnetic force be H in c. G. s. units, then 

 the strength of the current in C.G.S. units will be 125,0" tan a/36?r. 



35. A galvanometer coil of n turns is in the form of an anchor-ring described by the 

 revolution of a circle of radius b about an axis in its plane distant a from its centre. 

 Shew that the constant of the galvanometer 



= ^ /^cn 2 u dn 2 u du (k = b/d) 



= (8w/3 2 a) [(1 +F) E- (1 - F) K]. 



