168 Prof. J. Trowbridge on Methods of Measuring 



the movable part of the apparatus ; and it was found that read- 

 ings could be taken quicker than by galvanometric methods. 



Theory of Instrument* '. 



2 _ Ftanfl 



"" Gg cos ft ' 

 where 



C = current, 



F = directive force, 



G and g = constants of fixed and movable coils, 



ft = angle of coil with magnetic meridian. 



If the torsion-head of the instrument can be adjusted so that 

 the deflection is zero, and 6= —ft, we have 



The value of F was determined by several methods. Since 



F* 2 = 7T 2 A, 



where 



t is the time of vibration, 

 and 



A= moment of inertia. 



it is necessary to determine both the time of swing and the 

 inertia. The times of swinging were obtained by means of a 

 chronograph upon which seconds were recorded by the side of 

 the records produced by breaking an electric circuit at the in- 

 stant the movable coil passed the middle of its vibration. 

 The moment of inertia was first determined experimentally by 

 adding known cylindrical weights and determining the new 

 time of vibration. 

 We thus have 



P 



(('♦S<-') 



and 



k 



- (*+■'?)' 



where k = moment of inertia of added cylindrical weights, 

 w = weight of cylinders in milligrammes, I = distance of 

 point of suspension of cylinders from axis, r = radius of cy- 



* Maxwell's 'Electricity,' vol. ii. p. 329. 



