4 Mr Searle, A method of measuring the [Nov. 11, 



where K is the moment of inertia of the coil and <d its initial 

 angular velocity. Thus since JHdB = 4>tt W we have 



TV R 

 W=^Ka> (3). 



qAn 



Let T be the time of a complete vibration of the coil, 6 its 

 greatest angular displacement or "throw" and rj the restoring 

 couple exerted by the suspension per radian of displacement. We 

 have, by the Conservation of Energy, 



Initial kinetic energy = Potential energy at extremity of swing, 



so that %Ko) 2 = % v d 2 (4). 



Also T=2ttJ- (5). 



From (4) and (5) we find 



Ka = ^ (6 > 



Thus from (3) we have 



w =NBnT e (7) . 



The quantity rj/q is easily determined as follows : — 



Let currents i 1} i 2 flow in the suspended and the fixed coils of 

 the electrodynamometer respectively and let <f> be the observed 

 steady deflection, then 



qiii 2 = couple = r)(f> (8), 



so that rj/q = i^' 2 /</>. 



If there is any appreciable damping the expression must be 

 multiplied by 1 + |-\, where X is the logarithmic decrement. We 

 have finally 



F =W-^ 1 + * X ) < 9 >- 



Thus the "throw" of the coil is proportional to the energy lost 

 in hysteresis. 



It is evident that this affords a very ready means of testing the 

 hysteresis of iron under different physical conditions for a given 

 maximum value of H, the magnetic force, for all that has to be 

 done after the constant of the instrument has once been determined 

 is to observe (by means of a mirror and lamp and scale) the throw 

 produced by a single double-reversal of the magnetizing current. 

 For instance, a steady electric current may be maintained through 

 the iron and the value of W for each value of this current may be 



