276 Dr. A. D. Denning on a Simple Method of 



If the torsional couple per radian exerted by the suspension 

 is /&, then the couple on the fibre in a steady state would be 



But the observations are taken during the motion of the 

 galvanometer-coil, i. e. when the change of temperature is 

 not steady. 



In this case J2A jq 



where I = the moment of inertia of coil, 



K = the viscosity coefficient of the air, 

 T = the initial difference of temperature ; 



that is 



ig+xg + <rf=*T, 



putting K/I = k and m/i = m2 _ 



the equation of motion becomes 



Now, if T be approaching a steady value A, according to 

 the exponential law we have 



T=A(1 -«"**) ; 



consequently 



the solution of w r hich gives 



k 2 

 The second term is evidently oscillatory so long as n z > -j ; 



but if k be large it dies away rapidly, or to such an extent 

 that it can be eliminated by plotting, and we have only to 

 deal with 



n 2 e~P t \ 

 p 2 — kp-\-n 2 J 



6=\A(1 



differentiating 



dS 



dt r ' ' p 2 — kp — n 2 > 



but from the differentiation of the exponential equation, 



