Moment of Inertia of Magnets, 139 



Weight of ; period Value of 



suspended feystem. O. 



Cradle only 27 grms. 1-2426 



Cradle+No. 1 •. 92 „ 3-6612 1813-5 



Cradle+Xo. 4 96 „ 37749 181f " 



Cradle+No. 1+No. 4 ... 161 .. 5-1094 



Cradle only 27 .. 08713 



Cradle+So. 10 92 .. 2-5713 3691-3 



Cradle + No. 7 95 „ 2"6318 36916 



Cradle+No. 7+No. 10 ... 160 .. 3-5744 3692-2 



Assuming in each case that the change in the rigidity o£ 



the suspending fibre is proportional to the load, we obtain the 

 following values for the ratio o£ the change in c for an 

 increase in load from 27 grains to 95 grains to the value of c, 



that is the value of — 



c ' 



From the observations with the first fibre . « '00017 

 From observations with the second fibre . . '00022 

 The effect of this increase in the rigidity of the suspension 

 fibre with increase in load on the ratio deduced for the 

 moment of inertia of the bars, will increase as the difference 

 in the moment of inertia (and mass) of the bars increases. 

 To obtain an idea of the magnitude of the effect we may 

 calculate the correction in the case of the bars numbers 

 1 and 2, which are the lightest and heaviest bars respectively. 

 The numbers obtained are : — 



Ratio of moments of inertia obtained when c is 



assumed constant 1255 7dL 



Ratio of the moments of inertia obtained when 

 corrected for the change in c produced by 



change in load « 125568 



It will be seen that the only effect of applying the correction 

 is to alter the value of the ratio by one part in 20000, so 

 that it may be neglected. 



§ 2. The Effect of the Inertia of the Air on the Period of 

 a Vibrating Magnet. 



In order to test the influence of the air carried along by 

 the bars when swinging on their period, a vacuum-chamber 

 was prepared which was capable of enclosing the whole 

 instrument. This chamber consisted of a wooden box ; the 

 sides having a thickness of two inches, which was coated 

 outside with thin tinplate,, the joints being soldered up. Two 

 windows were fitted, one of considerable size and easily 

 removable, through which the bars could be changed, and the 



