Moment of Inertia of Magnets. 



137 



have, subject to variations due to errors of experiment, that 

 the quotient 'K 1 /(t 1 2 — t 2 ) ought to be the same for all the 

 bars. The following table gives the value of the quotient for 

 the different bars : — 



I 



Number of Bar. 





Difference from 

 mean. 



1 



45944 



47 

 40 

 21 

 31 

 23 

 41 

 44 

 37 



+ •008 

 + •011 

 + •004 

 -015 

 -•005 

 -013 

 + •005 

 +•008 

 + •001 



2 



3 



4 



5 



6 



7 



8 



9 





Mean 45-936 4 



±•0078 



The mean value for 



K, 



is 45"936 4 . I do not, however. 



consider that this mean is the best value to take, as owing to 

 the extra care taken in making the three bars numbers 7, 8, 

 and 9, I consider it advisable to give the results with these 

 bars treble weight. Doing this the mean becomes 



45-938. 

 By means of this mean value we may calculate the moment 

 of inertia of the standard bar No. 10. The value obtained in 

 this way at 15° C. is ki 70 - 



Owing to the fact that the rigidity of fused silica increases 

 as the temperature rises, the period changes very little with 

 temperature. Thus for the cradle alone the percentage 

 increase of the period for a fall of temperature of one degree 

 centigrade is '0036. With a brass bar of the dimensions used 

 in these experiments, the percentage increase of period for a 

 fall of temperature of one degree is '0035. As long, there- 

 fore, as care is taken to have the temperature uniform 

 throughout the instrument, the correction for changes of 

 temperature can be made without difficulty. 



In order to obtain some idea as to the accuracy with 

 which the moments of inertia of bars may be compared by the 

 method employed, we may take the differences from the 

 respective means of the determinations of period given on 

 page 136, taken irrespective of sign. The number obtained 

 is 0"000 14. This corresponds to a mean difference from the mean 

 of 0*0037 per cent. This value, if we neglect the error 



