11. 



12. 



13. 



14. 



15. 



268-61 



26847 



268-61 



268-61 



268-54 



148 Moment of Inertia of Magnets. 



Dr. Chree having been kind enough to supply me with the 

 mean values of the moment of inertia of their standard 

 magnet, as deduced by comparisons with these bars, also 

 with the dimensions which they had used in calculating the 

 moments of inertia, I have been able, taking 45*938 as the 

 mean value of c/4:7r 2 , to calculate from my comparisons of 

 the Kew bars the moment of inertia of the Kew standard 

 magnet. The results obtained for the moment of inertia of 

 the magnet at a temperature of 0° C. by the intermediary 

 of the different bars are shown below, no correction having 

 been applied to allow for the effect of the air. 



Number of Inertia 



Bar used. 

 Moment of inertia 



of the Tvew 



magnet at 0° C. 



In the case of the bar 15 the results are very doubtful, as 

 the bar was damaged between the observations at Kew and 

 those at South Kensington. Dr. Chree writes "... the bar 

 suffered some abrasion .... subsequent to the swings whose 

 results I send you, but prior to the time when it was sent to 

 you/'' As a result of the abrasion the bar changed in weight 

 from 68*770 grams to 68*756 grams. If, as is likely, the 

 abrasion did not take place uniformly throughout the length 

 of the bar, then it is impossible to allow for the effect it 

 would produce on the comparison. If, for this reason, we 

 omit the results with this bar, the mean value obtained for 

 the moment of inertia of the Kew magnet at a temperature 

 of 0° C. is 



268*58. 



Four of my own bars have been used by the Observatory 

 for deducing the moment of inertia of the Kew magnet. The 

 results obtained are as follows : — 



Number of Inertia Bar used .. 

 Moment of inertia of the Kew j 

 magnet at C. 



Mean. 



1. 



3. 



8. 



268-48 



268-65 



268-42 



268-49 







Taking the two sets of results together, the mean value of 

 the moment of inertia at 0° uncorrected for the air effect is 



268'54:. 



Or, applying a correction of 0*049 per cent., which has to 

 be added on account of the air carried by the inertia-bar, 

 the final value is 



268-61. 



