Moment of Inertia of Magnets. 



147 



measuring the bars, however, I discovered that their form 

 was so far from uniform as to make any argument based on 

 the assumption that they were regular cylinders of relatively 

 slight value. Thus in the case of bars 13, 14, and 15, the 

 ends were inclined to the axis by at least 40'. In the case 

 of bar No. 11 the diameter varied between '9937 and '9946. 

 Bar No. 15 is the old bar belonging to the Kew Observatory 

 standard instrument. Its diameter is very variable, being 

 •9962 cm. at the middle and near one end, and '9934 cm. 

 near the other. Although, on account of these irregularities, 

 the numbers are of little value for determining the value 

 of the moment of inertia of my standard bar, yet as 

 the bars had been compared by the Observers at Kew 

 with the standard magnet of their unifilar, all the bars were 

 swung in the comparison apparatus, and the observed periods 

 are given in the following table : — 



No. 11. 



No. 12. 



No. 13. 



No. 14. 



No. 15. 



3-6841 

 3-6841 

 36841 

 3-6838 

 3-6837 



3-6777 

 3-6777 

 3-6781 

 3-6777 



3-6998 

 3-6995 

 3-6998 



3-6577 

 3-6577 



3-7925 



3-7927 

 3-7929 



3-6840 



3-6778 



3-6997 



3-6577 



3-7927 



From these values and the moments of inertia calculated 

 from the mean dimensions of the bars, the following values 



for 



were obtained. 



Number of Bar... 

 Value of Kl 



11. 



45-966 



12. 



45-934 



+14 



13. 



45-937 



+11 



14. 

 45-955 



-7 



15. 



45971 



('I —"to 



Difference from "I 



raean(exclud- > —18 

 ing bar 15)... J 



Although, for the reasons given, these values are cf no use 

 as far as determining the value of the constant c/47r 2 to be 

 used in determining the moment of inertia of my standard 

 bar is concerned, yet they are of very considerable interest in 

 that they show the kind of variation to be expected in the 

 bars such as they have been turned out by makers hitherto, and 

 incidentally the magnitude of the errors likely to arise if the 

 moment of inertia of such bars is calculated from their 

 dimensions, and such calculated moment used to determine 

 the moment of inertia of the magnet. 



L2 



