Moment of Inertia of Magnets. 



135 



The diameters were measured at three places along each 

 bar by means o£ a Brown and Sharp micrometer-screw 

 gauge which had been tested against the standard metre, 

 measurements being taken along different diameters at each 

 place. 



The masses were obtained by means of a Bunge balance, 

 which reads easily to a tenth of a milligram. The method 

 of double weighing was employed, the weights used having 

 been cnlibrated and compared with a standard 100 gram 

 weio-ht the error of which has been determined at Sevres. 

 Except in the case of the silver bar no correction has been 

 applied for buoyancy, as all the weights, except the fractions 

 of a gram, were of brass. 



In the following table are given the dimensions of the bars 

 and the calculated values of the moment of inertia : — 



No. of Bar. 



Length 



at 15°. 



cm. 



Diameter ,, 



at 15°. Mass ' 

 grams. 

 cm. ^ 



Logarithm of 



the Moment 



of Inertia 



at 15°. 



\ 



9-9755 

 9-9811 

 99998 

 100044 

 10-0017 

 9-9878 

 9-9964 

 9-9837 

 9-9856 



•9909 65-230 

 •9974 81-814 

 •9975 69-204 

 •9979 69-437 

 •9985 69-391 

 •9972 68-516 

 •9994 63-745 

 •9973 68-414 

 •9976 68-481 



2-73633 

 2-83525 

 2-76416 

 2-76602 

 2-76549 

 2-75878 

 2-76099 

 2-75779 

 2-75837 



1 



3 



4 



5 



6 



7 



8 



9 





All the observations of period used in the comparisons of 

 the moments of inertia of the bars were made at temperatures 

 not greatly differing from 15° C. In order to be able to 

 allow for the small departures from this temperature, a series 

 of observations were made with the whole instrument heated 

 in a well-stirred air-bath to a temperature of 30°. The change 

 of period wdth change of temperature is very small, owing to 

 the fact that the rigidity of quartz increases with rise of 

 temperature, and thus produces the opposite effect to that due 

 to the increase in the dimensions of the bars owing to expan- 

 sion. In the case of the copper bars the coefficient of increase 

 of period with temperature is 0*00004, and in the case of brass 

 0*000035. By means of the coefficients the observations of 

 period recorded in the following table have been reduced to 

 what they would have been at 15°, the actual temperatures at 

 which they were made being included between 14° and 18°. 



In the case of bar 1, since the observations with this bar 



