166 



lengths are.- 



A. TANAKADATE AND H. NAGAOKA. 



rt ft' = 39.0759 cm. at 0" c. 

 h h' = G7.0VJ6 cm. ,, „ ,, 



The distance between the edges of the magnet was next measured, 

 both in the direct and in the reversed positions. The mean length 

 of the magnet from one edge to the other was found equal to 6.9905 

 cm. at 0'' C'. Thus we have for the distances of the centre of the 

 mngnet from the magnetometer magnet. — 



r, = 30.0196 cm. at 0" C. 

 r,^23.0332 „ „ „ „ 



It is to be noticed that the distances of the mao-net frijm the 

 magnetometer mngnet will be slightly different unless the latter be 

 accurately centred. Since the centering of the magnetometer magnet 

 can be easily done with suflicient accurac}^, arid since the correction 

 te be made from this cause would be ordy of the order of the square 

 of this error, it was quite negligible in our observations. 



The distances of the magnet for two j)ositions, a and b, had to be 

 corrected for the expansion of the rod as well as fov that of the 

 magnet. If the expansion coefficient of l)rass be f^-, and that of steel 

 be /5, it can be easily shown that in place of the fictor (l + 3«t) 

 (see specimen sheet in tlic former report), used in the calculation of 

 3I/H in tlie former survey, we must use. — 





where / denote the half length of the mag-nets. 



Taking «=0.0000189, /5 ==0.0000106, / =7/2, /•, = 30, r,=23, 

 the value of the coefficient becomes — 



1 + 3.08 oi] 



