472 PROCEEDINGS OP THE AMERICAN ACADEMY 



was 0.3 GO cm. This is what was used in the tables. After com- 

 pleting the experiments upon the hollow ring, it was sawed open, in 

 order to see that it was hollow, and to measure the thickness of the 

 shell. The diameter of the hollow was almost, but not exactly uni- 

 form, and the average of the measured values of its radius was a little 

 larger than 0.360 cm. It seemed that the hollow had been made a 

 little larger than the drill at and near the place where the ring had 

 been forged. This would make B, T, and M a trifle larger in Tables 

 ]X. and X., and would make the magnetic induction and the perme- 

 ability more nearly equal to those of the solid ring in Tables VJI. 

 and VIII., while it would make the difference in the temporary mag- 

 netism of the two rings greater. 



The mirror galvanometer used in Tables VII. and IX. was so sensi- 

 tive that resistances had to be placed in the induced current circuit. 

 This made the deflection of the earth inductor very small, and may 

 probably have introduced an error. But as these tables were made 

 especially to get the difference between the solid and the hollow ring, 

 and as they were tried under exactly similar circumstances, the same 

 errors would appear in both tables. 



As far as this single experiment goes, it shows that the temporary 

 magnetism is greater in the hollow than in the solid ring, consequently 

 the permanent magnetism has a larger value in the solid ring. It also 

 shows that the axis of the curve of the hollow ring is more inclined to 

 the axis of M than that of the solid ring is. 



Jefferson Physical Laboratobt. 



