OP ARTS AND SCIENCES. 471 



with thin asbestos paper, aud each turn of the coil was separated from 

 the next by an asbestos string wound around the ring. Only one 

 layer of wire was used. The small uncovered portions of the ring 

 allowed it to take the temperature of the paruffine without materially 

 affecting the uniformity of the winding. 



From the curves of the other rings there does not seem to be any 

 definite difference between the values of M due to the variation of the 

 diameter or of the cross-section of the rings. 



The temper of the metal seems to exercise a much greater influence 

 upon the magnetic permeability than any other physical condition. 

 This is shown by an inspection of Tables IV., V., and VI. for iron, 

 aud XI. aud XII. for cobalt. 



The iron ring 4 was heated to red heat and allowed to cool slowly, 

 and it gave Table IV. It was then heated and plunged into cold water, 

 and then gave Table V. We wished to see if it would regain its per- 

 meability on being softened again. It gave Table VI., in which the 

 maximum value of M is greater than after the first annealing. 



The maximum value of M for soft iron is about three times its 

 maximum value for the same iron hardened. 



The maximum value of M for soft cobalt is ten times as great as the 

 maximum value for the same cobalt hardened. 



Shielding of Magnetic Influence. 



"We next wished to determine whether or not the outer layers of 

 the ring shielded the inner layers from magnetic influence. 



Professor Bosanquet * endeavored to decide this question by com- 

 paring the magnetic induction, for equal magnetizing forces, in rings of 

 different cross-section. He said there was no shielding. It is evident 

 from the preceding tables, that the quality of his metal would make 

 such a large difference in the values of the induction as to completely 

 mask the shielding effect if there were any. 



"We think the method tried by us more suitable to determine the 

 question. A solid and a hollow ring were made from the same bar 

 of iron, and of nearly the same diameter. 



The hollow ring was made by cutting off a bar of the proper length, 

 and drillincf a hole alon^ its axis. Then the tube thus formed was 

 bent around and forged without closing the hollow. The radius 

 cross-section was 0.698 cm., and the radius of the hollow, calcu- 

 lated from the equation tt r^ — tt r- = weight divided by the density, 



* Phil. Mag., February, 1885. 



