1888.] Magnetic Properties of an Impure Nickel. 



319 



3. The magnetisation of my impure nickel disappears about 

 310° C. 



4. A little below the temperature of 310° C. the induction 

 diminishes very rapidty with increase of temperature. 



5. At lower temperatures still the induction increases with rise of 

 temperature for low forces, diminishes for high forces. This fact has 

 been observed by several experimenters. 



Specific Heat. — The object here was simply to ascertain whether or 

 not there was marked change at the temperature when the nickel 

 ceases to be magnetic. It appeared that this question could be best 

 answered by the method of cooling, and that it mattered little even 

 if it were roughly applied. A cylinder of nickel (fig. 2, Plate 13) 

 was taken, 5*08 cm. diameter, 5*08 cm. high, having a circumferential 

 groove, 15'9 mm. deep and 6'35 mm. wide. In this groove was wound 

 a copper wire, well insulated with asbestos, by the resistance of which 

 the temperature was determined. The cylinder was next enveloped in 

 many folds of asbestos paper to insure that the cooling should be slow, 

 and that consequently the temperature of the nickel should be fairly 

 uniform and equal to that of the copper wire. The whole was now 

 heated over a bunsen lamp till the temperature was considerably 

 above 310° C. ; the lamp was next removed, and the times noted at 

 which the resistance of the copper wire was balanced by successive 

 values in the Wheatstone's bridge. If 6 be the temperature, and t be 

 time, and if the specific heat be assumed constant, and the rate of 



d9 



loss of heat proportional to the excess of temperature, h — + = 



at 



or h log 6 + (t— 1 ) = 0. In curve 21 the abscissae represent the time in 

 minutes, the ordinates the logarithms of the temperature ; the points 

 would lie in a straight line if the specific heat were constant. It will 

 be observed that the curvature of the curve is small and regular, 

 indicating that although the specific heat is not quite constant, or the 

 rate of loss is not quite proportional to the excess of temperature, 

 there is no sudden change at or about 310° C. Hence we may infer 

 that in this sample there is no great or sudden absorption or libera- 

 tion of heat occurring with the accession of the property of 

 magnetisability. 



