378 On the Magnetic Properties of Nickel. 



The temperature at which the permeability practically 

 vanishes seems, on the other hand, to be higher the greater 

 the magnetizing force. As for the above-mentioned magne- 

 tizing forces, we have for this temperature the values 333°, 

 392°, and 412° C. respectively. In this respect the behaviour 

 of nickel resembles that of iron, as is evidenced by the curves 

 in fig. 3 ; and it is probably partly for this reason that different 

 observers have obtained different results for the temperature 

 of ^'/-permeability. Another cause for the discrepancies in 

 this respect between the observations of different experimen- 

 ters is probably to be found in the want of purity of the metal ; 

 for it is evident that if iron be present as an impurity, the 

 point of raV-perrneability may be rendered very much higher*. 

 The observations made by the author were not in any case 

 carried right up to the vanishing-point of the permeability, as 

 the nickel used by him was, though nearly, not quite puref; 

 and it is not at all unlikely that, had this been done, the ter- 

 minations of the curves would more nearly resemble those of 

 Berson, shown in fig. 2, where it will be noticed that there is 

 a point of inflexion before 320° C. The author does not 

 believe that, even with perfectly pure nickel, the magnetic 

 permeability would entirely vanish; but the following ex- 

 periment will show that, with a thin layer of pure nickel, 

 the permeability practically vanishes w r ith great sudden- 

 ness : — A brass wire 1 millim. in diameter was coated with 

 a thin layer of nickel by electrolysis, and a piece of it, about 

 2 centim. in length, was suspended horizontally in a cradle of 

 platinum between the pole-pieces of a powerful electromagnet 

 in a direction perpendicular to the lines of magnetic force ; 

 the platinum cradle was supported by a platinum wire 12 

 inches long and T J- inch in diameter. The nickel-plated 

 brass wire was heated by a burner to a temperature of visible 

 red, and the electromagnet was then excited by five Grove- 

 cells. Not the slightest effect of the intense field seemed to 

 be experienced by the nickel, even after it had been cooling 

 for several seconds. The magnetizing circuit was now broken, 

 leaving only the residual magnetism in the pole-pieces, when, 

 after a few seconds of further cooling, and without the least 

 preliminary warning, the wire set axially with startling rapidity. 



* The temperature at which the permeability of iron vanishes seems, 

 from Ledeboer's curves, to lie between 750° and 770° C. 



t The wire was procured from Messrs. Johnson and Matthey, who 

 informed the author that they found it impossible to draw pure nickel 

 wire. Mr. G. S. Johnson, the Demonstrator of Chemistry at King's 

 College, London, hns kindly furnished an analysis of the wire. It con- 

 tains 97"5 per cent, of nickel and only 0-C>7 per cent, of iron. 



