54 



Mr, Shelford Bidicell 



[Feb. 2r 



nature of the changes of length undergone by certain rods of iron, 

 cobalt, and nickel, when magnetised. 



* Maximum increment. 



t Maximum decrement. 



It was shown by Professor J. J. Thomson, a year or two ago, that 

 the elongations and contractions of iron under magnetisation are 

 intimately connected with the phenomenon which has been referred 

 to as the Villari reversal. With a knowledge of the Villari elfect, 

 the elongation and subsequent contraction of an iron rod under 

 magnetisation miyht have been predicted, and vice versa. Now, since 

 the elongations and contractions of cobalt are of the opposite cha- 

 racter to those of iron. Professor Thomson's reasoning would lead 

 us to expect a Yillari etfect in cobalt, which would also be of the 

 opposite character. Quite recently, Mr. Ohree, at Professor Thomson's 

 suggestion, made some experiments to test the accuracy of this pre- 

 sumptiou, and found the Villari reversal which was anticipated. 

 Again, the circumstance that nickel is always shortened by mag- 

 netisation, and never lengthened, indicates that there is no Villari 

 reversal in that mi tal ; and, in fact, though one has been looked for by 

 Professor Ewing and others, it has never been found. 



A few words in conclusion with regard to the effect of heat. Iron, 

 when gradually made very hot, loses its magnetic susceptibility quite 

 suddenly at a low red heat, and practically becomes a non-magneti sable 

 metal. Pure nickel loses the greater part of its magnetic quality at a 

 much lower temperature, perhaps about 300^ C. Both metals again 

 become magnetisable when cold. Dr. Hopkinson has lately dis- 

 covered a very remarkable effect of heat upon the magnetic pro- 

 perties of an alloy of iron and nickel. If a bar or wire of this alloy 

 be made red hot, and then allowed to cool, it is rendered permanently 

 non-magnutic, although the metals of which it is composed are by 

 themselves both strongly magnetic. But if this non-magnetic 

 material be cooled to a temperature a little below the freezing point, 

 and then again allowed to resume the ordinary temperature of the 

 air, it will be found to have become almost as strongly magnetic as a 



