454 Prof. A. Gray on the Dynamical 



above P a balance is taken from the medium to make up 

 the electrokinetic energy. 



It seems to me that these results are very intimately con- 

 nected with and confirm Prof. E wing's theory of Magnetiza- 

 tion, published in the Philosophical Magazine for September. 

 In the steep part of the curve beyond the first bend we have 

 work done by the electromagnetic forces (and probably 

 converted into heat) at a much greater rate than elsewhere in 

 the curve. Here, according to this theory, magnets undergo 

 almost all the turning of their axes they receive. Hence the co- 

 efficients of mutual induction must increase here most quickly, 

 and consequently also the work done by electromagnetic 

 forces which depend, as shown above, on the changes in these 

 coefficients. 



Prof. Ewing supposes that the molecular magnets having 

 been swung round into new positions of stable equilibrium 

 vibrate about that position, generating Foucault currents in 

 the surrounding medium, thus heating it. JSIow Baur, Row- 

 land, Hopkinson, and others have shown that iron has its 

 magnetic susceptibility increased by rise of temperature so 

 long as a certain high magnetizing force and a certain high 

 temperature are not exceeded. The conclusion which follows 

 from this by thermodynamic theory is that increase of mag- 

 netization should cool the iron. Although of course we cannot 

 reason, rigidly for an unclosed series of changes, owing to our 

 w r ant of knowledge of the molecular changes, it does seem 

 probable that the iron is really heated in the part of the curve 

 for which the curvilinear area Q S is greater than the 

 triangle Q S. The hypothesis would, however, explain this 

 seeming contradiction ; for, as I understand it, the thermo- 

 dynamic theory does not take into account any expenditure in 

 producing heat of the electromagnetic work done in altering 

 the magnetic state. If the molecules could be laid hold of by 

 external forces, against which they might do work in going 

 round to their new positions, so as to be guided to rest in 

 these positions, there would be no vibration, and the iron 

 would show a cooling, according to thermodynamic theory. 

 In fact the heating due to the Foucault currents, supposed to 

 be thus generated, must be less than it otherwise would be by 

 the thermodynamic cooling. 



In the upper part of the curve it seems probable that there 

 is a marked cooling. At present I am engaged in a careful 

 experimental investigation of heating-effects for the whole 

 curve. 



Another point which may be here remarked is that (as 



