550 PROFESSOR KNOTT ON THE STRAINS PRODUCED IN 



portant influence upon the detailed character of the strain, appreciably affecting the 

 magnitude of the volume change. 



But this conclusion is placed beyond a doubt when we come to examine the be- 

 haviour of the tubes with combinations 020 and 1001. Of all the combinations, the 

 combination 020 gives the most variable distribution of induction ; and with the higher 

 currents this non-uniformity is sufficient to make up for the diminished magnetisation. 

 It is with this combination, accordingly, that the iron tube experiences the greatest 

 volume change. Also, the manner in which the volume change is growing with increase 

 of current shows that, in all probability, with stronger current the volume change pro- 

 duced with combination 020 would still more markedly exceed the volume changes with 

 the other combinations. This remark applies with equal force to the case of nickel, for 

 which, however, the current is not strong enough to produce a volume change with com- 

 bination 020 greater than the volume changes with the three first combinations. With 

 both iron and nickel, the smallest volume change with currents 3 and 4 is produced 

 with the combination giving the least variable distribution of magnetisation ; and the 

 probability is that, with strong enough currents, the combination (020), which gives the 

 most variable distribution of magnetisation, would produce the greatest volume change. 



The effects produced with the last combination of all (100001) are comparatively 

 insignificant, especially in the case of the iron. The magnetic bias of pre-existing 

 magnetisations was very apparent, and the results were of an extremely doubtful 

 character. In the case of nickel, however, fairly consistent results were obtained, the 

 most interesting feature being the comparatively high field in which the volume change 

 altered sign. 



Interesting and suggestive though these varied effects are, the general similarity 

 of behaviour of each tube in the different distributions of field is of still greater interest 

 and importance. For it seems to show that, however complex the strain may be in its 

 smaller details, it is in its broader characteristics essentially the same in distributions of 

 field which are very dissimilar. We may accordingly regard the strain as mainly com- 

 posed of longitudinal and transverse elongations, which, for purposes of comparison, we 

 may assume to be fairly uniform along the tube. 



§ 14. Conclusion and Summary. — So far as I am aware, Joule, Barrett, and 

 Bidwell are the only investigators who have studied the volume changes due to magnet- 

 isation. They all, however, deal with the volume change of the bar, rod, or wire itself, 

 and do not discuss the internal changes of volume of tubes. Nevertheless, there ought 

 to be some relation between their results and mine. 



The most recent and elaborate investigations are those of Mr Bidwell. In a paper, 

 read before the Royal Society of London on March 2nd, 1894, he describes certain 

 experiments made on iron rings. These were magnetised in the usual way by anchor 

 ring coils wound round them, and the corresponding changes in length of the diameter 

 of the ring and of the thickness of the metal forming the ring were carefully measured. 

 From these measurements, the elongation (k) along the lines of magnetisation, and the 



