284 



The Influence of Stress and Strain, tyc. [Jan. 26, 



there is little or no perceptible difference in either the increase of 

 resistance or the induction current due to magnetisation, whether a 

 current is or is not at the time flowing through the metal. 



In the paper curves are shown exhibiting the connexions between 

 increase of resistance, magnetisation, and induced magnetism. From 

 these curves, and from the fact of the above-mentioned formula 

 holding good, it is assumed that the resistance will go on increasing 

 with the magnetising force even when the latter is so great that 

 further increase of force does not produce perceptible increase of 

 magnetism. 



In some remarks made in the paper on the nature of the alteration 

 of resistance which is produced by magnetisation, it is stated, that 

 " had the nature of the change of resistance been the same for longi- 

 tudinal mechanical stress as for longitudinal magnetisation in the case 

 of all metals, there is nothing in the actual amount of alteration that 

 might not lead us to suppose that the change of resistance from the 

 latter cause is due to mere rotation of the molecules considered simply 

 as molecules without regard to the electric currents, which, according 

 to Ampere's hypothesis, are constantly circulating round these 

 molecules. But when we find, that with nickel, longitudinal traction, 

 which must also cause to a certain extent rotation of the molecules, 

 but without magnetic polarity, actually produces decrease of resistance, 

 we are probably right when we conjecture, that the change of 

 resistance resulting from magnetisation is in a great measure due to 

 the fact that the current used in the ' bridge ' is encountered by a 

 set of molecular currents circulating all more or less in the same 

 direction, and in planes more or less at right angles to the direction 

 of the ' bridge '-current, according as the magnetism induced is 

 o-reater or less." These molecular currents would cause the current 



o 



passing through the substance to flow spirally, and the effect would be 

 aided by the action of the magnetising force itself, which action would 

 go on increasing with increase of force, even when no appreciable 

 further increase of induced magnetism took place. Since Professor 

 Hall has proved that such action is possible, and that nickel, iron, 

 and cobalt are very conspicuous in this respect, we have some support 

 tor this view. 



The " circular " magnetisation which any magnetic substance 

 undergoes when a current is conducted through it, seems to have very 

 little or no appreciable effect on the electrical resistance of the sub- 

 stance, so that, if we compare the resistances of iron and platinum, the 

 ratio of the two will be independent of the electromotor employed in 

 the "bridge." 



The effects of temporary stress on the alteration by magnetism of the 

 resistance of an iron or nickel wire are of a somewhat similar nature 

 to those caused by the stress on the magnetic inductive capacity of 



