398 Mr. H. Tomlinson on the Villari 



well annealed, but was subsequently loaded to very nearly 

 its breaking-point, and was in consequence permanently 

 lengthened by about 10 per cent., whereas the wire referred 

 to in fig. 1 was not loaded sufficiently to produce any sensible 

 permanent extension. For a magnetizing force of 3 C.G.S. 

 units the curve cuts the load-line twice, but for higher forces 

 the first cutting-point disappears and we have only the 

 second one. 



The maximum decrease of permeability is greater the 

 greater the magnetizing force up to a certain limit of the 

 latter, which limit other experiments (not illustrated by the 

 figure) proved to be about 16 C.G.S. units. Beyond this 

 limit the maximum decrease begins to become slowly less, as 

 is shown by the dotted curve of the figure for which the 

 magnetizing force was 73. 



The curves for nickel can never be made to cut the load- 

 line twice, for the wire of fig. 2 was stretched within one or 

 two per cent, of its breaking-point, suffering thereby a per- 

 manent elongation of about five per cent, of its original 

 length. The curves, however, show a maximum decrease of 

 permeability with a load of about 24 kilos. 



The observations hitherto recorded were only made at the 

 different stages of loading, but subsequent ones showed that 

 with nickel the readings on unloading were very nearly the 

 same as those en loading, there being very little lagging of 

 mechanical strain behind stress. Prof. Ewing has pointed 

 out also that nickel exhibits very little magnetic hysteresis. 

 With iron there is a very sensible lagging of mechanical 

 strain behind stress, but the only effect of taking a mean 

 between the observed changes on loading and unloading 

 would have been to make the curves more parabolic in shape 

 and not to alter their main character. 



For the next two figures observations were made both on 

 loading and unloading, and though the magnetizing force 

 employed was here very minute, the curves show pretty 

 nearly the same amount of lagging of mechanical strain 

 behind stress which is experienced when much higher forces 

 are used. For these very minute forces, of course, the 

 apparatus had to be made much more sensitive, and the 

 number of turns in the secondary coils was very great. 

 Fig. 4 refers to the same kind of wire as that used in the first 

 experiment. The wire in this case was, however, previously 

 loaded to nearly its breaking-point. If the mean be taken of 

 the percentage alterations of permeability on loading and 

 unloading, the curve becomes exactly parabolic, with its axis 

 vertical and its vertex at the point where the vertical from 



