278 Prof. J. A. Ewing and Mr. W. Low on the 



Compressive stress produced by external load ha9 an influ- 

 ence of the same kind. The effects of compression are, 

 however, complicated by the fact that this stress reduces the 

 permeability of the solid metal (for moderate magnetizing 

 forces*). Hence on a cut bar it exerts two antagonistic 

 influences, tending to increase the permeability of the bar by 

 its action at the plane of section, and to diminish the perme- 

 ability by its action on other parts ; and when the planes of 

 section are numerous enough, the former influence swamps the 

 latter. Fig. 2 shows that when the bar was cut into two parts 

 the negative effect of load predominated ; the dotted curve 

 (with load) lies below the plane curve (without load). When 

 the bar was cut into four parts, the two effects came near to 

 neutralizing each other. "When the bar was cut into eight 

 parts the positive effect of load was the stronger ; the dotted 

 curve then lies above the plane curve. The following table 

 gives the inductions found in the last case and in the solid 

 bar, and the thickness of the apparent air-space at the planes 

 of section. 



Table II. — Influence of Compression (226 kilogs. per square 



centim.). 



Magnetizing 

 force, $. 



7-5 

 10 

 20 

 30 

 50 

 70 



Induction, 23, under load 



of 226 kilogs. per square 



centimetre. 



Thickness of 

 equivalent 

 air-space, 

 mean for 

 seven cuts. 



Solid bar. 



Bar cut in 

 eight. 



7500 

 10000 

 13900 

 15200 

 16500 

 17200 



3600 



4900 



8300 



10700 



13750 



15700 



millim. 

 0020 



0-019 



0-018 



0017 



0011 



0-007 



The influence of compression is well seen by comparing the 

 thickness of the air-space here with its thickness for corre- 

 sponding magnetizing forces, when there was no externally 

 applied load (Table I.). 



* This effect of compression becomes reversed when the magnetization 

 is strong : the reversal corresponds to the " Villari" reversal of the effects 

 of longitudinal pull from positive to negative when the magnetization is 

 sufficientlv intense. 



