ON THE MAGNETISATION OF COBALT. 
337 
cyclic changes of magnetisation, in which the magnetisation, whether induced or 
I’esidual, was least when the tension was “ on.” 
The effects of pressure on a nickel bar were exactly the opposite of those of tension, 
and the increase in the susceptibility was extremely large, especially near the 
“ Wendepunkt ”—or point where 3/'§ is a maximum. Professor Ewing looked for a 
ViLLARi point in low fields but found no trace of its existence. 
Theoretics I Considerations. 
§ 13, The experimental results already stated will enable Professor J. J. Thomson’s 
theoretical conclusions to be understood. 
Let e denote the strain in an isotropic elastic cylinder parallel to its axis, f and g 
the strains in the cross section perpendicular to and along the radius vector. Also let 
3 and K denote respectively the strength of the magnetic field, the intensity of mag¬ 
netisation, and the coefficient of magnetic induction, assumed everywhere the same 
throughout the cylinder. 
In the case of a uniform longitudinal stress g —f. Thus, assuming 
^3 ^ 
dg df 
0). 
Professor Thomson obtains for this case, on his p. 51, two equations (48), of which 
the first may be put in the slightly altered form 
= C 
, ^ cIk 
1 — -V tC^ 
d^ 
d3_^^d^ 
de df 
( 2 ). 
Here C denotes a positive constant depending only on the elastic properties of the 
cylinder, and cr is Poisson’s ratio. 
All the differential coefficients must of course apply to some one definite state of 
the cylinder, and in determining d^jde and d’^jdf the strength of the field must be 
kept constant. 
It is pointed out by Professor Thomson that according to Ewing’s experiments 
1 — g/c/cZ 3 is positive for iron. It is obviously positive for any magnetic metal in 
any field exceeding that which answers to its ‘‘ Wendepunkt.” It also appears to be 
positive for nickel and cobalt throughout the experiments of Professor Rowland.'^'' 
Thus it is probably safe to regard it as essentially positive. 
Under a uniform longitudinal stress 
g =/= - o-e. 
MDCCCXC.—A. 
* ‘ Phil. Mag.,’ vol. 48, 1874, p. 321. 
2 X 
