200 
MAGNETIC STRESS AND MAGNETIC DEFORMATION IN NICKEL. 
diminution of magnetisation caused by adding two kilogs. to the load, and the diffe¬ 
rence of ordinates of A and C represents the effect of adding seven kilogs. to the load. 
In fig. 3 curve D represents the observed total contraction — a, curve E the 
residual contraction after the field H has been removed, F the least contraction 
attainable by reversing the current after the field H has been removed, G the value 
of hljl calculated from equation (3), and K the difference a — hljl, all as functions of 
the field H, the load being 4'9 kilogs. The curve K therefore represents the 
contraction in nickel corrected for the effects of known stresses. 
In the last columns of Tables I and II, the corresponding values of I^ are given. 
It will be seen that these numbers are approximately proportional to the numbers in 
the preceding column representing the values of a — §?//. 
This is also shown in fig. 4, where the abscissae of the points . . . -j—1—h are 
proportional to P, and the ordinates to the corrected contraction — (a — ^Ijl) for the 
load 4‘9 kilog. The j^oints all lie, to within about 5 per cent., on a certain straight 
line passing through the origin. An error of 5 per cent, in the values of P and 8I/8P 
might be caused by much smaller errors in the measurement of I. Especially was this 
the case with the other load 2’4 kilog., for the change of magnetisation 81 in this case 
(Table I) was smaller, and therefore more difficult to measure accurately, though even 
in this case there is no regular deviation from proportionality between Pand a — 8//?. 
With the load 4'9 kilog., however, the change of length a observed in the nickel 
wire is closely represented by the equation 
a = cP + 
dvr 1 -r 6 
3E‘ I +~2A* 
___L AH 
2E(I -t- 2^) ^ ^ 8P ’ 
where c has the value — '056 X 10“^^ at the temperature 6°C., and where the 
second and third terms on the right-hand side are numerically very small in com¬ 
parison with the first and fourth. 
[Added May 15.—The result that the magnetic contraction in nickel, corrected in 
the manner described above for the effects of Kirchhoff’s stresses, is under certain 
conditions approximately proportional to P, is at present to be regarded as purely 
empirical, and without further experiments it cannot be said to be generally true. 
It is, therefore, proposed to continue the investigation by repeating the experiments 
on the nickel wire under different conditions, especially as regards temperature. 
It should be borne in mind that in the deduction of the theoretical value of the 
magnetic contraction, the material is supposed to be perfectly “soft,” and no account 
is taken of hysteresis. Some experiments made by Nagaoka on an ellipsoid of 
nickel (‘ Wied. Ann.,’ 53, p. 496, 1894) seem to show, however, that the contraction 
in this metal depends only on the value of the magnetisation, being almost inde¬ 
pendent of the manner in which that value has been reached. It is, therefore, 
unlikely that any considerable discrepancy can arise in consequence of hysteresis.] 
