MAGNETIC STRESS AND MAGNETIC DEFORMATION IN NICKEL. 
195 
Measurement ofl and 8I/SP. 
The magnetisation was measured by observing the deflection of the galvanometer 
needle when the magnetising current was reversed. The sectional areas of the nickel 
wire and of the coil wound on it being known, and the galvanometer being frequently 
standardised, the induction in the wire, and hence the magnetisation, were calculated. 
In order that equation (3) should hold, it was necessary that the nickel wire should 
be in the saine magnetic state in the magnetisation and in the elongation experiments. 
Hence, both experiments were made with increasing reversals of magnetisation. Also, 
since during the reversals of magnetisation the magnetic stress is alternately applied 
and removed, the weight used in measuring 8I/8P was added and removed several 
times before readings were taken. An initial weight of 1 kilogram was always kept 
in the scalepan and the effect observed at several fields of adding a few kilograms, 
the magnetisation being measured before and after the additional load was applied. 
An increase of tension always caused a diminution of magnetisation, which was not 
in general proportional to the weight added.^ [Added May 15.—This depends on 
the field-strength. At low fields, within the range of tensions used in the present 
experiments, the effect of tension in reducing magnetisation diminishes slightly as 
the tension is increased, at stronger fields the opposite is th ecase ; in other words, at 
low fields 0H/0P^ is positive, at high fields negative. This may partly explain the fact 
observed by Bidwell that at low fields increase of tension diminishes the magnetic 
contraction in nickel, at high fields increases it. For if part of the contraction is 
represented by 
_ g _ Ijj 
^ P0 ’ 
then 
0c 0“I 
^ = - 2-bL ^2 , 
which is negative at low, positive at high fields,] It could, however, be assumed that 
the change of magnetisation, divided by the increase of tension per unit area of section 
of the wire, gave the value of 8I/8P for the mean load. The curve of increasing 
reversals for the mean load was then determined. 
Several loads were tried, but the effects of adding 2 and 7 kilograms respectively 
sufficiently show the nature of the results. Thus, the curves of increasing reversals 
of magnetisation, and the corresponding values of 8I/8P, were determined with 2 and 
4’5 kilograms in the scalepan, the total tensions being 2’4 and 4’9 kilograms. 
Since the dimensional ratio of the nickel wire was about 500, the mean demagnet¬ 
ising forcet was '00018 I. This was never more than about O'l per cent, of the 
magnetising force due to the coil. 
* See Ewing, ‘ Magnetic Induction in Iron and other Metals,’ p. 196, 1893. 
t Du Bois, ‘The Magnetic Circuit in Theory and Practice,’ p. 41, 1896. 
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