ON THE MAGNETISATION OE COBALT. 
347 
The total removal of this disturbing agency would require the complete demagneti¬ 
sation of the rod. The method of demagnetisation employed at first consisted of 
applying a few reverse currents gradually diminisliing in intensity. Tlie magnetisation 
left in the rod seemed infinitesimal, but the tinsatisfactory character of the result will 
be easily seen on reference to Tables I. and II. The metliod finally adopted was to 
expose the rod to a succession of diminishing reverse currents—the first current in the 
case of a weak field considerably exceeding that in existence during tlui ]>ressure 
cycles—and then to tap it vigorously. The first attempt was by no means always 
successful, and the loss of time was a distinct objection to the method. 
It is, of course, impossible to be sure that the rod was ever demagnetisetl, in the 
sense of being restored to its condition prior to its first magnetisation, but it was, at 
all events, reduced to such a condition that successive experiments made with the 
same field showed an excellent agreement. In strong fields demagnetisation appeared 
to be, at least for many purposes, of very little importance. 
§ 23. The second disturbing agency, the heating of the coil and thence of the rod, by 
means of the current, was troublesome only in fields over 400 C.G.S. units. The 
heating of the coil wiies increases their resistance, and so tends to diminish the 
strength of the field; while the heating of the rod increases its permeability, as luts 
been shown by Professor Howland.'" For a rise of temperature of 22.^)° C. he found 
the maximum value of the coefficient of induced magnetisation to increase by about 
70 per cent., and to appear in a distinctly lower field. 
This explains wliy, in some of my stronger fields, the scale reading kept altering in 
a direction indicating a progressive increase in the rod’s magnetisation. This did mt, 
of course, annul the cyclic changes accompanying C 3 mlic clianges of pressure, but it 
rendered impossible any very great accuracy in the determination of their magni¬ 
tudes. The determination of the effect of the first application of pressure was found 
particularly difficult. 
Lest it should be supposed that the cjmlic changes in tlie magnetisation may in 
reality be due to the heating and cooling of the rod produced by the application and 
removal of pressure, I would here point out that the cyclic changes of magnetisation 
alter in sign in a comparatively low field, while the increase in the permeability 
accompanying heating was observed by Howland in fields of all strengths, from 50 to 
1470 C.G.S. units. 
First Series of Experiments. 
§24. With the new apparatus four principal series of experiments were performed. 
The first series took place in December, 1888, and the results are given in Tables I. 
and II. The general order of conducting these experiments was as follows ;— 
The rod, after being demagnetised by reverse currents alone withoitt tapping, was 
removed to a distance. The number of stora 2 :e cells and the bridge resistance were 
O O 
* ‘ Pliil. Mag.,’ 4tli series, vol. 48, 1874, p. 321. 
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