ON THE MAGNETISATION OF COBALT. 
353 
during which an increase of 22‘9 units of magnetisation appeared, the current was 
bi’oken, and after a short interval re-made, before the rod had cooled much. During a 
second almost equally numerous series of pressure cycles an increase of only 1'3 unit of 
magnetisation appeared. At the commencement of the second part of the experiment 
most of the coil was doubtless colder than the contained rod, so that the temperature of 
the rod would not alter much for some time after the re-make of the current. This 
last experiment indicates, I think conclusively, that in fields of this strength the 
permanent change in the magnetisation brought about by pressure cycles must be 
small, whatever its sign may be. 
The values given for the First “ on” in the strongest fields cannot claim great 
accuracy. The rod began to get heated before the oscillations of the magnetometer 
needle following the introduction of the rod into the coil had sufficiently subsided to 
permit of a reading being taken. The progressive increase of magnetisation was, in 
fact, going on m.ost rapidly when the first pressure was applied. There had thus 
occurred a considerable progressive change before the next scale-reading could be 
taken, so that it was very difficult to deduce the true effect of the first pressure. 
The values obtained in these high fields for the intensity of the induced magnetisa¬ 
tion must also have been to some extent unduly raised by the heating, so that the 
curve of fig. 1 is, in its higher portions, not so flat as it ought to be. 
§31. Table IV. gives merely certain average results, calculated from the large 
number of observations actually made on the residual magnetisation. On comparing 
the results after the conclusion of the experiments, it became apparent that the 
magnitudes of the several phenomena varied so little with the strength of the pre¬ 
existing field within the limits of the observations, that trustworthy deductions as to 
the exact modes of their variation were rendered impossible by the small variations in 
the distances of the pieces of the apparatus which the method of experiment was sure 
to introduce. The measurements of the effects of pressure are, in addition, exposed to 
possible irregularities in the magnitude of the pressure, but this ought not to affect 
sensibly the average results recorded in the Table. 
Third Series of Experiments. 
§32. In the next series of experiments, extending from January to February, the 
residual magnetisation alone was under investigation. Every single piece of apparatus 
was bolted or screwed to the supporting beam, so as to prevent any relative move¬ 
ment. The magnetometer needle was distant 82 cm. from the scale and 38 cm. from 
the middle of the rod. 
First of all a set of observations were taken of the initial amount of the residual 
magnetisation as the strength of the pre-existing field was raised by steps from 1 ‘7 to 
79-4 C.G.S. units. The rod was apparently very nearly demagnetised by reversed 
currents previous to the first experiment, but no subsequent demagnetisation was 
MDCCCXC.—A. 2 z 
