ON THE MAGNETISATION OP COBALT. 
371 
over 30 C.G.S. units the retentiveness is least when the current is broken with 
pressure “ on,” and in fields over 50 units the difierence is not inconsiderable. In 
fields over the Villari point, of 120 C.G.S. units, the Induced magnetisation is greater 
for curve c than for curve d, so that in such fields the excess of the ordinates of curve 
c over those of curve d would, relatively to their absolute magnitude, be somewhat 
less in the case of fig. 12 than in the case of fig. 11. 
Similar considerations lead to the conclusion that in fields over 130 C.G.S. units 
the curve h would be, if anything, slightly above the curve c. In other words, in 
fields over 130 C.G.S. units the effect of pressure cycles is, if anything, slightly to 
diminish the retentiveness. This effect is, howevei", insignificant compared to that 
proceeding from the existence of pressure “ on ” during the break of the current. 
The curves h and d cross in a field of about 40 C.G.S. units. The curve d would 
then become the lower, but it would be for a time so close to h that it is not attempted 
to show it separately in the figure. 
The conclusions which the preceding considerations lead to are the following ;— 
In fields below 30 C.G.S. units the application of cycles of pressure “ on ” and “ ofi‘” 
during the flow of the current, and the existence of pressure on ” during the brea.k 
of the current, botli tend to increase the retentiveness, the former agency being in 
fields below 10 C.G.S. units far the more important. In fields between 30 and 
120 C.G.S. units, or thereby, the application of cycles of pressure still increases the 
retentiveness, but the effect continually diminishes as the strength of the field is 
raised. In fields over 130 C.G.S. units up to at least 300 or 400 C.G.S. units the 
effect of pressure cycles is very small, but is, if anything, a diminution of the reten¬ 
tiveness. In fields from 30 C.G.S. units up to at least 300 or 400 C.G.S. units the 
existence of pressure “ on ” during the break of the current diminishes the reten¬ 
tiveness, and the eftect is of the same order of magnitude as that due to the same 
cause in fields below 30 C.G.S. units. In fields between 30 and 40 C.G.S. units, or 
thereby, the increase in the retentiveness occasioned by the pressure cycles exceeds 
the diminution occasioned by the existence of pressure “ on,” but in stronger fields 
the latter effect is the larger, and its superiority appears continually to increase with 
the strength of the field. 
§ 51. Curve h of fig. 12 is repeated in Curve I. of fig. 13, in order that its form in 
fields over 75 C.G.S. units may be seen. There appears a continual but very slow 
diminution in the retentiveness as the field is raised to 400 C.G.S. units. Curve II. 
of fig. 13 will be discussed presently. 
§ 52. In fig. 14 the ordinates give the absolute amount of the residual magnetisa¬ 
tion which seems to disappear with the first alteration of stress after the break of the 
current. 
Comparing the curves h and c, in which the loss of magnetisation accompanies the 
application of pressure, with the curve cZ, in which it accompanies the removal of the 
pressure existing during the break of the current, we see that the effect must be to a 
