O^'T THE MAGXETISATIOX OF COBALT. 
:3G7 
critical field exists, where the effect changes sign a second time, its value must very 
considerably exceed 700 C.G.S. units. 
It should be noticed that the ordinates in fig. 7 are drawn on double the scale of 
those in fig. 5. Thus, comparing in those figures the two curves a, which are based 
on the same series of experiments, we see that, in fields below the Villaei point of 
fig. 7, the total effect of the first pressure is invariably very considerabiy larger than 
the cyclic effect of pressure^ The maximum value attained by the former effect is 
about thrice that attained by the latter. 
§ 46. In fig. 8 the ordinates give the ratio of the cyclic increase in magnetisation 
accompanying pressure “ on” to the amount of the induced magnetisation existing 
when the cyclic state is reached. They thus show what proportion of the magnetisa¬ 
tion takes part in the cyclic change. 
The curves a and c are derived from the same experiments as the corresponding 
curves a and c of fig. 7. The form of a, near the vertical axis, shows how much the 
relative importance of the cyclic effect increases as the strength of the field is reduced. 
In comparing the relative magnitude of the cyclic effect and of the shock effect of 
the first pressure, it should be noticed that the ordinates of fig. 8 are drawn on a scale 
ten times that on which the ordinates of fig. 6 are drawn; Thus, in the weakest 
fields, the shock effect is fully ten times the cyclic effect. 
Residual Magnetisation. 
§47. In figs. 9 and 10 (Plate 16) the ordinates give the amount of the residual 
magnetisation existing immediately after the break of the current. The curves h and d 
are shown in both figures, but on a different scale; The curve a is based on Table V. 
I’he rod was here demagnetised before exposure to the weakest field, but not subse¬ 
quently, and remained entirely free from pressure. The curve h is based on Table VI. 
The rod was here demagnetised by reverse currents and vigorous tapping before each 
introduction into the coil, but no pressures were applied while it was under the 
influence of the magnetising currents. The curves c and d are based on Tables VII. 
and VIII. respectively. In both cases the rod was demagnetised, as in the case of 
curve 5, but was subjected to six pressure cycles while under the influence of 
the magnetising current. In the case of c the pressure was “ off” when the current 
was broken, whereas in the case of d the pressure was “on.” The individual obser¬ 
vations are indicated by dots for the curves 6, by circles for the curve c, and by crosses 
for the curves a and d. With the scale of fig. 9 the curve c would be indistinguish- 
able from the curve d in weak fields, and from the curve h in strong fields. 
Effects of Pressure on the Amount of the Residual Magnetisation. 
§ 48. The curves a and h of fig 10 agree in showing only a small amount of 
residual magnetisation on the break of weak fields ; but in fields below 5 C.G.S. units 
