364 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
lion-cyclic effects. The individual observations are indicated by dots, whose distances 
from the curve are in no case serious. 
The curve shows that the total effect of the first pressure attains a distinct 
maximum in a field of about 45 C.G.ki. units—which soinewhat exceeds the Wende- 
punkt—and then diminishes somewhat rapidly as the strength of the field is raised. 
An unmistakeable critical field, where the effect vanishes, appears at about 160 C.G.S. 
units, and in stronger fields the totA effect of the first pressure is a diminution of 
magnetisation. From the form of the curve it would appear that the diminution does 
not increase indefnitely as the strength of the field is raised. But the experiments 
leave it uncertain whether this diminution of magnetisation attains a maximum in a 
field of about 350 C.G.S. units, or whetlier it continually approaches an asymptotic 
value. 
§ 42. In curve h of fig. 5, the ordinates give the loss in the induced magnetisation 
accompanying the removal of the rod from the coil, after the application of the 
pressure cycles, and its reintroduction. The curve is based on the column 3^ — ^3 of 
Table IX. 
The ordinate corresponds more or less closely to the non-cyclic effect of the sum of 
the pressure cycles applied during the flow of the current prior to the rod’s removal. 
There are two rea^sons however, why it cannot be regarded as an exact measure of this 
effect, more especially in the weaker fields. In the first place, even when no pressures 
are applied it is well known that the magnetisation of iron is not exactly the same on 
the second exposure to a certain field as it is on the first. Some occasional observa¬ 
tions on the cobalt itself gave a somewhat greater magnetisation on a second exposure 
than on the first. The difference was, however, unimportant, and in fields over the 
Wendepunkt it was, if existent, extremely small. In the second place part of the 
effect of the pressure cycles unquestionably survives a removal of the rod froin the 
mao-netic field. As will be seen from 5 65, this residual effect is of considerable 
importance in fields below 30 units. Owing to both these causes the effect of the 
pressure cycles in fields below 30 or 40 units must be decidedly greater than the 
curve h would indicate. No observations exist to show whether or not the curve h 
eventually crosses the axis of abscissae. Not improbably this would be a very difficult 
point to settle owing to the heating of the rod. 
The reason for draw ing a curve for 3^ ~ Ss i>J preference to one for 33 — 3i> is that 
only a few minutes elapsed between the taking of the readings giving 33 and 33 ? while 
the reading which gives 3i might have been taken over an hour previously. Thus 
under ordinary circumstances the variation of the current could not but be utterly 
insignificant in the one instance, whereas in the other it occasionally reached a 
measurable quantity. 
In Table IX. omitting of course the fields marked 'p, the values of 33 — 3i and 
33 — 33 show a very fair agreement. In fields below the Wendepunkt the former 
quantity is, as the above reasoning suggests, distinctly the larger ; but in stronger 
