366 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
given in the eleventh and twelfth columns of Table X. Comparing these with the two 
previous columns, it will be seen that in weak fields the non-cyclic effect of the 
removal of the original pressure, though much less important than the corresponding 
effect of the application of the first pressure, is by no means a negligible quantity. It 
may, however, be easily overlooked, because in the weaker fields, where its importance 
is greatest, it is in the opposite direction to the cyclic effect of the removal of pressure. 
Also in fields between 25 and 95 C.G.S. units, as appears from the thirteenth column 
of Table X., the cyclic part of the effect is numerically the larger, and so between 
these limits the remov^al of an original pressure appears to be accompanied by a 
diminution in the magnetisation. In fields over 120 C.G.S. units the non-cyclic 
effect has the same sign as the cyclic, and so the increase in magnetisation accom¬ 
panying the removed of an original pressure is fairly conspicuous. 
Cyclic Effect of Presswe on the Induced Magnetisation. 
§ 45. In fig. 7 the ordinates give the cyclic change in the induced magnetisation 
accompanying the pressure cycles. The thick line a, in which the individual obser¬ 
vations are indicated by dots, is based on Table X., the broken line l> with individual 
observations indicated by crosses, on Table I., and the dotted line c with individual 
observations indicated by circles, on Table HI. 
These curves agree in showing a critical or Villaei field of about 120 C.G.S. units. 
The magnetisation is greatest or least when the rod is under pressure according as the 
strength of the field is less or greater than that of the critical field. 
It will be seen from fig. 1 that the magnetisation of the rod in the critical field is 
about 520 units. According to both the curves, a and h, the absolute magnitude of 
the cyclic effect attains a maximum in a field of about 35 C.G.S. units. This, it vviU 
be remembered, is the field found for the Wendepunkt in §39. In general form the 
two curves a and h could hardly agree better than they do, and the difference in the 
absolute lengths of their ordinates might well be due to a difference between the 
standard pressures adopted at the times. Both curves, it will be noticed, pass 
through almost all their experimental points. Near the critical field all three curves 
woidd lie so close together that only the experimental points of c are there shown. 
The highest field in Table IX. is 351 C.G.S. units, and the form of the curve a 
leaves it uncertain whether the cyclic effect attains an algebraic minimum in a field 
somewhat higher than tliis and then diminishes numerically, or whether it continually 
approaches an asymptotic value. The former alternative is unquestionably supported 
by the form of the curve c, which extends to a field of 725 C.G.S. units. The devia¬ 
tions of the experimental points from this curve are, however, so large that it would 
be rash to attach much weight to its precise form. Probably all we are entitled to 
infer is that, if an asymptotic value exists for the cyclic effect, its numerical value 
cannot much exceed that found in a field of 400 G.G.S. units; whereas, if a second 
