378 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
identical in the two cases. The rod was not, as in the experiments discussed in the 
last pai’agraph, in a freshly demagnetised condition, but had just heen exposed to 
numerous pressure cycles in a field of the same strength and sign. 
This column agrees with fig. 4 in showing in weak fields an increase of magnetisation 
due to the existence of pressure. An unmistakeable critical field, however, ensues, the 
effect eventually changing sign. Thus the character of the effect of the mere existence 
of pressure varies in the same way as does that of the joint effect of the application 
and existence of pressure, which we have already discussed in § 41. 
In this case an exact basis of comparison exists, because in the third column of 
Table X., under the heading First “on” Ao, we have the combined effect of the 
application and existence of pressure when the previous treatment of the rod was the 
same as in the experiments of the seventh column of Table IX. 
A comparison of these two columns shows that the fields in which the effects attain 
their maxima, and the critical fields must be nearly the same, if not absolutely 
identical in the two cases. The magnitudes of the effects are, however, decidedly 
different. There can Idc no question that in the weaker fields the effect is very 
considerably greatest when the application of the pressure succeeds the introduction 
of the rod into the coil, and this effect of the application of the pressure clearly 
continues until the field of 128 units is reached. From this to somewhat over the 
critical field neither the application nor the existence of pressure has effects large 
enough for satisfactory determination. In the fields of 198 units and upwards the 
diminution of magnetisation due to pressure was in every case fonnd greatest when 
its application preceded the introduction of the rod. 
Thus in all fields up to 128 C.G.S. units the actual application of pressure during 
the flow of the current produces in the condition of the rod during these experiments 
an unmistakeable increase in the magnetisation, and not improbably this tendency to 
increase the magnetisation exists in all the fields within the range of the experiments. 
In the eighth column of Table IX are a few results from experiments on the same 
point. The circumstances of these experiments differed from those of the preceding 
column, only in that the previous fields of equal strength in which pressure cycles had 
been applied, were of opposite sign to those in which the effects were observed. The 
phenomena are clearly of the same general character as those of the previous column, 
and the existence of a critical field is unmistakeable. The observations are in¬ 
sufficient to determine the exact position of the critical field, but it is obviously not 
far from the critical field of the preceding column. 
The Polar or Non- 2 oolar Character of the Effects of Pressure. 
§ 64. Another very important question is whether the effects of the application and 
removal of pressure on the magnetisation are of a ^oolar or a non-polar character. 
The meaninef of these terms will be best understood from an illustration. 
o 
