382 
MR. C. OHREE ON THE EFFECTS OF PRESSURE 
reintroduced into the coil after being subjected to a series of pressure cycles in a 
previous field of the same strength. So far as they go the results are very similar to 
those of the previous column, from which it follows that the residual polar efiect is, at 
least in its general character, independent of the state of the reintroduced rod as to 
pressure. 
§ 67. Further results bearing on the polar character of the residual effects of 
pressure are given in the second, third, and fourth columns of Table X. These give 
the total algebraic increase of the induced magnetisation due to the first application 
of pressure. The headings A;^, A^, show the condition of the rod as explained in 
§37. 
Comparing the columns headed and Ao, we see that in fields below 50 or 60 
C.G.S. units the increase in the magnetisation caused by the application of the first 
pressure is very considerably greater when the rod is in a freshly demagnetised 
condition than when it has recently been subjected to a numerous series of pressure 
cycles in a field of the same strength and sign. In fields between 60 and 90 C.G.S. 
units the effect of pressure is still distinctly larger in the freshly demagnetised rod, but 
whether considered absolutely or relatively the difference between the results in the 
two columns is small. In stronger fields the numerical value of A^, whether it signify 
an increase or a decrease of magnetisation, appears almost invariably greater than 
that of Ao. Both quantities, however, are then small, and in the strongest fields the 
effects of the first pressure were difficult to measure exactly, as the irregularities in 
the column headed A^ would suggest. It would thus be unsafe to build too much 
upon the differences between the two columns in fields over 90 C.G.S. units. The 
evidence, however, clearly supports the conclusion that even in the strongest fields the 
numerical value of the change in the magnetisation due to the first pressure is, ceteris 
'paribus, greatest in a freshly demagnetised bar. 
The observed differences between the columns headed A^ and Ag are exactly of the 
character we should have expected from the already discussed experiments on the 
residual effects of pressure. In the weaker fields, we saw that the residual effect is 
very considerable. Thus the rod on its second exposure to a given field, under the 
circumstances of the experiment, is more or less in the state of a rod already subjected 
to pressure. Thus its condition, when the pressure Ag is applied, is somewhat inter¬ 
mediate between its conditions before and after the application of the pressure A^. 
Now the effect of the second j^ressure, after the introduction of the rod into the coil, 
was invariably, in the weaker fields, small compared to the effect of the first pressure. 
Consequently, in such fields, the pressure Ag would naturally have a smaller effect 
than the pressure A^, which is precisely the phenomenon observed. 
Comparing the fourth column of Table X. with the previous two, we see that 
in fields up to 60 or 70 C.G.S. units, the increase in the magnetisation of the reintro¬ 
duced rod, due to the first pressure, is distinctly greater when the field is of the 
