ON THE MAGNETISATION OF COBALT. 
383 
opposite sign to that existing during the pressure cycles, tlmn when it is of the same 
sign. In stronger fields, the sign of the field existing during the pressure cycles 
appears practically immaterial. 
A difference between the effects of the pressures and Ao, or A^ and Bo, might be 
due either to a polar or a non-polar residual effect of pressure, bat a difference 
between the effects of the pressures A^ and Bg must be due entirely to a polar residual 
effect. Thus the experiments on the first pressures afford independent proof of the 
existence of a residual polar effect in fields up to 60 or 70 C.G.S. units. This is 
in accordance with the results of § 65. 
There would also appear to be evidence of the existence of a non-polar effect in the 
weaker fields. In such a field, we have already shown the existence of a very 
considerable quasi polar residual effect. Now the application of a pressure in a field of 
opposite sign, would unquestionably shake out part of this quasi polar effect, and might 
be expected to produce, in addition, a polar effect of its own, equal or nearly equal to 
that of the first pressure in the original field. Thus, if there were not a non-polar 
residual effect, we should expect the pressure Bg to have, in weak fields, a larger effect 
than the pressure A,. This does not, however, appear to be the case. 
The positions of the critical fields for the vanishing of the total effect of the first 
application of pressure, indicated by the second, third, and fourth columns of Table X. 
are in good agreement. Consequently, it would appear that the position of this 
critical field depends but little on the treatment of the rod since its last demag¬ 
netisation. 
§ 68. The results given under the heading First “off” in Table X., show that in 
the strongest fields of that table the residual polar effects, due to the application 
of pressure cycles in pre-existing fields of the same strength, have no appreciable 
influence on the magnitude of the increase of magnetisation which accompanies the 
removal of the pressure existing during the re-introduction of the rod into the 
magnetising coil. 
Combining this phenomenon with the close agreement in the stronger fields of the 
magnitudes of the effects of the first pressures Ag and B 2 , we seem entitled to conclude 
that the residual polar effect of pressure cycles in a strong field, of whose existence the 
two last columns in Table IX. give clear proof, is not sensibly diminished by the 
application or removal of a single pressure in a subsequent equal field, whether of the 
same, or of opposite sign. 
Resemblances betiveen the Phenomena shoivn by Iron amd Cobalt, 
§ 69. Most of the following resemblances between the phenomena shown by the 
cobalt rod and those shown by the stretched iron wires of Professor Ewing’s experi¬ 
ments, have been already incidentally noticed, but a formal statement and comparison 
may be useful. In the following statements, stress signifies tension in the case of the 
iron wires, and pressure in the case of the cobalt. 
