ON THE MAGNETISATION OF COBALT. 
365 
fields the one is sometimes tlie larger and sometimes the other. In no single case was 
a negative value obtained for either quantity, though in fields over 200 C.G.S. units, 
both become unquestionably very small. 
The only safe conclusion appears to be that if a critical field existed for the non- 
cyclic or shock-effect of the pressure cycles it exceeded 275 C.G.S. units. Considering, 
however, that the critical field for the cyclic effect of the pressure cjmles is only about 
120 C.G.S. units, this seems an important result. 
§ 43. In the curve of fig. 6, which is based on Table X,, the ordinates give the 
ratio of the increase in the induced magnetisation produced by the non-cyclic part of 
the first pressure relative to the amount of the induced magnetisation existing 
previous to the pressure. The ordinates may thus be held to measure the relative 
importance in fields of various strengths of the non-cyclic portion of the effect of the 
first pressure. 
The curve shows in a striking manner how the relative importance of the non-cyclic 
portion of the effect of the first pressure continually diminishes as the strength of the 
field increases. In the weakest field employed, viz., 1’8 C.G.S. units, the first pres¬ 
sure permanently increases the magnetisation by one half its original value, whereas 
in a field of 160 C.G.S. units the increase is certainly less than oiie part in two 
hundred. 
In stronger fields, as appears from the ninth and tenth columns of Table X., this 
shock-effect becomes extremely small, and continues to be so within the range of the 
experiments. 
The experiments cannot be said to settle conclusively the sign even of the effect, 
but the evidence is decidedly in favour of its remaining positive in fields of at least 
270 C.G.S. units. It will, in fact, be observed that in the tenth column of Table X., 
a decrease of magnetisation was in no single case obtained. Now in the higher fields 
the results in this column are calculated from the mean of two or three observations, 
whereas those in the preceding column answer, of course, to only one observation. 
Ejfects of the Removal of Pressui'e. 
§ 44. It might ajipear at first sight that the increase in the magnetisation invariably 
found to accompany the application of the sum of the pressure cycles in fields up to 
270 C.G.S. units is in itself sufficient jiroof that the non-cyclic effect of the first 
pressure must be within the same limits an increase of magnetisation. The increase, 
however, in the former case might be due to the removals not the applications of 
pressure. 
The removal of pressure from a rod occasions relative displacements of its parts to 
pretty much the same extent as does the application of pressure, and so it too may be 
expected to have some permanent inffuence on the magnetisation. This effect was 
actually found to exist, and its magnitude relative to the original magnetisation is 
