ON THE MAGNETISATION OF COBALT. 
355 
experiment with a given strength of field. This variation in the order was adopted 
lest during the application for several hours of a succession of pressure cycles, 
interrupted only by the demagnetisations, the rod might become less responsive to 
pressure, developing a sensible amount of what may be termed fatigue. Under the 
actual conditions, if this did happen, it would not afiect the experiments of one type 
more than those of another. 
The strength of the pre-existing fields was raised step by step from 1 to 400 C.G.S. 
units, and until fields of 350 units, or thereby, were reached, the rod was demag¬ 
netised, before each single experiment, by reversed currents and vigorous tapping. In 
the stronger fields the rod was merely exposed to a preliminary reverse current, of 
the same strength as that about to be used in the experiment. During the actual 
observations the end A was invariably the north pole. 
In each experiment of each of the three types, with the exception of some of the 
weakest fields, the following operations were conducted. When the spot on the scale 
had become stationary, after the break of the current, a reading was taken. The pin 
was then withdrav/n and the rod carefully removed, being kept at right angles to the 
magnetic meridian. The scale reading when stationary having been observed, the 
rod was gently restored to its place and the pin secured. With a little practice this 
operation was accomplished without shaking out any sensible amount of magnetism, 
as the coincidence of the scale readino's before the rod’s removal and after its 
restoration sufficiently testified. Pressure was then applied once and removed in the 
experiments of types L and M, being simply removed in those of type N, and the 
corresponding scale readings observed. Twelve pressures cycles were then applied, 
the effects of the last six only being observed. The rod was then removed and the 
constancy of the magnetometer zero tested. 
The various observations obviously supplied sufficient data for calculating the 
initial residual magnetisation, i\\e, first “on” ox first “of” of pressure, the cyclic 
effect off pressure, and the fined residual magnetisation, i.e., the residual surviving 
a definite number—13 for types L and M—of pressure cycles. 
§ 34. The results of the experiments of type L are given in Table VI. All the 
headings have been already explained, except next “ off.” This means the change 
in the magnetisation accompanying the removal of the first pressure applied after 
the break of the current. In the two weakest fields the amount of the initial 
residual magnetisation alone was observed, and in the three next weakest fields the 
effects of only one pressure cycle were taken. Thus no data existed for calculating 
the cyclic effect in these fields ; and the first three results given in the last column 
are unduly large, because the values there assigned to Sa' magnetisations 
existing after the application of only a single pressure cycle. 
In the fields over 90 C.G.S. units the changes in the properties of the residual 
magnetism accompanying the rise in the fields are so small that the small irregu¬ 
larities in the magnitudes of the pressures applied become important. Thus the 
2 z 2 
