ON THE MAGNETISATION OF COBALT. 
3G9 
in fields below 120 C.G.S. units, an excess in the ordinates of curve d over those of curve c 
in weak fields might reasonably have been expected. The difference, however, as will 
more fully be seen presently, is somewhat greater than might have been anticipated. 
The crossing of the curves c and d in a field of from 35 to 40 C.G.S. units, and 
thus far below the Villari field, appears a somewhat striking fact. In fields over 
120 C.G.S. units the curve c, if shown in fig. 9, could not be distinguished from 
curve 6, and the difference between its ordinates and those of curve d seems then 
truly remarkable. In fields over 150 C.G.S. units this difference remains nearly 
constant, and amounts to about 4 per cent, of the ordinates of c. If it be remein- 
bered that according to the last column of Table X. the percentage of the induced 
magnetisation, which is cyclic for the same pressure “ on ” and “ off,” is in fields 
between 150 and 400 C.G.S. units never in excess of '4, the significance of this 
result will be more fully understood. 
§ 49. il'he ratio of the residual magnetisation existing in the rod immediately after 
the break of the current to the numerical value of the strength of the pre-existing 
field will, for shortness, be here termed the residual s asce'ptibility. It is denoted by 
in Tables V. to VIII., and its values are represented graphically by the ordinates 
of the curves of fig. 11. In this and in the subsequent figures the letters a, h, c, d 
denote curves obtained from the same series of experiments as are the respective 
curves a, h, c, d of fig. 10. In fig. 11 and the subsequent figures, however, the indi¬ 
vidual observations are indicated by crosses for curves a and c, by dots for curve h, 
and by circles for curve d. 
In the absence of pressure, as shown by curves a and 6, the residual susceptibility 
is very small in weak fields, but increases rapidly, attaining a maximum in a field a 
little over 20 C.G.S. units. This, it will be noticed, is not much over half the field 
which gives the Wendepunkt for the induced magnetisation. The greatest value 
actually observed for Ki in the case of curve b, was only 1'36. After the maximum 
is passed there is a continuous, but gradual, diminution in the residual susceptibility. 
The difference between the oommencing portions of curves a and h shows, even 
more clearly than in fig. 10, the large effect in weak fields of the process of demagne¬ 
tisation. In fields over 50 C.G.S. units the two curves practically coincide, and are 
drawn as one. 
The crossing points for the various curves in fig. 11 are, of course, the same as in 
figs. 9 and 10. Within the range of the figure the curve c lies distinctly above the 
curve h. In fields over 55 C.G.S. units, the curve d is not drawn, as it could hardly 
within the remaining limits of the figure be distinguished from curve h. 
Commencing with the strongest experimental fields, the ordinates of the curves c 
and d increase gradually as the strength of the field is reduced, and show maxima 
values in somewhat lower fields than do the curves a and 6. As the strength of the 
fields is further reduced, the ordinates of c and d pass through distinct minima values, 
and then increase rapidly as the strength of the fields approaches the lowest experi- 
MDCCCXC.—A. 3 P, 
