3G2 
ME. C. CHREE ON THE EFFECTS OF PRESSURE 
removal of a pre-existing pressure. and Ag refer to the results recorded in the 
first two columns under the headings First “on” and First “off” The last heading 
has been already explained. As usual . . indicates that no observations were 
taken. When no algebraic sign is attached to a figure + is understood. 
§ 38. To a casual observer the fluctuations that appear in the results of the fore¬ 
going tables may seem excessive. It should, however, be borne in mind that most of 
the pressure effects are very small compared to the total amount of the induced 
magnetisation—sometimes as little as one-thousandth part. Thus the small irregu¬ 
larities in the magnetising current which are produced by the slightest want of 
constancy in the cells, or by the vibrations communicated to the resistance wires from 
shakings of the floor, may produce commensurable effects. Such irregularities were 
frequently recognised through a continual quivering motion of the spot on the scale, 
while the ammeter reading appeared steady enough. Of coui-se, when the unsteadi¬ 
ness was very marked, observations were suspended, but comparatively little of the 
woik done would have been accomplished in the time if observations had been taken 
only when absolute steadiness prevailed. 
§ 39. A separate discussion of the results of each table would involvm a good deal 
of repetition. It has thus appeared best, as a rule, to embody the most trustworthy 
results in curves, and to discuss particular points in connection with the features of 
the individual curves. In every curve the horizontal coordinate gives the strength of 
the field, <§, calculated after the manner explained in § 19. In the case of the 
residual magnetism, .§ is of course the strength of the pre-existing field. 
In fig. 1 (Plate 15), the ordinates give the initial magnetisation when the rod is intro¬ 
duced into the magnetising coil free from pressure. In fig. 3 the ordinates give the 
coefficient of induced magnetisation under the same conditions. The first portions of 
both curves, in which the individual observations are indicated by dots, are based on 
Table IX., while the second portions, in which the individual observations are indi¬ 
cated by crosses are based on Table III. 
These curves are of the same general character as those obtained by many observers 
for iron. In fields below 8 or 9 C.G S. units, the magnetisation increases com¬ 
paratively slowly as the strength of the field is raised. There then ensues, as is most 
clearly shown by the steepness of the commencing portion of the curve of fig. 3, a 
much more rapid increase of magnetisation. The rate of increase attains a maximum, 
as shown in fig. 3, in a field of about 35 C.G.S. units ; which, accordingly, is the 
Wendepunht for the specimen. The rate of increase of the magnetisation then falls 
off, but at first in a comparatively gradual manner. There is thus no very clear 
indication of an approach to “ saturation ” until the strength of the field approaches 
200 C.G.S. units. Even in the strongest experimental fields the rate of increase is 
by no means infinitesimal, though, as already stated, this is probably in part accounted 
for by the heating of the rod. 
§ 40. The smallness of the scale of fig. 1, does not allow the effect of pressure on 
