350 
MU. C. CHREE ON THE EFFECTS OF PRESSURE 
of the rod, and so bring the spot near the centre of the scale. In the absence of the 
I'od the spot was now off or nearly off the scale, so that accurate measures of the 
induced magnetisation were impossible. 
The effect of the changed position of the compensating coil was allowed for in 
reducing the observations. A similar use of the compensating coil was subsequently 
made in some of the higher fields. It had the disadvantage of making the scale 
reading more disturbed by slight irregularities in the current. In these strong fieltl.s, 
however, the cyclic changes were so small compared to the total induced magnetisation 
that the alternative of increasing the distances of the coils from the magnetometer 
possessed greater disadvantages. 
§ 26. In Table II., <§ is the field existing prior to the break of the current. 3/ is 
the residual magnetisation prior to tlie application of any pressure, 3/ tliat finall}’- 
existing after what was approximately a constant number of pressure cycles. A 
minus sign represents, as elsewhere, a diminution in the magnetisH.tion, and when 
placed at the head of a column it applies to all the entries in that column. The 
shoch-effect is obtained by subtracting from the algebraic value of the total effect of 
the first pressure tlie algebraic value of tlie cyclic “ on ” — “ 
The second last column gives the ratio of the shock-effect to the intensity of the 
residual magnetisation prior to pressure, while the last column gives the ratio of the 
cyclic “ on ” — “ off” to the intensity of the residual magnetisation in the cyclic state. 
§ 27. In both the Tables I. and II. the letter N signifies that a certain end of the 
rod, which we shall call A, was a north pole ; S, that it was a south pole. The numhers 
in brackets in the first columns give the order in which the experiments took place. 
It was Intended to apply as nearly as possible the same degree of pressure in every 
single case. Still, as the pressure was applied by hand, a certain amount of irregu¬ 
larity was bound to exist. The smallness in the variations shown by the individual 
observations of the cyclic effect in each separate field, and the smoothness of the 
several curves obtained prove that throughout any single series of experiments a 
pretty uniform standard must have been maintained. As a considerable interval of 
time elapsed between some of the successive series of experiments, notably those made 
in December and Januar}’’, the standard probably varied somewhat from one series to 
another ; fig. 7, at all events suggests that in February the standard pressure was 
somewhat less than in December. 
^ 28. The results of Table I. and IT. will be presently discussed along with the 
corresponding results from suhsequent tables, but certain peculiarities illustrative of 
the first difficulty stated in § 22, claim a special attention. 
A glance at the values of and in Table I., shows that the supposed demagnet¬ 
isation following the strong field (7) had not sufficed to remove all its effects. A 
comparison of the fields (14) and (15), or of (12) and (13), shows a much smaller 
susceptibility for currents making A a south pole, than for those making it as in (7) a 
north pole. From the results subsetpiently obtained with a more perfect system of 
