566 
PROFESSOR J. A. EWING ON EXPERIMENTAL 
It is in the early part of the curve, the part where (under ordinary conditions) 
the magnetism has not yet begun to rise rapidly, that the effects of vibration are most 
remarkable. At later points the effects are still considerable, until we reach the 
region of approximate saturation, where, as might be expected, vibration produces 
little change. 
The form should be noticed which the curve takes at each place when, after tapping, 
the process of increasing or decreasing 43 is resumed without mechanical disturbance. 
Like the initial part of a normal curve of magnetisation these portions seem to start 
off tangent to the direction along which 4? is measured, and it is only after a con¬ 
siderable change of 43 has taken place that the magnetisation begins to change at all 
rapidly. This characteristic of the curves affords strong confirmation of the idea that 
retentiveness in soft iron is chiefly due to a resistance to the rotation of Weber’s 
molecular magnets of a kind resembling the static friction of solid bodies. 
§ 50. Magnetisation of soft Iron with and without Vibration .—In the next experi¬ 
ment the same piece of soft iron was again tested by the ballistic method. A pair of 
“ on” and “ off” curves were taken in the ordinary way without vibration, 43 being 
raised by steps to 16*8 and reduced by steps to zero, while 93 was measured by the 
summation of throws. Then, after the residual magnetism had been removed by 
tapping, a second pair of “ on” and “ off” curves were taken, in which the wire was 
vigorously tapped at each value of the magnetising force, and the resulting magnetism 
determined by slipping off the induction coil. The former pah* are shown by fall 
lines, the latter by broken lines (-), in Plate 61, fig. 24. Finally, after the 
magnetisation was again raised to 8000, by tapping under a force of 0‘6, the process 
of magnetising was continued without vibration. This last part of the operation is 
shown by the dotted line (.) in the figure. 
It will be seen that the “on” and “off” curves with vibration are nearly coincident. 
The following readings refer to the “ on” curve with vibration :— 
A 
SB. 
0 
240 
(initial) 
0-04 
840 
0T5 
3,370 
0-31 
5,370 
0-62 
8,260 
0-96 
9,540 
1-60 
10,740 
2-92 
12,040 
5-04 
13,140 
7'00 
13,460 
16-8 
14,750 
The enormously great magnetic susceptibility which soft iron exhibits when 
vibrated is worthy of special remark. In the above example a magnetising force of 
