RESEARCHES IN MAGNETISM. 
599 
simple loop like abed, but at a higher part of the diagram, since during vibration 
the magnetisation has been permanently raised. 
§ 83. Effects of Stress on Residual Magnetism. —Throughout the foregoing exami¬ 
nation of the effects of stress the vertical component of the earth’s magnetic force 
(about 0’34 e.g.s. units) was acting longitudinally on the pieces tested. The effects 
observed were therefore due in part to the influence of stress on the inductive 
susceptibility of the iron, and in part to the influence of stress on the existing mag¬ 
netisation. In order to study the effects of stress on residual magnetism a solenoid 
was placed round the wire throughout its whole length, and a constant current from a 
gravity Daniell’s cell was kept up in the solenoid, its strength being adjusted, by 
the introduction of suitable resistance, to a value which produced within the solenoid 
a field just equal and opposite to the vertical component of the terrestrial field.* 
This left the wire free from longitudinal magnetising force, and the changes of its 
residual magnetism, caused by applying and removing loads, were then observed by 
the magnetometer as before. 
A piece of the same iron wire as before (0 - 79 mm. in diameter and 33 centims. long) 
was annealed and placed within the solenoid, in which, however, there was at first no 
current. The wire was loaded several times, up to 8 kilos., with the earth’s vertical 
force acting, and was also tapped, with the result that at the zero of load the magneto¬ 
meter showed a deflection of 450. Then a current neutralising the field was made in 
the solenoid, and the magnetism of the wire fell to 440.t This residue diminished 
quickly when loads were again applied, as follows, but several repetitions of the cycle 
0 — 8 — 0 brought about a cyclic state of matters, of the kind which has been already 
described as produced in annealed iron by loading and unloading under the action of 
a constant field. 
§ 84. Then the range of load was extended. Each addition caused a fall of residual 
magnetism; especially when (at 12 kilos.) the limit of elasticity -was reached and the 
wire began to draw. At 13 kilos., when the wire had drawn about 5 per cent., it was 
unloaded and the cycle 0 —13 — 0 repeated. The curve already described as charac¬ 
teristic of the effects of stress on a stretched wire in an inducing field reappears here 
as equally characteristic of the effects of stress on a stretched wire when there is no 
inducing field, and when the magnetism which is varied is wholly residual. The 
observations are as follows : they are also shown in Plate 63, fig. 39. 
* This adjustment was tested at intervals by removing from the solenoid the wire under examination 
and substituting in its place a wire of specially soft annealed iron. If tapping reduced the magnetism 
of this wire to zero the adjustment of the current was correct. 
f This proportion of residual to induced magnetism, namely, or 97'8 per cent., is higher than in 
any of the experiments described in the earlier part of this paper. It must, however, be noted that in 
this case the wire was tapped while the inducing field was acting, and then the field was removed with¬ 
out tapping. 
4 H 2 
