RESEARCHES IN MAGNETISM. 
567 
only 0'5 c.g.s. unit gave an induction (S3) between 7000 and 8000, or say half the greatest 
magnetisation which it is practicable to produce by the strongest forces. The sus¬ 
ceptibility is a maximum at or close to the beginning of the curve. There the value 
23 . 3 . 
of p. or -^-is not less than 20,000, and that of k or ^ is about 1600. 
$ 
The slow beginning and subsequent rapid rise of magnetism which is characteristic 
of the process of magnetisation when performed without mechanical disturbance is 
entirely absent from the curve taken with vibration, but reappears when, at any point 
in the vibration curve, we stop vibrating and continue the change of <§ with the metal 
in a state of rest. The great retentiveness shown by iron at rest has completely 
vanished. At the highest magnetising force reached the value of 93 given with 
vibration is only a little greater than that given without vibration. This difference 
would probably diminish and ultimately disappear (as it should do by Weber’s 
theory) if a very strong magnetising force were applied. 
§ 51. Effects of Vibration in hard-drawn Iron and in Steel .—It is only very soft 
annealed iron that exhibits the influence of vibration in so marked a way. A piece 
of iron wire cut from the same bundle as the above, but not annealed, was subjected 
to the same set of changes of <§, and behaved in the way shown in Plate 61, fig. 25. 
The processes were the same as in the example just quoted, and the figure explains 
itself. Here the retentiveness, though much reduced by vibration, is by no means 
destroyed, and a decided difference is found between the “ on ” and “ off” curves with 
vibration. The former, too, shows a certain amount of inflexion near its starting- 
point. As the metal is here in the hard-drawn state its susceptibility and retentive¬ 
ness (without vibration) are much less than when annealed (cf § 31). 
In steel, whether drawn, annealed, or tempered, the effects of vibration are similar 
in kind to those shown in fig. 25. 
[Note added March 25, 1886.—A point of practical interest in this connexion is 
the circular magnetisation of iron telegraph lines by the signalling currents which 
traverse them, and the consequent retardation of signals. It was pointed out by 
Fleeming Jenkin as early as 1865 * that the experiments of Guillemin showed a 
greater retardation of signals on land lines than could be accounted for, on Thomson’s 
theory, by the electrostatic capacity and resistance of the lines ; and it is now 
generally recognised that this additional retardation is due to the large amount of 
self-induction which a telegraphic circuit exerts on account of the circular magnetisa¬ 
tion of the iron wire. The results of the present experiments may be applied to 
estimate the magnitude of the effects in question. In circular magnetisation we have, 
of course, the condition of endlessness very perfectly realised. Moreover, the almost 
incessant vibration to which telegraph lines are subject must have the effect of 
increasing the magnetic permeability to values not far short of those found in the 
above experiments, where the wires were tapped during the application of magnetising 
* “On the Retardation of Electrical Signals on Land Lines,” Phil. Mag., June, I 860 . 
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