ME. Gr. GORE ON ELEOTEOTOESION. 
561 
wire along the axis of a piece of iron gas-pipe*, 1 yard long, of an inch in bore, 
and of an inch in thickness, and found, as he anticipated, that the length of the gas- 
pipe became diminished when the current was instituted, and increased when the cur- 
rent was stopped. 
4. Eesidual magnetism leaves residual changes of dimension in iron and steel of the 
same signs as those exhibited when magnetizing force is first applied or afterwards re- 
applied. 
5. Longitudinal pull f, if sufficiently intense, reduces to zero the magnetic extensions 
and contractions ; and if more intense still, puts the metal into such a state that oppo- 
site strains are produced by it. An iron or steel wire stretched vertically by a small 
weight becomes elongated by magnetization, but if kept stretched by a constant suffi- 
ciently heavy weight it is shortened by magnetization $. 
Now the passage of a current along a straight iron or steel wire of circular section 
gives rise to poleless magnetization in circles perpendicular to the length of the wire 
and with their centres in its axis. Let y be the strength of the current through the 
wire, reckoned of course in absolute units. If the wire be infinitely long, the resulting 
field of force (whether the wire be of iron or of any other metal) is fully specified by 
saying that the lines of force are circles in planes perpendicular to the axis and having 
their centres in this line, and that the intensity of the force is 
and 
2«y 
-Ji r for points in the substance of the wire, 
2y t 
for external points, 
* The bends of the insulated wire outside the gas-pipe in Joule’s experiment complicate the circumstances 
somewhat by superimposing upon the circular poleless magnetization, which a single straight wire along the 
axis of the pipe would produce, magnetization in which there is northern polarity along one semicylinder, and 
southern polarity along the other semicylinder of the outer boundary of the iron pipe, and fainter opposite 
polarities on the inner cylindrical surface. But if the wire had been continued straight for several inches 
outside the pipe at each end, and then carried away to the battery without ever being brought near the gas- 
pipe externally, it is clear that effects in the same direction, though of slightly less magnitude (by an almost 
infinitesimal difference), would have been observed. 
t Rankine’s nomenclature regarding stresses and strains (which is consistent with Huyghens’s celebrated 
ut tensio sic vis ) ought to be carefully followed. It is therefore necessary to introduce two nouns, pull and 
thrust, common enough in familiar language, but not hitherto much used in the theory of elasticity, to express 
longitudinal forces in the directions which would elongate or shorten the bar or wire. With reference to a 
stretched wire we ought to talk of the pull along the wire, and ought not to use the word strain or tension 
to express a stretching force. The only objection to the word pull is that some people might consider it too 
familiar; but surely it is not a valid objection to the mathematician or philosopher that a word, the use of 
which enables him to avoid ambiguity in scientific statements, is already understood by non-scientific people. 
According to Rankine’s nomenclature we must confine the word strain to a change of dimension or figure 
caused by stress ; thus the longitudinal strain of a wire or of a beam experiencing a pull or thrust is the 
(positive or negative) elongation produced by the force. 
J Hence the “ Young’s modulus ” of iron or steel is increased by longitudinal magnetization. 
4 F 
MDCCCLXXIV. 
