74 
SIR WILLIAM THOMSON ON THE 
respect to electric conductivity would cause the electric current, instead of flowing 
rectilineally along the wire, to flow in left-handedly helical lines in the case represented 
in fig. 2, and thus the central parts of the iron cylinder would become really magnetized 
by, as it were, an ordinary helix, but with very steep thread. The effect of such a helix 
is the same as that of a true solenoid superimposed upon a rectilineal current through 
the wire, and the direction of the current in the supposed circumstances is such that 
it would give a true south pole at the upper end of the iron rod in fig. 2. 
(Received and read May 23, 1878.) 
§§ 230-240. On the effects of longitudinal stress on the Magnetization of Nickel 
and Cohalt. 
§ 230. Through the kindness of Mr. Joseph Wharton, of Philadelphia, U.S., I 
was enabled to continue my experiments with malleable and cast bars of nickel, and 
of cast cobalt. 
[Note added July 8, 1879.—-A qualitative analysis of one of Mr. Wharton’s nickel 
bars, performed in the Chemical Laboratory of Glasgow University, by Mr. Donald 
Mackenzie, showed that the bar was not of absolutely pure nickel, but contained 
some carbonaceous matter and also a trace of iron. The amount of the latter, how¬ 
ever, was very small, and probably could not vitiate to any sensible degree the results 
of the experiments described below.] 
The apparatus used in the preliminary experiments and its arrangement are shown 
in* the annexed diagram (XXX.). Each end of the rod experimented on was inserted 
into a ferule-shaped clamp, C (shown also detached in plan and elevation), the outer 
surface of which was conical and screw-threaded. 
The clamp, which had in it three longitudinal slits, was then, by means of a conical 
nut working round it, made to grasp the rod tightly enough to admit of the application 
of great amounts of longitudinal pull, without much danger of pulling the clamps 
away from the rock One of the clamps was then hung from a pin in a strong cross¬ 
beam of a frame, so that the bar hung vertically downwards. 
A rope, It, made of copper wire, connected the other clamp with a point near the 
end of a long heavy lever, turning on a fulcrum at that end formed by a knife edge 
pressing upwards against a brass plate, which formed a bridge between two strong and 
rigid uprights attached to the floor of the room. A heavy weight of lead was hung on 
the lever, and could be slided along it to give different amounts of stress. The lever 
was graduated, and the effect of its own weight was measured, so that the stress 
applied at any time could be at once read off When the bar was in position but not 
under stress, the lever rested on a support high enough to allow the wire rope Pi to be 
slack, and was gently removed from this support when the pulling stress was applied. 
