46 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
clearly defined on the scale, though the latter was at a distance of 6 feet from the 
galvanometer. 
The adjusting magnet was almost always used to assist the directive force of the 
earth’s magnetism on the needles, as it was found that by so doing the shifting of the 
zero-point caused by the above-mentioned thermo-electric currents was considerably 
diminished, and at the same time the instrument was sufficiently sensitive. The wires 
S and 0, which were made as nearly as possible of the same resistance, were, after being 
suspended, allowed to remain in the air-chamber for some time; fi was then made 
100 ohms and a adjusted, first by the resistances in the box as far as y^th of an ohm, 
and finally by using the sliding-block H until no deflection of the image of the fine 
wire could be detected on closing the batterv circuit. 
As the needles of the galvanometer soon came to rest, and H could be very readily 
clamped and unclamped, it was possible to make the observations quickly ; an interval 
of one minute was, however, generally allowed to elapse between two consecutive 
readings, as, though the battery-powder was small and S and 0 of the same material 
and of the same section, yet in some cases the current evidently produced unequal 
heating effects in the two wires. Nor is this to be wondered at, as a difference of 
temperature of less than tooo° C. would produce a sensible difference of resistance. 
In ascertaining the temporary alteration of resistance caused by longitudinal traction, 
it was deemed advisable to adopt the same precautions as were used in determining 
the modulus of elasticity, as though the wires employed had been previously strained 
for the latter purpose and then allowed to rest for some weeks; they seemed, in some 
cases at least, to have become partially annealed, and it was found that the temporary 
alteration of electrical conductivity caused by loading was affected in precisely the 
same manner as the elasticity, by stress producing recent permanent extension. 
Great care was taken in loading and unloading the wire S, and in the experiments 
which were made with the first two or three wires a large but light pail was used 
instead of the scale-pan. Into this vessel measured amounts of water were allowed to 
flow slowly through a piece of caoutchouc tubing, when it was necessary to apply stress, 
and the unloading was accomplished by suffering the same water to pass gently out 
through another piece of tubing connected with an orifice at the bottom of the pail. 
During the loading this latter tube was hitched up by the side of the vessel. It was 
found, however, that with practice quite as accurate results could be obtained by using 
a scale-pan in the ordinary manner, and as time was thereby saved this method was 
finally adopted. 
j Formulae employed. 
The temporary alteration of resistance which was in any case produced was so small 
that it could be measured by the wire joining a to /3 . If, therefore, A and B denote 
* The distance of the lamp and scale from the galvanometer is not drawn to the same scale in fig. 6 as 
for the other arrangements, for the purpose of avoiding the taking up of too much space. 
