62 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
passed through the wire in determining the resistance might now do so ; but again 
the alterations of resistance were found to be the same as before.* Now if E denote 
the electromotive force of the Leclanche, x and y the resistances of the wire and the 
comparison-wire, a and (3 the resistances in the box, and B the resistance of the 
E 
battery and connexions, the current in x will be proportional to -, 
B(i+H|) + * + y 
a + p 
or since x and y were only '44 ohm each, whilst a and (3 were 100 ohms each, the 
E 
current in x will be nearly proportional to j , ~ + + - • 
E was nearly 1‘5 volts, and B -\-x-\-y in the case of the weakest current was made 
10 ohms ; therefore this current would in absolute measure of C.G.S. units be '015. 
Unless, therefore, the maximum difference of alteration of resistance caused by circular 
magnetization in the stretched and unstretched wires had been reached by a still 
smaller current than this, we cannot regard the curious behaviour of nickel in respect 
to the effect of stress on its electrical conductivity as due to circular magnetization. 
Again, another idea suggested itself, namely, that the wires being suspended ver¬ 
tically might cause the resistance of the stretched and unstretched wires to be altered 
unequally by the earth’s vertical magnetic force, but this latter, it will be seen, is of 
too small intensity to have any effect at all comparable with that observed in the 
stretching ; and, moreover, experiments subsequently made with the wire at right 
angles to the magnetic meridian, showed plainly that this was not the case. We must 
therefore regard the abnormal effect produced on the electrical resistance of nickel as 
not due to the earth’s magnetic force.t 
We have seen also that, in the case of other metals which have suffered permanent 
extension, rest increases the elasticity and diminishes the temporary increase of 
resistance which any load is capable of causing, and that with iron this effect is very 
apparent. It was therefore an interesting point to determine whether the decrease of 
resistance which moderate loading produces in nickel would be increased by rest. 
The next experiment shows that this is so. 
* Similar experiments with iron bad also previously sliown that with this metal there is no appreciable 
difference in the alteration of resistance produced by stretching when different current strengths are 
employed in the bridge. 
f After these experiments I re-tried iron with a view to ascertain whether very small loads might not 
produce decrease of resistance, but found that the smallest load that caused any effect whatever produced 
as before increase of resistance. 
