AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
101 
Remarks on Table XV. and Experiments XXVIII. to XXXII. inclusive. 
It should be stated that before making any observation of the resistance or length, 
the wire was stretched about 2 per cent, of the original length in order to render it 
sufficiently straight, and that the resistance and length after this stretching are 
taken as the starting points from which the percentage alterations are measured; 
in the cases, however, of German-silver and nickel this was not done, as they were 
deemed to be sufficiently straight without stretching, and, moreover, not capable of 
much extension before breaking. 
The curves are evidently of parabolic shape, except perhaps in the case of nickel, 
whose behaviour seems to be quite abnormal. With this metal the curve after 
passing to the left, showing that the specific resistance is diminished by. extension, 
takes a sharp turn round to the right, and, after passing almost vertically upwards, 
makes another bend afterwards, proceeding almost in a straight line. Experi¬ 
ment XXXII., as well as the curve for nickel also, show how much larger is the 
alteration of specific resistance, whether decreasing or increasing, in comparison to 
the alteration of length, than is the case with any of the other metals. 
By comparing Experiment XXX. with Experiment XXXI., we see the effect of 
leaving a load on the wire after the extension has taken place ; the decrease of specific 
resistance of iron produced by a moderate amount of extension is in this case replaced 
by an increase. Now we have seen in Part I. that the wire will not quite assume the 
length which it would have if entirely relieved from stress, but the change in resistance 
is much larger than can be accounted for by any change of dimensions, and in some 
instances the effect of a much slighter restraint than was produced by the stress of the 
lever sufficed to make a very appreciable difference in the specific resistance. The 
silver wire formed a remarkable example of the kind; with this wire the specific 
gravity was determined after each stretching, and the resistance was measured both 
before and after the former operation, the only load left on the wire being that of 
the pulley, which produced a stress of a quarter of a kilogramme on each half of the 
wire. The specific resistance after the different amount of stretchings recorded in 
Experiment XXVIII. was decreased '03, ’27, ’28 and - 2 per cent, by merely removing 
the pulley and taking the wire down for the purpose of weighing it in water. These 
alterations are, it is true, not absolutely large, but if we compare them with the total 
alteration of specific resistance given in the third column* of the experiment, we see 
that they are relatively very considerable. Moreover, in this case not the slightest 
alteration in the length of the wire caused by the removal of the pulley could be 
detected ; neither was the change brought about by the weighing in water, since 
a similar alteration was caused when the pulley was simply removed and replaced 
without any such weighing ; nor, again, was it due to the restitution of conductivity, 
* The values given in this column are calculated from the resistance determined after the pulley had 
been removed and then replaced. 
