AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
69 
to remain on for 10 minutes, when a decrease of ’0428 per unit was measured; after 
20 minutes the decrease amounted to '0557 per unit, part of the decrease produced 
after the first 10 minutes was permanent, but a considerable part was temporary. 
The foil was now tested with loads of 12 and 20 kilogs., the latter causing a decrease 
of resistance represented by 300 divisions of the platino-iridium wire, and the former 
180, that is, the decrease was exactly proportional to the load. From these results it 
was calculated that a transverse stress of 1 grm. per square centimetre would produce 
a decrease of resistance amounting to 12384 X 10 _n as against 4406 XlO -13 , the 
alteration caused by the same longitudinal stress in a zinc wire; the alteration in 
the former case is about 28 times that in the latter. Again, assuming that the 
values of e and cr are the same for the foil as for the wire, the decrease of specific 
resistance which would be caused by transverse strain sufficient to double the width 
of the strip would be 95 per unit; whilst the alteration for the same amount of 
longitudinal stress is about 2 per unit. The temporary decrease of resistance of 
zinc and tin appeared to be so very large, that it was suspected that the silk did not 
properly insulate the foil from the upper and lower clamps; but this did not seem 
to be the case, as the resistance was the same before and after clamping ; nor could 
the stress have temporarily impaired the insulation, as if so the same large decrease 
would have been obtained with iron. It would thus appear that, at any rate for the 
metals zinc and tin, the effect on the electrical resistance of stress perpendicular in 
direction to the line of flow of the current is the reverse in every respect of that 
of stress applied longitudinally. In the case of iron we have seen that though the 
resistance is on the whole decreased, the mere change of form would more than account 
for the decrease; and that unless the amount of lengthening which any given load per 
square centimetre can produce is much less with iron-foil than with iron wire the 
specific resistance is increased. 
One of the most remarkable features of these experiments is the large influence of 
time on the temporary alteration produced by the heavy loading of zinc and tin. A 
similar influence, though not to the same extent, was noticed with aluminium, zinc, and 
tin when great longitudinal stress was employed, and with these metals also for both 
directions of stress, the departure from proportionality between the load and the 
alteration produced thereby became very marked, though here again much more so 
when transverse than when longitudinal stress was applied. 
