AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
87 
Explanation of and remarks on Table IX. 
The curves are in most cases divided into two parts, the lower part representing the 
increase of resistance per cent, up to and a little beyond the first critical point, and 
the upper one the increase up to and beyond the second critical point. The abscissae 
represent the increase of resistance per cent, for the lower curves on a scale of 40 
millims. to unit increase per cent., and for the upper ones on a scale of 80 millims. to 
unit increase per cent. 
The ordinates represent the load per square centimetre in kilogrammes, and are on 
a scale of 40 millims. to each 1000 kilogs. for the lower curves and of 80 millims. to 
each 1000 kilogs. for the upper ones. 
The upper carves start from the horizontal lines drawn through their lower ex¬ 
tremities, and the starting points on these lines are set off at distances representing on 
the scale for the upper curves the increase of resistance already attained : also oppo¬ 
site each horizontal line is placed the number of kilogrammes per square centimetre 
already put on to the wire. In all cases the origin of coordinates is marked X, T. 
Thus, for example, the upper extremity of the lower copper curve has an abscissa 
= 32 millims. and an ordinate of 48 millims, therefore the load at this point is 
48X 1 £or 2400 kilogs. per square centimetre; and the increase of resistance is 
82 X to or § per cent. Similarly the upper extremity of the upper copper curve has 
an abscissa of 6 5'6 millims. and an ordinate reckoned from the horizontal line passing 
through the lower extremity of the curve of 56'8 millims., therefore the total load is 
56 - 8 X --§{}- +1080 or 1790 kilogs. per square centimetre, and the increase of resis¬ 
tance 6 5’6 X -§■ or 8‘2 per cent. 
In order to include lead and tin in the same table it was found necessary to reckon 
the load on a scale of 2 millims. to 1 kilog. per square centimetre, and the increase of 
resistance on the same scale as that used for the lower curves of the other metals. 
Moreover, to the load registered in the table there should be added for these metals 
30 kilogs. per square centimetre, this representing the permanent load on the wires. 
It will be observed that there is in most cases a considerable resemblance between 
i the forms of the upper and lower curves, and that in the case of iron the curve near 
i the two critical points becomes very nearly a vertical straight line. 
The critical points of nickel. 
We have seen that the electrical resistance of nickel is altered in a peculiar manner 
by temporary longitudinal traction, and we might expect, therefore, that the effect of 
permanent extension would perhaps be different in the case of this metal, both in 
character and extent, from what it is with other substances. This is found to be so, 
and Table X. shows that the total permanent alteration of resistance produced by 
permanent extension is in the first instance of the nature of a decrease, but that, after 
