ON STANDARDS OF ELECTRICAL RESISTANCE. 147 
as unit. These facts therefore seem to indicate that mercury is not yet 
proved to be a safe means of reproducing standards of electric resistance. 
The influence of temperature on the conducting power of mercury, between 
0° and 100°, is, comparatively speaking, small, being only 8-3 per cent., 
whereas that of the metals in a solid state decreases between those limits 
29-3 percent. This property would, of course, render the use of very accurate 
thermometers unnecessary ; for 1° would only cause a difference in the con- 
ducting power of about 0-08 per cent., and therefore 0-1 only 0-008 per cent., 
so that an error of 1 or 2 tenths of a degree might almost be overlooked. 
A fact has just come to our knowledge through Mr. Jenkin. He informs 
us that, having to make a report on the electric apparatus in the International 
Exhibition, he tested, amongst other things, several resistance-coils. Nowhe 
found two sets of coils made by the same firm, the one exhibited in the Prus- 
sian, the other in the English department. Both were said to be multiples 
of the mercury unit proposed by Siemens*, and their resistances determined 
by comparing a coil in each set with that of a tube filled with mercury. 
Taking each set by itself and comparing the coils in it with one another in 
the proper combination, they were found to be perfect; in fact, the adjust- ~ 
ment ef them was perfectly accurate. When, however, Mr. Jenkin compared 
coils of the two sets with each other, instead of being equal, they were found 
to show a difference of 1-2 per cent.T 
III. On those reproduced by a given length and section or weight, at a given 
temperature, of an alloy. 
The alloy on which we have to speak is that composed of two parts by 
weight of gold and one of silver. The reason why this alloy was proposed 
is that the use of (say) 1 per cent. more or less gold does not materially alter 
_ its conducting power. 
1. On its preparation.—It has been shown that the alloy may be made of 
commercially pure metals and have the same conducting power as that made 
from chemically pure ones; for the maximum differences in the conducting 
power between those made in different parts of the world are not greater 
than those of a pure metal, either in a solid or liquid state, prepared by 
the same experimenter. But it may be urged that part of the differences 
obtained by different observers is due to the different methods employed in 
determining their conducting powers, and therefore had the conducting 
power of these alloys being determined by different persons, much greater 
differences would have been found. In answer to this, we give, in the fol- 
lowing Table, the determination of the conducting power of several alloys by 
Thomson and Matthiessen ¢, independently of one another. The alloys were 
made by Messrs. Johnson and Matthey, 
Alloy. Thoimson. Matthiessen. 
1 100-0 100°05 
2 95°8 95:0 
3 102°9 102-7 
4 100°8 99:1 
5 98-1 97-7 
6 89:9 92:7 
7 80°6 80:06 
* Phil. Mag, Feb. 1861. 
+ This discrepancy may perhaps be attributed to some inaccuracy in the reproduction 
of the mercury standard. ‘ 
} Proceedings of the Royal Society, Feb. 1861. 
L2 
