162 : REPORT—1862. 
prevalent that the electrical resistances of wires of some, if not all, metals are 
far from permanent; and since these resistances are well known to vary as 
the wires are more or less annealed, it is quite conceivable that even the 
ordinary changes of temperature, or the passage of the electric current, may 
cause such alterations in the molecular condition of the wire as would alter 
its resistance. This point is treated at some length in the two Reports, 
B and C, appended, by Professor Williamson and Dr. Matthiessen. The ex- 
periments hitherto made have not extended over a sufficient time to establish 
any very positive results ; but, so far as can be judged at present, some, though 
not all, wires do appear to vary in conducting power. 
Mercury would be free from the objection that its molecular condition 
might change; but, on the other hand, it appears from Report C that the 
mercury itself would require to be continually changed, and that consequently, 
even if the tube containing it remained unaltered (a condition which could 
not be absolutely ensured), the standards measured at various times would not 
really be the same standard, A possibility at least of error would thus occur 
at each determination, and certainly no two successive determinations would 
absolutely agree. If, therefore, wires can be found which are permanent, 
they would be preferred to mercury, although, as already said, no conclusion 
has been come to on this point. 
Some further explanation will now be given of the resolutions passed from 
time to time by the Committee, and appended to this Report. 
Dr. Matthiessen was requested to make experiments with the view of 
determining an alloy with a minimum variation of resistance due to change 
of temperature. The object of this research was to find an alloy of which 
resistance-coils could be made requiring little or no correction for tempera- 
ture during a series of observations. A preliminary Report on this subject 
is appended (A), in which the curious results of Dr. Matthiessen’s experiments 
on alloys are alluded to, and, in particular, the following fact connected 
with the resistance of alloys of two metals is pointed out. 
Let us conceive two wires of the two pure metals of equal length, and 
containing respectively the relative weights of those two metals to be used in 
the alloy. Let us further conceive these two wires connected side by side, 
or, as we might say, in multiple arc. Then let the difference be observed 
in the resistance of this multiple are when at zero and 100° Cent. This 
difference will be found almost exactly equal in all cases to the difference 
which will be observed in the resistance of a wire drawn from the alloy 
formed of those two metal wires at zero and 100°, although the actual resist- 
ance at both temperatures will in most cases be very much greater than that 
of the hypothetical multiple arc. 
In order to obtain a minimum percentage of variation with a change of 
temperature, it was consequently only necessary to make experiments on 
those alloys which offer a very high resistance as compared with the mean 
resistance of their components. The results of a few experiments are given 
in the Report, but these are only the first of a long series to be undertaken. 
Hitherto an alloy of platinum and silver is the only one of which the conduct- 
ing power and variation with temperature are less than that of German silver. 
Professor W. Thomson and Dr. Matthiessen were requested to examine the 
electrical permanency of metals and alloys. A preliminary Report on the 
subject by Dr. Matthiessen is appended (B), in which he shows that, after 
four months, one copper and two silyer hard-drawn wires have altered, 
becoming more like annealed wires, but that no decided change has yet been 
detected 1 in the great majority of the wires. 
