ON STANDARDS OF ELECTRICAL RESISTANCE. 135 
Thirdly, with reference to alloys, they say that there is better evidence of 
the independent and accurate reproduction of a standard by a gold-silver 
alloy of certain proportions than by a pure solid metal or by mercury. They 
point out that annealing and changes of temperature have far less effect on 
alloys than on pure metals, and that consequently any want of homogeneity 
or any error in observing the temperature during an experiment is, with 
alloys, of little consequence, but that, on the other hand, the existence of 
cavities must be admitted as possible in all solid wires. They are of opinion 
that the permanence of jewellery affords strong ground: for believing that a 
gold-silver alloy will be quite as permanent as any solid pure metal; and in 
the course of the Report they point out some curious facts showing that a 
great change in the molecular condition of some pure metals and alloys may ° 
oceur without any proportional change in their conducting powers. 
Finally, they recommend that practical experiments should be made inde- 
pendently by several gentlemen to determine whether mercury or the gold- 
silver alloy be really the better means of reproducing a standard. 
The main resolution arrived at by the Committee, viz. that a material 
standard shall be adopted which at the temperature of 17° Cent. shall approxi- 
7 metre 
mate to 10 
seconds 
explained. It was not arrived at until after several meetings had been held 
and the merits of the various proposals fully discussed. 
This resolution was passed (unanimously) at a meeting when five out of 
the six members of the Committee were present. 
It was at the same time resolved that provisional copies should be distri- 
buted at the present Meeting; but the circumstances have been already 
explained which have prevented this resolution from being carriedjnto effect. 
~ It was thought desirable that an apparatus should be designed which could 
be recommended by the Committee for use in copying and multiplying the 
units to be issued, since it is certain that some of the glaring discrepancies 
in coils intended to agree must have been due to defective modes of adjust- 
ment. Mr. Fleeming Jenkin has consequently designed an apparatus for the 
purpose, of which a description is appended. Messrs. Elliott Brothers have 
kindly constructed a couple of these instruments, which were seen in action, 
at the Meeting of the Association, by members interested in this subject. 
The present Report was drawn up by Mr. Jenkin, and adopted at a 
meeting of the Committee on the 30th of September. 
, as far as present data allow, has been already fully 
Appendix to Report on Standards of Electrical Resistance. 
A. On the variation of the electrical resistance of alloys due to change of 
temperature, by Dr. Matthiessen, F.R.S. 
ie oo the electrical permanency of metals and alloys, by Dr. Matthiessen, 
C. On the reproduction of electrical standards by chemical means, by 
Professor Williamson, F.R.S., and Dr. Matthiessen, F.R.S. 
D. Professor Kirchhoff’s letter. 
_E. Dr. Siemens’s letter. 
F. Dr. Esselbach’s letter. 
G. Circular addressed to foreign men of science. 
es Description of apparatus for copying and multiplying the units of re- 
sistance. 
