ON STANDARDS OF ELECTRICAL RESISTANCE. 141 
ture such as may occur (for instance, from 0° to 40°), and then, should no 
change occur in their conducting powers, to lead a weak current through 
them, say, for a month; for it has been asserted that a current passing 
through wire causes a permanent change in its conducting power. 
If after these experiments the conducting power of the wires remains un- 
altered, the different forms of resistance-coils, made from those wires, which 
have shown themselves permanent will then be tested in order to prove 
which is the best form of coil for the British Association unit. 
Apprnnix C.—On the Reproduction of Electrical Standards by Chemical Means. 
By Professor Wiitrauson, F.2.S., and Dr. Marrutessen, F.R.S. 
In the following Report we have discussed, more especially from a chemical 
point of view, the relative merits of the different propositions which have 
been made to reproduce standards of electric resistance, and haye treated them 
under three heads :— 
I. Those reproduced by a given length and section or weight, at a given 
temperature, of a pure metal in a solid state. 
Il. Those reproduced by a given length and section or weight, at a given 
temperature, of a pure metal in a liquid state. 
Ill. Those reproduced by a given length and section or weight, at a given 
temperature, of an alloy. 
The points on which we shall speak will be— 
1. On their preparation in a state of purity. 
2. On their homogeneity and their molecular condition. 
3. On the effect of annealing on their conducting power. 
4. On the influence of temperature on their conducting power. 
I. Those reproduced by a given length and section or weight, at a given 
temperature, of a pure metal in a solid state. 
As type of this class we have chosen copper, for it has been more exten- 
Sively used as unit of electric resistance, both by scientific as well as by 
practical men, than any other metal or alloy ; but what we are about to say 
regarding copper will hold good in almost every case for all pure metals in a 
solid state. 
1. On its preparation in a state of purity.—As traces of foreign metals 
materially affect the conducting power of most pure metals, it is of the utmost 
importance that those used for the reproduction of units of electric resistance 
Should be absolutely chemically pure. The difficulty in obtaining absolutely 
pure metals even by chemists is very great. Thus, for instance, Becquerel* 
found the conducting power of pure gold at 0° equal to 68-9, compared with 
that of pure silver at 0° equal to 100; whereas, under the same circumstance, 
- Matthiessen and von Bose found it equal to 77:9,—showing a difference of 
about 12 per cent. in the values observed for the conducting power of gold, 
prepared pure by different chemists. This difference may be due to the silver 
not being pure, or to all of them being more or less pure. Now when we 
consider that these standards are required by electricians and other physicists 
who have little or no acquaintance with chemical manipulation, and that 
the cost of the preparation of absolutely pure metals by scientific chemists 
would be very expensive on account of the time and trouble they require, 
we think that this fact alone constitutes a very serious drawback to their use 
* Ann, de Chim. et de Phys, (1846) t. xvii. p. 242, + Phil. Trans. 1862, pt. 1. 
