150 REPORT—1862. 
2. That the standard tube cannot be kept full of mercury for any length 
of time, owing to the diffusion of impure metal, arising from the amalgamated 
terminals into the narrow tube; so that each time the standard has to be 
used, it must practically be remade. 
3. If the tube be broken during the process of cleaning or otherwise, it is 
not yet certain with what exactitude the standard could be reproduced. 
4. It is doubtful whether the resistance of a tube filled with mercury 
today will have the same resistance if filled a year hence; for we have no 
proof if the dimensions of the tube will not alter by being kept. It is well 
known that the bulbs of thermometers are liable to change, and are conti- 
nually changing, im capacity. 
In favour of the gold-silver alloy may be said :— 
1. That this material, when prepared and drawn by different persons, was 
found not to vary in its conducting power more than 1-6 per cent. ; whereas 
the variations found with the metals in a solid state, prepared and drawn by 
different persons, amounts to several per cent., and those found for mercury by 
different observers amount also in all cases to several per cent. 
2. That the homogeneity and molecular corfdition of this alloy are always 
the same. 
3. That the effect of annealing on the conducting power is very small, 
being only 0-3 per cent.; so that if a wire be partially hard-drawn, its con- 
ducting power will not suffer to any appreciable extent. 
4. That the influence of temperature on its conducting power between 
0° and 100°, viz. a reduction of 6-5 per cent., is smaller than either that of 
the metals in a solid state, viz. 29-3 per cent., or that of mereury, viz. 8:3 
per cent. 
And against it :— 
That the conducting power may alter by age, as the physical properties of 
alloys are more likely to change than those of metals. 
From the foregoing statements, based on facts at present known, it would 
appear that the best method of reproducing standards, for those who are un- 
able to procure copies of the British Association unit of electrical resistance, 
is that they should make, or have made, a certain amount of the gold-silver 
alloy (as described in the Phil. Mag., Feb. 1861), by two or three different 
persons, in order to ensure a correct result, and take a given length and sec- 
tion or weight of it, at a given temperature, which has been found equal in 
resistance to the British Association unit. We would recommend, in order 
further to test what we have stated in the foregoing Report, that three or more 
scientific men and electricians be requested to compare the resistances of pure 
mercury, obtained by them from the best sources they are able, and of the 
gold-silver alloy (made in the manner described in the Phil. Mag.) with a 
German-silver standard supplied to them by your Committee. If this be 
done, results would be obtained which would put an end to many disputes 
on the subject, as well as decide which of the above means is practically the 
best for reproducing standards of electrical resistance where no copies of the 
British Association unit can be obtained. 
Apprnpix D.—Professor Krrcnnorr’s Letier. 
- To Fleeming Jenkin, Esq. 
Heidelberg, June 8, 1862. 
Dear Srr,—I have the honour to acknowledge the receipt of your letter of 
the 31st of May, in which you inform me of the labours of the Committee 
appointed by the British Association, to try and bring about the general 
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