ON STANDARDS OF ELECTRICAL RESISTANCE. 155 
ducting power of all solid bodies is too dependent on their molecular struc- 
ture. 
The same objection renders the adoption of the gold-silver alloy proposed 
by Dr. A. Matthiessen equally incapable. 
Another disadvantage in the way of a solid metal unit is the impossibility 
to solder thick connexions into the ends of a defined length of any wire 
without altering its resistance. 
Should the adoption of the mercury unit be deemed advisable, I would 
place at the service of the British Association any further information or assist- 
ance in my power. 
I have the honour to be, Gentlemen, 
Your most obedient Servant, 
W. Sremens. 
Apprnvix F.—Extracts from a Letter addressed to Professor W1Lttamson by 
Dr, EssELpacu. 
The two objections against the practical applications of Weber’s absolute 
unit haye been sufficiently pointed out as being— 
1. Its minuteness; and 
2. That the electromotive force of galvanic elements does not allow of vari- 
ation (as strength of current, tension, and resistance do), but that we have 
to accept certain constants as nature has fixed them. 
I take it for granted that the standard of absolute unit would not lose in 
authority if a plain multiple of it were adopted. I need not point out that 
the French metre itself is only a submultiple, iamomth of a natural unit—the 
earth’s quadrant. The multiple of the natural electro-magnetic unit I am 
about to suggest for practical use is 10”°, therefore very simple (which is of 
no little importance); and it is a multiple which leads us to those standards 
which are practically used. 
M. Bosscha gives the electromotive force of his Daniell’s cells in absolute 
measure as 
1025-80 . 10°, 
and calculates the one used by Mr. Joule to be 
1045-1 . 10°. 
It will therefore be practicable to determine such concentration of sulphuric 
acid as to make the electromotive force equal to 
10. 10"; 
and I believe the concentration required would be very near what is actually 
used in telegraphy. 
Resistance—The different copies of Jacobi’s étalons are well known to 
differ as much between each other as Daniell’s cells ; and if Siemens had done 
nothing else for galvanometry than to give us copies which agree among 
themselves within a quarter per cent., the progress is obvious, 
Weber’s copy of Jacobi’s étalon is” 
598 . 10’; 
and that of M. Bosscha was 
607 . 107 
in absolute measure. 
Other statements (of Kirchhoff and others) give a much smaller value. 
Tn comparing Mr. Siemens’s mercury standard with three copies of Jacobi’s 
étalon in his possession, I found two of them agreeing tolerably well with 
