152 REPORT—1862. 
unit), which was afterwards proposed by Weber in his “ Electrodynamie 
Measurements.” Professor Weber has subsequently had the kindness to 
compare the copper wire whose resistance I measured, with those whose 
resistances he himself had determined (Pogg. Ann. vol. Ixxxii. p. 360); he 
thereby found the resistance of my wire about one-seventh greater than I had 
found it. The reason of this want of agreement consists partly in the im- 
perfection of the instruments which I had used, and partly in the fact that 
in my experiments the temperature was little above 0° R., while in Weber’s 
experiments it was about 20° R. 
Allow me, my dear Sir, to record the very great respect with which I have 
the honour to be, 
Yours very truly, 
G. KircHnorr. 
Appenpix E.—Dr. Sremens’s Letter.—Suggestions for the adoption of a 
Common Unit in measurement of Electrical Resistance. 
To the Committee appointed by the British Association to report on Standards 
of Electrical Resistance. 
GrntLEMEN,—I beg to acknowledge, with thanks, the honour you have done 
me, in requesting me to furnish you with suggestions in furtherance of your 
endeavours to procure the adoption of a common unit of electrical resistance. 
I proposed in Poggendorff’s Annalen (vol. cx. p. 1) to supply this want by 
the adoption of the conducting power of mercury as unit, and of the resist- 
ance which a prism of that metal a metre long, and a square millimetre 
section, at 0° C., opposes to the passage of a current, as unit of resistance. 
The method by which I constructed standards in this unit was as follows : 
From the ordinary glass tubes of commerce, pieces were selected whose 
calibre was found to vary most regularly. After the selected tubes had 
been ground to the length of a metre, they were carefully cleaned and filled 
with pure mercury—the temperature being measured. The contents were 
then weighed, and the values reduced to 0° C. for expansion of glass and 
metal. The resistances of the tubes were calculated by the formula 
a 
wale 14+ vat va, 
g 3 
which represents the resistance to a current in the longer axis of a prismatic 
conductor either in the above unit or in 0-001 unit, according as / is ex- 
pressed in metres and g in grammes, or /in millimetres and g in milligrammes 
respectively. o=13-557, the specific gravity of mercury, at 0°C. 
1 
1+ fa+ Va 
3 
is the coefficient for conicalness, which in good tubes equals 1, very nearly. 
a is the ratio of the greatest to the least transverse section of the tube. 
All the data therefore necessary for the value of W are exact measures of 
length and weight. Measurements of the same tube, at different times, gave 
results corresponding within 0-01 per cent. with each other. 
The first objection which is raised against the adoption of mercury as unit, 
“that the tubes cannot be made of uniform or similar wires, and that the 
