154 REPORT—1862. 
trodes of considerable surface might be employed; but I believe that the 
removal of the copper connexions after each test, and the removal of the old 
mercury from their surfaces before using them again, are a sufficient safeguard 
against error arising from this source. Besides, it is easy to fill the spiral 
with fresh mercury whenever it is suspected to have dissolved any quantity 
of copper, or even on every occasion when a measurement with it is to be 
made. Nor .does mercury change its resistance in the least by standing in 
the air. This I have proved by keeping a spiral six months filled without 
changing the mercury, and found its resistance to be constant. 
The material which I have extensively employed in copying this measure, 
viz. German silver, may be classed under the same head as the expensive 
gold-silver alloy of Dr. A. Matthiessen, over which it has, however, the con- 
siderable advantages of a greater specific resistance, and that its resistance 
varies less with temperature variations. 
As a preventive against alteration of resistance by the influence of the air, 
I have usually had the resistances made of this metal covered with a coating 
of silk and lac. 
Intermediate between the resistances to be measured and the measure 
itself, I have introduced resistance-scales. These contain each a series of 
resistances (multiples of the unit), and are so arranged that each resistance is 
exact when it stands stopped alone in the circuit. When carefully made, these 
scales may be depended on to 0-1 per cent. 
Being convinced of the sufficiency of the method I have described of repro- 
ducing a standard of electrical resistance, I have the honour to suggest to you, 
1st. To recommend the universal adoption of the conducting power of 
mercury as unit, and of the resistance which a prism of that metal, a metre 
long, and square millimetre section, at 0°C., opposes to a current of electri- 
city as common unit of resistance. 
2nd. To have the value of this measure ascertained, with the greatest pos- 
sible exactness, in absolute units. 
3rd. To have copies of this unit constructed in mercury contained in glass 
spirals for preservation in scientific repositories. 
In the event of my suggestions being adopted, the mercury unit should be 
determined again with the greatest possible care, and with all the help which 
pure and applied science offers, and copies of it made with equal exactness. 
According to a late determination by Weber, the mercury unit is only about 
23 per cent. greater than 10" absolute units, or one mercury unit at—26° C, 
would equal 10,000,000,000 absolute units. 
Since those cases in which the expression of resistances in absolute measure 
is of advantage in facilitating calculations occur only very seldom, and only 
in purely scientific exercises, a single determination of the relation of the two 
measures would be amply sufficient. Should the absolute unit or any mul- 
tiple of it be adopted as common unit of resistance, there would still be 
wanted a unit for expressing the conducting powers of bodies; and mercury 
is indisputably the best calculated for this purpose. And for practical pur- 
poses, which in adopting a universal unit should be principally taken into 
consideration, it is indispensable to define the resistance-measure as a geo- 
metrical body of that material which is selected as unit of conducting power. 
Every other definition would not only burden unnecessarily the calenlations 
which occur in common life, but also confuse our conception of the measure. 
The reason why the arbitrary unit proposed by Jacobi (a length of copper 
only approaimately defined) found no admittance into general use is to be 
sought in the fact that it failed to fulfil this condition, and because the con- 
