ON STANDARDS OF ELECTRICAL RESISTANCE. 153 
standard once broken is lost for ever,” is clearly untenable, since the tubes 
are not required to be uniform, and the breakage of the standard involves 
only the necessity of anew tube, and the determinations of length and weight 
anew, to put the operator in possession of a new standard, whose agreement 
with the broken one will depend solely on his own handiness in manipulating. 
Every standard, of whatever material, is liable to injury ; but the breakage of 
a glass is infinitely to be preferred to the treacherous results of a bruised 
wire. 
Mercury is, of all metals, that which is best suited to supply a reproducible 
standard. 
In the first place, it is procurable pure in sufficient quantities. I heated 
for some hours samples of commercial mercury under sulphuric acid con- 
taining a few drops of nitric acid, and found their conducting powers after- 
wards to be precisely the same as that of a quantity of chemically pure mer- 
cury reduced from the oxide. 
Secondly, mercury has always the same molecular structure, and has there- 
fore, at the same temperature, always the same resistance. 
From these two grounds it is possible to couple with this unit a geome- 
trical conception which is indispensable in practice. 
Thirdly, of all metals capable of being used for resistances, mercury has the 
lowest conducting power ; and of all pure metals capable of the same applica- 
tion, its resistance varies least with variations of temperature. 
Having formed such original standards, it only remained to copy them in 
a convenient form for employment in practice. This I have done,— 
1. In mercury contained in glass spirals, and 
2. In German-silver wire. 
The resistance-bridge which I made use of in these measurements, with a 
reflecting galvanometer in its circuit, enabled me to attain a precision of 
within 0-01 per cent. 
The mercury spirals, as may be seen by the accompanying drawing*, are 
provided with cups at their ends. for convenience of filling and for receiving 
the contacts of the measuring apparatus. They are either of known resist- 
ances, approximating only to a multiple of the unit, or may be adjusted to 
an exact multiple by boring out one of the ends of the tube, which, in this 
case, must stand up half an inch inside the cup. The resistances of the bridge 
must then be arranged so that no current passes through the instrument only 
when the desired resistance in the fourth side is reached. When the spiral 
is filled, a vulcanized india-rubber ring is put round the cups, and the spiral 
is suspended in a vessel of ice-water or water kept in circulation by passing 
a current of air through it, and the temperature measured by a delicate 
thermometer. 
The electrical value of each spiral which I have made has been determined 
by comparing it with at least two of the straight normal tubes, both being 
kept during the measurement in ice-water. The greatest differences which 
I have found between such determinations do not exceed 0-05 per cent., to 
which limit the copies may be trusted. 
In answer to the objection that an admixture takes place between the 
mercury and the solid metal used for the terminals, I must remark that I 
have found this occasion really less inconvenience than is generally believed. 
I kept the copper connexions immersed in the mercury a whole week, but 
could not perceive the slightest decrease in its resistance. Platinum elec- 
* The drawings have been omitted, the descriptions being intelligible without them. 
