ON STANDARDS OF ELECTRICAL RESISTANCE. 145 
bear in mind that supposing the zero-point of the thermometer is correct to- 
day, we are not at all justified in assuming that it will be so in six months 
time ; so that we ought to redetermine the zero-point of the thermometer be- 
fore using it for the above purpose. Again, it has been proved that the in- 
fluence of temperature on the conducting power of wires of the same metal is 
not always the same*. Thus, for the conducting power of annealed copper 
wires the following values were found :— 
d, No. 1. No. 3. 
0 100-0 100-0 
20 92°8 92-4 
40 86°3 85-6 
60 80:4 79-6 
80 751 74:4 
100 70:5 70-0 
showing therefore that if standards of pure metals be used, the influence of 
temperature on the conducting power of each would have to be ascertained. 
It must also be borne in mind that it is not at all easy to maintain a stand- 
ard, even in a bath of oil or water at a given temperature, for any length of 
time. 
II. Those reproduced by a given length and section or weight of a pure metal 
in a liquid state. 
The only metal which has been proposed to be used in a liquid state for 
the reproduction of units of resistance is mercury. We shall only have to 
speak of its preparation in a state of purity, and on the influence of tempe- 
rature on its conducting power. For a tube, carefully filled with mercury, 
will certainly form a homogeneous column, and its molecular condition will 
always be the same at ordinary temperatures. 
On its preparation in a pure state—Although this metal is one of the 
most easily purified, yet the use of it as a standard is open to the same objec- 
tions, although in a less degree, as have been advanced against the use of 
pure metals in a solid state when speaking of their preparation. We there 
stated that metals prepared by different chemists conducted differently. Now 
although the same manipulator may obtain concordant results in purifying 
metals from different sources, yet that by no means proves that the results of 
different observers purifying the same metal would show the same concor- 
dance. Thus we find that the values obtained by one experimenter} for the 
resistance of mercury, determined in six different tubes, varied 1:6 per cent. 
This difference, he says, is not greater than was to be expected. The resist- 
ances found were as follows :— 
Tubes. ie II. ITI. IV. aVic VI. 
Experiment... 101652 427-28 555-38 217-73 194:70 11423 
Calculated .... 1025°54 427-28 555-87 216-01 193-56 1148-9 
Again, the values found for the conducting power of different preparations 
of pure hard-drawn gold, by the same observer ¢, were found equal to 
* Phil. Trans. 1862, part 1. 
+ Phil. Mag. Jan. 1861. The same experimenter (Dr. Siemens) states, however, in a 
later paper (Pogg. Ann. cxiii. p. 95), that he is able to reproduce standards of resistance by 
means of mercury with an accuracy equal to 0:05 per cent., but does not indicate what 
other precautions he takes (see remarks on the above, Phil. Mag. Sept. 1861). 
{ Phil. Trans. 1862, p. 12. 
1862. L 
