i 
296 LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE ABSOLUTE 
1 mercury unit='9337 X 10 9 C.G.S. 
differing nearly 1 per cent, from the value (’941) obtained by ourselves. 
§ 2. Under the conviction that this method offers in some respects important 
advantages, and influenced also by the fact that the arrangements for producing and 
measuring the uniform rotation necessary were ready to our hands, we determined to 
give it a trial, in the hope of obtaining confirmation of the results already arrived at 
by ourselves and by Glazebbook with other methods. At first the intention was to 
follow Lobentz in using for the resistance a glass tube full of mercury, with two 
points of which contact would be made by platinum wires passing through the glass. 
It appeared, however, that there would be difficulty in making the measurements 
with the degree of accuracy aimed at. If the wires were sealed into the glass, the 
section would probably be rendered irregular. An attempt was made to avoid this 
difficulty by using a tube from which the ends had been cut with the aid of heat. 
After small nicks had been filed sufficiently deep to receive the platinum wires, the 
ends were replaced in their original positions and secured with shellac. In this way 
a satisfactory uniformity of section near the points of derivation could be attained, but 
the measurement of the distance between these points, which is required to be known 
with full accuracy, was rendered difficult by the presence of the cement. It is possible 
that these difficulties might have been overcome, but at this point a method of 
shunting occurred to us, allowing the use of mercury to be dispensed with. Merely 
for the purpose of connecting the mercury unit with the B. A. unit or other standard 
of resistance, it would not be desirable to use tubes of such large bore.* This problem 
may more conveniently be taken by itself, and has already been treated by us in a 
former communication to the Society.' 1 ' 
| 3. In the shunt method the greater part of the main current y passes on one side 
through a relatively small resistance a (see fig. 1), and the difference of potentials 
at the points of derivation B, C, is due to the passage of a small fraction only of the 
total current, the resistance (h-f-c) being great compared with a. If at the same 
time b be small relatively to c, the difference of potentials is doubly attenuated. Its 
value for a given main current y is found at once from the consideration that the 
current divides itself between the two branches in the inverse ratio of the resistances. 
The current through b is thus + c y, and the difference of potentials at the points 
* If the distance between the points of derivation were 1 metre, R—‘002 mercury unit would require 
a section equal to 500 square millims. 
f Phil. Trans., 1883, p. 173. 
