182 
LORD RAYLEIGH AND MRS. SIDGWICK 
down to 0°. This portion of the tube was about 2 centims. at each end, or about 
5 per cent, of the length in the case of tube I., and about 2 per cent, in the case of 
tube IY. What the exact temperature of this part of the tube was it is impossible 
to say, but it was ascertained that the temperature of the mercury in the terminals 
with the copper connecting rods in situ was not higher than 5° or 6°, depending in 
some degree on the extent to which the ice was piled up round the cup. The mean 
temperature of the parts of the tubes not directly exposed to ice can hardly have been 
so high as 2°. Supposing it to have been 2°, and taking the case of tube I., where 
the largest proportion of the whole length was within the terminals, the effect would 
be an overestimate of r by about '00008. In the case of tube IY. the error in r would 
be less than the half of this. 
The tubes were connected with the resistance balance by copper rods, well amalga¬ 
mated, of which one end stood on the bottom of the vulcanite terminals, so that a 
considerable portion of the amalgamated copper surface was in contact with the 
mercury. The rods were kept at a little distance from the ends of the tubes. 
Dr. Matthiessen brought flattened copper rods up against the ends of his tubes, but 
this plan appeared open to objection, since it would be very difficult to secure complete 
contact between the copper and glass all round the edge of the orifice, especially under 
an opaque fluid like mercury; and any defect in such contact would render necessary 
an unknown correction. We preferred, therefore, to let the ends of the tube open 
without obstruction into the mercury cup, which may be regarded as of infinite extent 
by comparison. The correction necessary to take account of the resistance of the 
mercury beyond the ends of the tube has already been considered. 
The resistance of the rods used to connect I., II., and III. with the bridge was 
about ’00215 B.A. unit. With tube IY. an additional rod had to be introduced to 
get the necessary length. This brought the resistance of the rods up to '00291. 
The other end of the rods fitted into mercury cups on the resistance balance. 
The balance used was one designed by Professor Fleming (Phil. Mag., ix., p. 109, 
1880), in which Professor Carey Foster’s method is employed of interchanging the 
resistances in the two arms of the balance containing the graduated wire, so that the 
difference between them is expressed in terms of the wire. One thousand divisions 
of the graduated wire are stated by Professor Fleming to equal '0498 B.A. unit, • 
and experiments of our own also showed it to be about '05. The wire is of platinum- 
iridium, and as it has a high temperature coefficient compared with the platinum-silver 
of the standard coils, we thought it undesirable to use much over 100 divisions of it. 
In order to avoid this in the case of tubes I. and IY. it was necessary to introduce coils 
from a resistance box in multiple arc. The resistance box employed was one by Messrs. 
Elliott Brothers. With tube I., 20 ohms from the box were used in multiple arc 
with the standards against which the tube was balanced, and in the case of tube IY. 
24 ohms were used in multiple arc with the tube itself. Tubes II. and III. were 
balanced against the standard coil belonging to the British Association and deposited 
