286 MR. £. H. GRIFFITHS ON THE LATENT 
as a shunt, and the agreement between the results was satisfactory. The whole wire 
was 80 centims. long and had a total resistance of about 0°4 ohm. For convenience, 
and to avoid thermal effects, a similar wire connected with the galvanometer was laid 
alongside it, and the sliding-piece was fitted with a screw so arranged that a small 
turn of the serew-head made contact with both wires. 
“The wire and contact maker were covered by a thick copper shield (the screw- 
head projecting through a narrow slot) passing from end to end of the bridge. Thus 
the temperature of the wire was kept uniform. By means of a vernier, the divisions 
on the scale could be read to 3/5 millim., which with this wire and thermometers AB 
and CD indicated at 50° C. a temperature difference of -600915° C. The temperature 
coefficient of this wire was found to be (00029. The resistances of the different parts 
of the wire were, after correction for the errors of individual coils, &c., merely expressed 
in terms of the mean box ohm, the absolute value being of no consequence so long as 
the fixed points were determined in terms of the same standard. The remaining two 
arms of the bridge were constructed of german-silver. They were wound together, 
boiled in paraflin, placed in a bottle, and I expended much care in finally adjusting 
them until equal. Their resistance was about 5 ohms, and that of the galvanometer 
about 8 ohms, which, assuming the resistance of the thermometers as about 20 ohms 
each, would give nearly the maximum of sensitiveness. A single storage-cell was 
always used, and a resistance of 40 ohms was placed in the battery circuit when the 
thermometers were in ice. A table was then calculated which gave the resistance 
necessary in the battery circuit when the thermometers were at any temperature in 
order that C?R (where R is the thermometer resistance) should be constant. Thus 
the rise in the temperature of the thermometer-coils due to current-heating was 
always the same, and consequent errors were eliminated, a point to which I attach 
considerable importance. 
“The value of R, — R, in thermometer AB was 6°88815; therefore a difference of 
1 ohm at 50° C. indicated a difference of 14°°5177 C., and as 
dpt 2t — 100 
ii = 1 — 8 i002 i, 

the degree value of any bridge-reading at other temperatures can be deduced. 
“There was no difficulty, in the arrangement above described, of reading with 
certainty a difference of yo'99° C., and, as an illustration, | may mention the fact that 
if the thermometers were placed in separate hypsometers side by side on the bench 
and one of the hypsometers was then removed to the ground (about 3 feet below), 
the difference in the bridge-wire reading thus caused slightly exceeded °4 millim.” 
In the paper above referred to, further particulars are given regarding the 
standardisation of these thermometers. During the experiments there described 
they were used to determine the rate of rise in the temperature of the calorimeter, 
whereas, in the work I am now describing, they were used chiefly as detectors of any 
