288 MR. E. H. GRIFFITHS ON THE LATENT 
change of 3°05 millims.; thus, entirely independent methods agree in giving the 
following formula as a sufficiently close approximation for the bridge-wire null-point 
at any temperature @, viz., 
N.P. = 598°35 =F ‘030. 
True, the coefficient of @ should be slightly larger, but I did not profess to read the 
vernier to nearer than ‘05 millim. during the calibration of the wire, and therefore 
any closer approximation would be useless. 
As even a small difference between the average temperature of the calorimeter 
and the surrounding walls would have some effect in experiments lasting more than 
an hour, the agreement between the results obtained by the different methods above 
described was extremely satisfactory. In addition [ carried out a series of observa- 
tions with both thermometers strapped together and immersed in the rapidly- 
stirred tank water, the temperature of which was gradually raised from 15° to 54°, 
and the position of the bridge-wire null-point was found throughout that range to 
be accurately given by the above formula. 
The galvanometer was observed by means of a microscope fitted with a micrometer 
eye-piece by Zeiss. The swing was very ‘dead beat” and very uniform for a 
given D.P. The ;'oth of a division could be estimated without difficulty by anyone 
accustomed to the instrument, and it was found more convenient to read and enter 
swings with 75th of a division as unity rather than to express the fractions as decimals. 
Thus a swing of 2°5 divisions was entered as 25. I mention this as otherwise 
the swings given in the tables might appear curiously large. Occasionally the 
swing due to a change of 1 millim. in the bridge-wire reading was observed, but as 
the E.M.F. used was constant, and the galvanometer control-magnet was not 
re-adjusted during these experiments, the swing for a given change was found 
to remain practically constant when the temperature of the thermometers was 
unchanged. As the resistance of two arms of the bridge altered when @ altered, the 
swing varied slightly when @ changed, but the arrangement, above described, for 
keeping C’R constant, diminished the extent of this change. A swing of 90 corre- 
sponded to a change of 2 millims. in the bridge-wire reading when 6 = 40°, and the 
swing at 30° was about 94. 
The swing was of course obtained by reversing the battery connections and was 
thus independent of thermal effects, changes in the position of the galvanometer 
zero-point, &¢, 
Now, as above stated, a change of 1 millim. of bridge-wire about the reading 
60 centims. indicated a difference of ‘0006300 box ohm. If therefore the bridge- 
wire reading was at the null-point a swing of 10 indicated a difference of § of 
‘0006300 = -000140 ohm in the resistance of the two thermometers. 
The following Table (calculated from the constants of the thermometers by means 
