MEASUREMENT OF TEMPERATURE. 
217 
legal ohm-box by Elltott, nominally correct at 18° C., to which temperature the 
resistances are reduced. The temperature coefficient of the box was determined to be 
'00023 per 1°C., the coils being of platinum-silver wire. The relative values of the 
resistances were correct to more than 1 in 10,000. 
The resistances observed are given in legal ohms, but have to be corrected for the 
temperature of the box It. The correction for the resistance of the connecting wires 
is \ + '008. 
By observing also the resistance R 34 , of the capillary tube electrode, the mean 
temperature of the corresponding 15 cm. of the capillary tube could be determined, 
and an allowance for its varying temperature approximately made by reckoning part 
with the bulb and part at air temperature. In most cases the temperature was 
varying; and an exact balance was observed with the rocking commutator as previously 
described, and the simultaneous reading x of the U-gauge was recorded. When the 
temperature was too steady to admit of this a fifth figure was obtained by observing 
galvanometer throws and interpolating. The column headed a?' gives the reading of 
the level of the mercury by the scale S. In taking observations at constant pressure, 
mercury was allowed to run out from the bulb A (see fig. 6, Plate 12) and weighed in 
a beaker. The observed weight in grammes W. of the mercury and beaker is given in 
the manometer column; the manometer was of course unnecessary in taking 
observations at the atmospheric pressure. The temperature indicated by a delicate 
thermometer immersed in the water surrounding the bulb A is then given in the x 
column. The readings given of the thermometers and barometer are mean or typical 
readings : all are not given, to save space. The observations are numbered to 
correspond with Plate 13. 
No permanent change of resistance of the platinum wire was with certainty 
detected in this series of experiments, in spite of the prolonged heating to which the 
spiral was subjected, and although it was in contact with the glass of the bulb for a 
considerable portion of its length. The apparent change of temperature coefficient 
fromPj/Ro = P3391, Series IV., to 1'3395, Series V., was probably due to the fact that 
the position of the loose comparison-electrode was shifted so as accurately to correspond 
with the single electrode inside the capillary tube ; it had been accidentally misplaced 
through an error in measurement in Series IV. The apparent change in the value of 
P 0 in Series A.-v. from 19'674 to 19'668 (the latter value exactly was observed again 
one month latter, on April 18) was probably partly due to a change in the comparison- 
electrode, the copper portion of which was accidentally fused in re-coating with hard 
glass on March 4, so that it had to be re-made, and its resistance was slightly increased ; 
and partly, perhaps, to a change in the plug contacts in the 10-ohm arm of the bridge, 
which were thoughtlessly altered on that day to measure another resistance. The 
value R, c = 19'668 should, of course, be used in reducing the observations from 
March 4 onwards. 
On March 4, while the bulb of the air thermometer was immersed in steam, it was 
MDCCCLXXXVII.—A. 2 F 
