MEASUREMENT OF TEMPERATURE. 
209 
this arrangement of the junctions thermo-electric effects were rendered almost inappre¬ 
ciable in spite of the great sensitiveness of the apparatus and the low resistance of 
the galvanometer. The correction for resistance of connections, owing to the use of so 
much fine wire, amounted to nearly 3 per cent., but it was measured on each occasion, 
and could be very accurately applied. 
With the instrument above described upwards of 130 independent observations in 
several sets, at various steady temperatures, were taken during the days August 7 
to 12, 1886. They were worked out on much the same plan as that explained below 
on page 218. 
To save space, only the observations at steady temperatures are given in Table A.-iv. 
The experiments were interrupted on August 12 by the cracking of one of the glass 
joints of the gauge ; an attempt to mend it only made matters worse. It was sent to 
the maker (Muller, of 89, Whitfield-street) for repairs; and meanwhile other 
experiments were undertaken. The breaking of the gauge was particularly unfor¬ 
tunate, as I had so far taken no observations at constant pressure, for which the 
gauge had been specially designed. A great many observations were taken with the 
temperature varying to test the working of the apparatus ; these on reduction were 
never found to differ by more than a degree from the curve, and often happen to fall 
exactly on it. To insert them would only confuse the diagram. The earlier observa¬ 
tions of Series IV., which were taken just after filling, before the apparatus had reached 
a steady state, give results which are too high by two or three degrees. This is an 
error of the air thermometer, and not due to change of the resistance of the platinum ; 
for the observations in ice and steam show that the platinum spiral suffered no change 
during this series of observations ; but the value of the constant mk fell slightly, 
probably owing to surface absorption. The air thermometer was filled with dry air on 
the 9th August, at a temperature of 350° C., after repeated exhaustion ; and it 
seemed probable that there would be a tendency for the air pressure to fall rather fast 
at first, owing to surface condensation; between observations (3) and (4) the gas was 
left unaltered for 3k hours. The change in the difference column from 7‘9 to 7T is in 
the direction of absorption, but condensation probably took place most rapidly as the 
instrument cooled at lower temperatures. The historical element seems to enter here : 
the dates and times of the observations are therefore given ; heating gradually 
dissipates the air-film, which may take some time to condense again, so that an 
observation (e.g., No. 12) taken at a lower after a higher temperature may give 
results which are too high ; but this requires further investigation. 
The later experiments seem to throw doubt on the suggestion offered a year ago 
(p. 181) as to the value of oq — cr 0 explaining discrepancies in the values of the 
coefficient of dilatation according to different observers : the apparatus is not 
particularly suited for determining the absolute value of the constant 0 0 , which 
depends on the difference of pressures in ice and steam, on which the probable error 
with my apparatus may amount to a quantity of the order of one in a thousand. 
MDCCCLXXXVJI. — A. 2 E 
