MEASUREMENT OF TEMPERATURE. 
180 
Observations.—Series III. 
Since lead-glass begins to soften at about 450° C., it was considered advisable to 
make an instrument of harder glass. Air thermometer No. 3 was therefore con¬ 
structed of combustion tubing. Since hard glass cannot well be joined on to other 
tubing, the capillary tube was formed by drawing out a piece of combustion tubing, 
which formed the bu 1 b ; the platinum spiral was then inserted and the other end drawn 
out, and the whole cleaned and calibrated. 
To avoid leakage, one of the electrodes passed out through the whole length of the 
capillary tube, and the fine wire itself was sealed through the glass at the other end, 
a length of 1 centim. being perfectly coated with glass. A capillary tube formed in 
this way is of course exceedingly fragile, and some delicacy of manipulation is required, 
but it appears that the instrument cannot be otherwise made of hard glass without a 
glass furnace in which a bulb can be blown. 
This thermometer was suitably mounted and filled with dry air at a temperature of 
470° C., and the series of observations recorded in Table A-ill. were taken with it. 
It was heated in a thick iron tube constructed to fit it, and the temperature maintained 
steady by regulating the gas supply. 
It showed no tendency to leak, till at the conclusion of observation (10) it was un¬ 
fortunately cracked by too sudden exposure to the air after heating in the steam 
bath. 
The chief objection to this thermometer was the small volume of the bulb, and the 
large volume correction, which necessitated a third approximation ; the value of j (see 
equation 5, p. 181) being large, all the uncertain effects of surface condensation are 
unduly magnified. For instance, for observation (7) we obtain for the value of the 
Q 
temperature 397'6 or 398’3, according as we use the first or second value of °. The 
Po 
mean deviation of observations (l), (6), (7), from the curve (e) is 2 0, 5. 
For this reason, I made a larger and better instrument on the same plan, but it was 
unfortunately broken just as it was completed. In spite of this accident, I hoped to 
make another series of experiments with a better thermometer, but a most unfortunate 
accident has completely disabled my right hand for the last month and put a stop to 
my experiments. 
Details of Air Thermometer No. 3. 
Volume of bulb. V o =15'02 cub. centims. 
Capillary and gauge tube.—u = '0442 cub. centim. per centim. 
X— 1 centim., v=‘270 cub. centim., 
£C— 10 centims., r=-6fi7 cub. centim., 
/3= -0178 
/3~ '0442 
6 '=e"= 288°. 
