MEASUREMENT OF TEMPERATURE. 
223 
value of t may be in error to the extent of about 1°, as it depends on the air thermo¬ 
meter observations. These, however, may be rendered still more accurate, and the 
uncertainty further reduced ; they do not affect the claim of the platinum thermometer 
to rank as a practical thermometric standard. 
The platinum air thermometer gives us the means of comparing theory with experi¬ 
ment, and of testing the accuracy of the formula 
pv = mk 6 
at high temperatures by taking observations at various pressures in rapid succession, 
while the temperature is maintained nearly constant. The platinum spiral may at 
least be relied on to give any small changes of temperature which may occur. Never¬ 
theless, for experiments of such delicacy it was felt to be necessary to maintain the 
conditions as uniform as possible, because any slight variation might introduce errors 
comparable with the differences that might be expected to exist between the indications 
of the air thermometer at constant pressure and at constant density. 
Consequently, when on my return from India in February, 1887, I set to work 
again with the same glass air thermometer, a small gasometer was ordered in order 
to keep the gas pressure perfectly constant during any observation. This want had 
not been so much felt in August, when there were few students at work and little ga.s 
was being used ; but the unsteadiness of the gas pressure was a great source of 
annoyance during the series of observations given in Table A.-v., which were taken 
during February and March in full term time, pending the arrival of the gasometer. 
On one occasion the temperature rose 100 ° in five or ten minutes, owing to change of 
gas pressure. Such changes of temperature would have been less material, had the 
platinum spiral been uniform ; so that its mean temperature would have been more 
nearly that of the air enclosed. The gasometer arrived about the end of April; it 
leaked badly, and had to be mended. After that it performed satisfactorily, and a 
few observations were taken at steady temperatures one morning with its aid. 
Unfortunately, the observations at constant density were pushed too far this time, 
and the capillary tube gave way at about 550° C. under two atmospheres’ pressure. 
Porcelain Air Thermometer. 
For higher temperatures it is necessary to use an air thermometer of porcelain. 
The difficulties to be encountered are that the wire must not touch the glaze and 
cannot safely be fused through the porcelain air-tight at any point exposed to the 
high temperature ; and that the expansion of the material must be determined with 
some accuracy. To meet these difficulties, a somewhat peculiar type of instrument 
has been made. 
A cylindrical bulb of porcelain, AB, Plate 12 , fig. 8 , 3 cm. diam., 200 c.c. capacity, 
opens at either end into thick side-tubes AC, BD, of small rectangular bore, 3 mm. 
