THERMAL EMISSIYITY OF THfN WIRES IN' AIR. 
3 77 
was subsequently compared with a Kew standard thermometer. Considerable diffi¬ 
culty was introduced into the carrying out of these comparisons from the fact that it 
is, or, at any rate, was not three years ago, possible to obtain a standard thermometer 
from the Kew Observatory to read, say, from 200° to 300° C., with a short wide 
chamber at the base in which the mercury expanded below 200° C. All that could 
be obtained was a long thermometer which had been carefully tested between 0° C. 
and 100° C., and the remainder of whose tube had been simply calibrated for 
uniformity of bore. The consequence was that when we desired to compare one of 
our thermometers reading say, from 200° C. to 300° C., with the Kew standard 
thermometer, their bulbs were very far apart when they were both immersed in oil so 
that the top of the mercury column of each was just at the top of the oil; secondly, 
whereas we had kept each of our thermometers with its bulb close to the wire whose 
resistance was being tested, and, therefore, at a fixed distance below the surface of 
the oil while it was being used, the Kew standard thermometer had to be continually 
lowered further and further into the oil as the temperature rose in order that correct 
reading’s could be obtained. 
By adopting, however, the following device, a satisfactory, although very laborious, 
comparison between the four thermometers and the Kew standard thermometer was 
finally carried out in the autumn of 1889, by three of the students of the Central 
Institution, Messrs. Muller, Stephens, and Wightman. The thermometer to be 
tested was placed in a deep oil bath in exactly the same position, relatively to the 
surface of the oil, as that in which it had previously been used. To obtain uniformity 
of temperature throughout the oil bath, the oil was kept constantly agitated by means 
of a stirrer driven by an electromotor, and further, the heat was applied not merely 
at the bottom of the bath but along the whole of its long vertical sides, which were 
covered with several layers of asbestos cloth to prevent the flames warming one part 
of the surface more than another. The bulb of the thermometer was surrounded by 
a bobbin of wire, the resistance of which was very carefully measured for many 
readings of the thermometer. Then the Kew standard thermometer was inserted in 
place of the thermometer to be tested, but now, as the temperature rose the coil and 
the standard thermometer were depressed together so as to keep the coil always 
surrounding the bulb of the thermometer, and so that the level of the mercury in the 
thermometer tube was always only just above the level of the oil. The temperatures 
were then read off on the standard thermometer which caused the coil of wire to have 
exactly the same resistances as before, and which were, therefore, the true temperatures 
corresponding with the readings of the thermometer to be tested. In consequence of 
the unwieldy length of a Kew standard thermometer reading to 300° C., it was 
necessary to use an oil bath 80 centims. deep for this experiment. 
Much thought and labour was given to the piece of apparatus, and many devices 
were introduced into it which it is not necessary to describe here, first because after 
we had completed this part of the investigation we learnt that much more suitable 
standard thermometers might have been obtained from abroad than could be purchased 
mdcccxcii.—A. 3 c 
