244 REPORT—1899. 
Tn order to reduce to the standard scale of temperature the indications of any 
platinum thermometer, it is necessary to know the law connecting pt and T. 
These are identical at 0° and 100°, but the determination of the relationship 
between them at other temperatures is a matter for experiment. 
The work of Callendar established for a particular sample of platinum the 
relation 
ne 
a= 1 - pt = {199 - 00 | 
| 
over the range 6° to 600°, T being measured on the constant pressure air-scale, 
and 6 being a constant. 
Later experiments by Cailendar and Griffiths showed that this relation holds 
for platinum wires generally, provided that they are not very impure. They 
propose that the value of 6, the constant employed in the formula, should be 
determined by taking the resistance of the thermometer in the vapour of sulphur, 
and a new determination by them of the boiling-point of this substance, under 
normal pressure, gave 444°°53 on the air scale. 
The present communication gives a short account of some experiments which 
are the outcome of the collaboration of the Kew Observatory Committee and the 
authorities of the Bureau International des Poids et Mesures at Sevres, for the 
purpose of carrying out a comparison of some platinum thermometers with the 
recognised International Thermometric Standards. A. full account of the 
work will shortly appear in the ‘Philosophical Transactions of the Royal 
Society’ and in the ‘ Travaux et Mémoires du Bureau International des Poids et 
Mesures.’ 
A new specially designed resistance-box, together with several platinum 
thermometers, and the other accessories needed, were constructed for the Kew 
Committee, and after their working had been tested at the Kew Observatory, 
they were set up at the Sévres Laboratory in August 1897. The resistance-box 
in its general design was very similar to the one previously described before this 
Section by Mr. Griffiths, but the plugs were replaced by a special form of contact 
maker, and the coils were of manganine instead of platinum-silver. The methods 
adopted for the standardisation of the apparatus only differed in a few details 
from those of Callendar and Griffiths. 
The comparisons made between the platinum thermometers and the standards 
of the Bureau may be divided into several groups. The first group of experi- 
ments covers the range (— 23° to 80°), and consists of a large number of com- 
parisons between each platinum thermometer and the primary mercury standards 
of the Bureau, whose relation to the normal hydrogen scale had previously been 
studied by one of us. 
Above 80° the mercury thermometers were replaced by a gas thermometer, 
constructed for measurements up to high temperatures. 
We at first attempted to use hydrogen as the gas for these measurements, but, 
owing probably to a slow chemical action taking place between the gas and the 
glass reservoir in which it was enclosed, we were afterwards compelled to sub- 
stitute nitrogen, which we have not observed to exert any action on the material 
of the envelope up to a full red heat. 
The comparisons between 80° and 200° were made in a vertical bath of stirred 
oil, heated by different liquids boiling under varying pressures. For work above 
200° a bath of mixed nitrates of potash and soda was substituted for the oil tank. 
In this bath comparisons of the two principal platinum thermometers with the 
gas thermometer were made up to 460°, and with a third thermometer, which was 
provided with a porcelain tube, we were able to go up to 590°, the glass reservoir 
of the gas thermometer being replaced by one of porcelain, whose dilatation had 
previously been measured by the Fizeau method. Comparisons of the platinum 
and gas scales were carried out at over 150 different points, each comparison 
consisting of either ten or twenty readings of the different instruments. 
By the intermediary of the platinum thermometers a determination of the 
