PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 243 
the temperatures pt, 0°, and 100°C., respectively. The melting-pomt of ice is taken 
as the zero of this scale in accordance with common usage.) 
(2) That the scale of temperature ¢ deduced from the standard platinum scale 
by means of the parabolic difference formula, 
t—pt=d(t | 100—1) t] 100, 
which has been proved to give a very close approximation to the true or thermo- 
dynamic scale, be recommended for adoption as a practical standard of reference, 
and be called the British Association Scale of Temperature. 
(Note.—The gas thermometer would still remain the ultimate or theoretical 
_ standard, and the exact relation of the British Association scale to the absolute 
scale would be the subject of future investigation. In the present state ot 
experimental science, the difference between the two scales over the greater part 
of the range is less than the probable errors of measurement with the gas 
thermometer, and the possible accuracy of measurement with a platinum thermo- 
meter, especially at high temperatures, is of a much higher order than with the 
gas thermometer. Measurements directly referred to the British Association scale 
would therefore be of greater permanent value, because they could be subsequently 
corrected when the relation between the scales had been more accurately 
determined.) 
(3) That the value of the difference-coefficient din the parabolic difference- 
formula be determined for the British Association standard thermometers by 
reference to the boiling-point of sulphur as a secondary fixed point in the manner 
described by Callendar and Griffiths, ‘ Phil. Trans. A, 1891.’ 
(Note.—It is probable that this method gives the best results over the whole 
range at temperatures above —100°C. At very low temperatures there appear to 
be singularities in the resistance variation of metals which require further investiga- 
tion. The boiling-point of liquid oxygen would be a more convenient secondary 
fixed point to choose for low temperature research, especially for testing thermo- 
meters the construction of which did not permit their exposure to a temperature 
as high as that of boiling sulphur.) 
(4) That the temperature of the normal boiling-point of sulphur under a 
pressure of 760 mm. of mercury reduced to 0° C., and latitude 45°, be taken for the 
purposes of the British Association scale as 444:53° C., as determined by Callendar 
and Griffiths (doc. cit.), with a constant pressure air-thermometer, 
(Note.—Until the relation between the various gas-thermometer scales, and 
the expansion of glass and porcelain, have been more accurately determined, it 
does not appear that anything would be gained by changing this value to which 
so much accurate work has already been referred.) 
APPENDIX III. 
A Comparison of Platinum and Gas Thermometers made at the Interna- 
tional Bureau of Weights and Measures at Scvres. By Dr. P. 
Cuappuis and Dr. J. A. Harker. 
Professor Callendar in 1886 investigated the method of measuring temperature 
based on the determination of the electrical resistance of a platinum wire. 
He pointed out that if R, denote the resistance of the spiral of a particular 
platinum thermometer at 0°, and R, its resistance at 100°, we may establish for 
the particular wire a scale, which we may call the scale of platinum temperatures, 
such that if R be the resistance at any temperature T°, this temperature on the 
R-R, 
R,- RB 
platinum scale will be x 100 degrees. For this quantity Callendar em- 
ploys the symbol pt. : 
R2 
