24.2 REPORT—1899, 
symmetry about the axis is only approximately satisfied. The question whether 
both helices may be replaced by the corresponding current-sheets is to be 
answered in the negative, as may be seen from consideration of the case where 
there are two helices of the same pitch on cylinders of nearly equal diameters. 
In one relative position of the cylinders the paths are in close proximity through- 
out, and the value of M will be large, but this state of things may be greatly 
altered by a relative rotation through two right angles. 
But although in strictness the helices cannot be replaced by current-sheets, 
the complication thence arising can be eliminated in experimental applications by 
a relative rotation. For instance, if the helix to which the field is supposed to 
be due be rotated, the mean field is strictly symmetrical, and accordingly the mean 
M is the same as if the other helix were replaced by a current-sheet. A further 
application of Professor Jones’s theorem now proves that the first helix may also 
be so replaced. Under such conditions as would arise in practice, the mean of 
two positions distant 180°, or at any rate of four distant 90°, would suffice to 
eliminate any difference between the helices and the corresponding current-sheets, 
if indeed such difference were sensible at all. 
The same process of averaginz suffices to justify the neglect of spirality when 
the observation relates to the mutual attraction of two helices as employed in 
current determinations. 
APPENDIX II. 
Proposals for a Standard Scale of Temperature based on the Platinum 
Resistance Thermometer. To be submitted to the Electrical Standards 
Committee. Drawn up by Professor H. L. Catuenpar, JLA., FBS. 
The following proposals are submitted in consideration of the importance of 
adopting a practical thermometric standard for the accurate verification and 
comparison of scientific measurements of temperature. The gas thermometer, 
which has long been adopted as the theoretical standard, has given results so 
discordant in the hands of different observers at high temperatures, as greatly to 
retard the progress of research. 
The arguments in favour of the adoption of the platinum resistance thermo- 
meter as a practical standard were given by Professor H. L. Callendar, in a paper 
‘On the Practical Measurement of Temperature,’ communicated to the Royal 
Society in June 1886, and published in the ‘ Phil. Trans.’ in the following year. 
These arguments have since been confirmed and strengthened by the work of 
many independent observers, 
The Electrical Standards Committee of the British Association has done so 
much in the past with reference to the adoption of the present electrical standards, 
and more recently in connection with the adoption of the jowle as the absolute 
unit of heat, that it would appear to be the most appropriate authority for the 
discussion and approval in the first instance of proposals relating to an electrical 
standard of thermometry. 
The suggestions for the standard scale of temperature here proposed may be 
embodied in the following resolutions :— 
(1) That a particular sample of platinum wire be selected, and platinum 
resistance thermometers constructed to serve as standards of the platinum scale of 
temperature. 
(Note.—A degree centigrade of temperature on the scale of a platinum resist- 
ance thermometer corresponds to an increase of resistance equal to the hundredth 
part of the change of resistance between 0° and 100°C. In other words tempera- 
ture pt on the platinum scale is defined by the formula 
pt=100 (R—R°) | (R’--R°), 
in which the letters R, R°, and R’ stand for the resistances of the thermometer at 
