FORCE OF THE WESTON NORMAL CELL IN SEMI-ABSOLUTE VOLTS. 
193 
VIII. The Resistances. 
The two ohms used as the standard in this investigation were made of platinum- 
silver wire wound on mica frames, annealed after winding, and immersed in an oil bath. 
Prof. H. T. Barnes, who constructed them in 1898, gives a detailed account of their 
special design on p. 173 of his paper “ On the Capacity for Heat of Water between 
the Freezing and Boiling Points.At that time their resistance in series was 
found to be 2'00218 true ohms at 20° C. with a temperature coefficient of +0'000248. 
The present value of their resistance was obtained from a comparison with a 
manganin standard, No. 4086, which was sent to the Bureau of Standards for 
calibration. It was certified that its resistance could be expressed to within 0'005 per 
cent, between 15° C., and 35° C. by the formula 
R t = U00032 + 0-000018(/:-20)-0-0000005 (£-20) 2 . 
Each of the two platinum-silver ohms was compared with No. 4086 on a Carey- 
Foster bridge,!' and the comparisons gave the following mean values (reduced to 
20° C.) at different times during the investigation :— 
The resistance of 
(a) Platinum-silver Coil No. I. 
(^) 55 55 5 5 
( C ) 55 5 5 5 5 
(^) 5 5 5 5 5 5 
Mean 
= that of No. 4086 + 0'00138 ohms. 
= „ +0-00140 
= „ +0-00137 
= „ +0-00138 
+ 0-00138 
and the resistance of 
(a) Platinum-silver Coil No. II. 
(^) 5 5 5 5 55 
(^) 5 5 5 5 5 5 
(^) 5 5 5 5 5 5 
Mean 
that of No. 4086 + 0"00011 ohms. 
,, ,, +0 00007 ,, 
,, ,, +0"00014 ,, 
„ „ +0-00013 
+ 0-00011 
Hence 
Coil I. = 1 "00170 international ohms at 20° C. 
„ II. = 1-00043 
Coil I. + Coil II. = 2-00213 
* Barnes, ‘ Phil. Trans.,’ A, vol. 199, p. 173, 1902. 
t The same bridge is described and its accuracy indicated on p. 175 of the paper quoted above. 
VOL, CCXIV.-A. 2 C 
