MEASUREMENT OE TEMPERATURE. 
229 
R/R 0 = l 4- 0-0045346* + 0-000007034* 3 ,.II. 
R/R 0 = 1 + 0-0045658* + 0'000007082* 3 ,.I. 
both of which correspond exactly to the formula— 
fe — t — 13-43 {(*/100) 3 - (*/100)}, 
represent the resistance-variations of the wires I. and II. with an accuracy of about 
1 per cent. 
Benoit gives for Iron— 
R/R 0 = 1 + 0-004516* + 0-000005828* 3 ; R^Rq = P5099 ; 
for Steel— 
R/R 0 = 1 + 0-004978* + 0-000007351* 3 ; RJR 0 = P5713. 
These correspond to 
fe — t— 1U43 {(*/100) 3 — (*/100)} . (Iron) 
and 
fe — * = 12-87 {(*/100) 3 — (*/l00)} . (Steel) 
respectively. 
Matthiessen, by observations between 0° and 100° with a mercury thermometer, 
found for very pure iron the formula 
R 0 /R = 1 — -0051182* + -000012915* 3 , 
whence R^Rq = P6197. But this kind of formula is quite inadmissible for extra¬ 
polation, as it makes R a maximum when * = 198°. Between the limits 0° and 
100° C. the formula of the parabolic type which agrees most nearly with it is 
fe—t — 12-74 {(*/100) 3 — (*/100)}. 
He similarly found for two specimens of commercial platinum, between 0° and 100°, 
R 0 /R = 1 — -0027225* + ‘000005776* 3 . . (1); whence RJR 0 = U2730. 
R 0 /R = 1 - -0025777* + '000005054* 3 . . (2); whence R/Rq = R2613. 
Wire (1) at 50° gives pt = 50'70. No. (2) gives p* = 50"20. These differ by half 
a degree, whereas the pure wires of Tables P give pt — 50*40 approximately, and differ 
by only one or two hundredths of a degree. Matthiessen’s formula (l) makes R a 
maximum when * — 236°, and is of course inadmissible for extrapolation. 
Comparison of Platinum and Platinum-Silver Alloy . 
This is of little interest, except that the platinum-silver wire is used for standard 
resistance-coils. It is quite unsuited for thermometry, because of its small coefficient 
and variable composition. 
