19G 
MR. H. L. CALLEXDAR OX THE PRACTICAL 
generation: meanwhile, the wire may be used as a standard, and preserve the 
record for comparison with future determinations. And, since the range of the 
platinum thermometer is very great, and different wires may be accurately compared 
throughout them range, we may expect that by adopting such a standard we shall at 
any rate secure consistency in place of the discrepancies at present prevailing. 
[An exponential formula of the type log R/R 0 — atjil + fit)* was found to represent 
the comparison of platinum and iron very approximately (see p. 202 ). If we assume 
from Series A.-m. the data *= 100 °, R/R 0 = 1*3460: t = 500°, R/R 0 = 2 6400, 
and calculate the values of the constants for platinum, we obtain a = ’0034259, 
/3 = ’0015290. By an odd coincidence it happens that this formula satisfies almost 
exactly the observations at varying temperature in Series A.-ii. Values of B calcu¬ 
lated by this formula (<?) are therefore given in Tables A.-n. and A.-m. for com¬ 
parison.] 
Comparison of Different Wires. 
Different specimens of wire have very different coefficients; when, however, the 
resistance-variation of one specimen of wire has been determined by direct com¬ 
parison with an air thermometer as above described the resistance-variations of 
other wires may be compared with the standard by a method as accurate as the 
comparison of resistances, and practically independent of all knowledge of tempera¬ 
ture. It is only necessary that the temperature should be approximately uniform and 
steady enough to allow observations to be taken. The method consists in winding 
the wires to be compared side by side on a cylinder of non-conducting material, such 
as clay, porcelain, or asbestos. By this arrangement we may be sure that their mean 
temperatures are the same, and we have only to measure their resistances several 
times in rapid succession when the temperature is steady, or oscillating slightly about 
a maximum, and take the mean of the observations. The resistance of the standard 
wire gives the' mean temperature, which is the same for both coils. If, however, the 
resistance-variation of the standard wire be not considered known with sufficient 
accuracy, this does not affect the accuracy of the comparison as such ; the resistance- 
variation of the others is simply referred to temperature on the scale of the standard 
wire. 
The simultaneous resistances are measured by precisely the same apparatus and the 
same method as before. They are therefore liable to the same errors and corrections ; 
that is to say, they are corrected for the resistance of the connecting wires and the 
temperature of the resistance-box : the latter is less material, since it affects both 
wires proportionately. The resistances are reduced to a unit at 0° C.; thermo-electric 
effects are especially to be avoided, and are, of course, eliminated as before. 
That changes of structure may not occur, producing permanent changes in the 
resistance, the wire must be first annealed. To prevent chemical changes such as 
* 1 Memoires de l’lnstitut,’ torn. 21, p. 619. 
