THE MECHANICAL EQUIVALENT OF HEAT. 
371 
lure-coefficient was practically zero. The reasons which led us to reject these alloys 
and adopt a platinum wire will, we think, be found sufficient (see section VII.). 
The value of R was first determined by a dial-box (legal ohms) constructed by 
Messrs. Elliott.* Mr. Glazebkook has been so kind as to perform a complete 
standardization of this box by means of the B.A. standards. The resulting correc¬ 
tions have been applied, and the values of R expressed in true ohms, as defined by 
the B.A. Report, 1892. 
The method by which we ascertained the true temperature, and therefore, the 
resistance of the wire when traversed by a current, is fully described in Section VII. 
We found that the difference in temperature between the wire and that indicated by 
the thermometer could be accurately determined, and, other conditions being equal, 
varied as C^. 
The arrangement of our connections enabled us to dispense with exterior resistance, 
as the ends of the coil itself were maintained at a constant potential-difference. 
(3.) Current (C).—The current was maintained by storage cells of the (L) type, 
supplied to us by Messrs. Bailey and Geundy. The E.M.F. of these cells was 
wonderfully constant. This constancy, although not a necessity, was a convenience. 
As previously pointed out, C was not directly measured. 
(4.) Time (T).—An electrical clock with a seconds pendulum was used as our 
standard for time. It was carefully compared at intervals with a chronometer by 
Dent. A chronograph was controlled by this clock. The rate of the clock was a 
losing one until August 21st, but after that date its error was less than 1/25,000, 
and no correction was necessary. 
(5.) ff/ass (M).—A balance sensitive to a change of 1/100,000 of the least mass 
measured by us, and a set of Oeetling’s weights,! were used in our determination of 
mass. 
(6.) Temperature {9 ).—At the time of writing (December, 1892) our thermoraeti’y is 
based on measurements made by platinum-thermometers. We propose to make, at an 
early date, a direct comparison of our standard thermometer with the air thermometer 
by means of the apparatus described by Mr. Callendae, j who has been so kind as to 
promise his assistance. 
In a previous paper§ the details of a careful comparison of the platinum and the air- 
thermometer have been given. It was then shown that the platinum-air difference 
curve 8[0/lOOj'^ — d/100] gave values of 9 at all temperatures from 0° to 100° C., accu¬ 
rate within ’01° of the real value of 9, and that discrepancies appeared to have an 
experimental origin. The experience of two years’ work with platinum-thermometers 
has hut increased our confidence in them.|| Should a direct comparison with the 
* Particulars of this box have been given in a previous paper. ‘ Phil. Trans.,’ A., 1891, p. 44. 
t These weights were re-standardized by Messrs. Oertling in August, 1892. 
+ * S'Oy- Soc. Proc.,’ Jan., 1891. 
§ See ‘Phil. Trans.,’ 1891, A, p. 155. 
II ‘ Philosophical Magazine,’ December, 1892. 
3 B 2 
