sources of constant error which could be foreseen.
The results of the light trials with 400 ft.-pounds on the brake
differ slightly from those with 600 ft.-pounds. This might be merely
accidental, or it might indicate some constant difference in the
conditions requiring further investigation. It would have been
desirable, if possible, to have tried the effect of a larger range of
variation in the experimental conditions of load and speed, with a
view to detect the existence of constant errors; but owing to the
limitations imposed by the use of a steam-engine, and the difficulty
of securing steady conditions of running, this proved to be
impossible. There can be no doubt, however, that the final result is
the most accurate direct determination of the value of the mean
calorie between 0 deg. and 100 deg. C. in mechanical units. Expressed
in joules per calorie the result is 4.1832, which agrees very closely
with the value found by Rowland as the mean over the range 15 deg. to
20 deg. C. The value 4.183 is independently confirmed in a remarkable
manner by the results of the electrical method described below, which
give 4.185 joules for the mean calorie, if Rowland's value is assumed
as the starting-point, and taken to be 4.180 joules at 20 deg. C.

[Illustration: FIG. 7.]

S 11. _Electrical Methods._--The value of the international electrical
units has by this time been so accurately determined in absolute measure
that they afford a very good, though indirect, method of determining the
mechanical equivalent of heat. But, quite apart from this, electrical
methods possess the greatest value for calorimetry, on account of the
facility and accuracy of regulating and measuring the quantity of heat
supplied by an electric current. The frictional generation of heat in a
metallic wire conveying a current can be measured in various ways, which
correspond to slightly different methods. By Ohm's law, and by the
definition of difference of electric pressure or potential, we obtain
the following alternative expressions for the quantity of heat H in
joules generated in a time T seconds by a current of C amperes flowing
in a wire of resistance R ohms, the difference of potential between the
ends of the wire being E = CR volts:--

H = ECT = C^2RT = E^2T/R (1).

The method corresponding to the expression C^2RT was adopted by Joule
and by most of the early experimentalists. The