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Callendar’s Apparatus for Determining the Mechanical Equivalent of Heat. (Fig. 31.)

FIG. 31.

This apparatus comprises a cylindrical, calorimeter of thin brass, whose axis is horizontal, containing a known quantity of water and is rotated at a moderate speed, either by hand or by means of a water or electric motor. Unequal weights are suspended from the ends of a silk belt slung over the cylinder, and so arranged as to make one and a half complete turns round the cylinder. Stability of equilibrium is secured by the addition of a light spring balance which acts in direct opposition to the lighter weight. Since this spring balance contributes only a small (positive) term to the effective difference of load at the two ends of the belt, small errors in its readings are relatively unimportant. The weights are adjusted by trial to suit approximately the friction of the belt, which is found to be very nearly independent of the speed, and the final adjustment is effected automatically by the spring balance. A counter registers the number of turns which have been given to the calorimeter, while the rise of temperature is read by means of a bent mercury or platinum thermometer, inserted through a central opening in the front end of the cylinder.

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