Page:Elementary Text-book of Physics (Anthony, 1897).djvu/211

§ 170] '''169. Method of Comparison.'''—The method of comparison consists in conveying to the substance of which the specific heat is to be determined a known quantity of heat, and comparing the consequent rise of temperature with that produced by the same amount of heat in a substance of which the specific heat is known. In the early attempts to use this method, the heat produced by the same flame burning for a given time was applied successively to different liquids. A more exact method was the combustion, within the calorimeter, of a known weight of hydrogen. The best method of obtaining a known quantity of heat is by means of an electrical current of known strength flowing through a wire of known resistance wrapped upon the calorimeter.

170. Method of Cooling.—The method of cooling consists in noting the time required for the calorimeter, in a space kept constantly at zero, to cool from a temperature $$t'$$ to a temperature $$t,$$ when empty, when containing a given weight of water, and when containing a given weight of the substance of which the specific heat is sought. The thermo-calorimeter of Regnault, represented in Fig. 67, is an example. It consists of an alcohol thermometer, with its bulb $$A$$ enlarged and made in the form of a hollow cylinder, inside of which- the substance is placed. The thermometer is warmed, and then placed in a vessel surrounded by melting ice. It radiates heat to the sides of the vessel, and the column of alcohol in the tube falls. Let $$x$$ be the time occupied in falling from the division $$n$$ to the division $$n'$$ when the space $$B$$ is empty. Let the times occupied in falling between the same two divisions, when the space $$B$$ contains a mass $$P$$ of water, and when it contains a mass $$P'$$ of the substance of which the specific heat $$c'$$ is sought, be respectively $$x'$$ and $$x''.$$ Let $$M$$ be the water equivalent of the instrument. We then have $$\frac{M}{x} = \frac{M + P}{x'} = \frac{M + P'c'}{x''},$$ since, under the conditions of the experiment, the heat lost per second must be the same in each case.