To measure the specific heat capacity of a liquid
A copper calorimeter together with a copper stirrer is weighed, first empty, and then when about three-quarters full of liquid. A heating coil is placed in the liquid and is supported by a cover which has holes to take the stirrer and a thermometer. The calorimeter is highly polished and is supported on a poor heat conductor inside a polished outer jacket to reduce heat losses from radiation, conduction, and convection. The coil is connected in series with a d.c. supply, an ammeter, a rheostat and a
plug key or switch. A voltmeter is connected directly across the coil terminals (Fig. 42.2).
The voltage of the d.c. supply used will depend on the resistance of the heating coil, but the current should be adjusted to give a temperature rise of about 10 K in not more than 5 minutes. It is first necessary to carry out a preliminary test for the purpose of adjusting the current to the chosen value. The current is then switched off and the liquid well stirred. Having taken the initial temperature of the liquid, the current is now switched on again, and simultaneously a stopciock is started. The voltmeter reading is also noted
and a steady current maintained for 5 minutes. Meanwhile, the liquid is stirred continuously. At the end of the 5 minutes the current is switched off and the final steady temperature of the liquid is noted. The results may be recorded, as shown, symbols being replaced by actual readings.
Note. Owing to the fairly long time of heating suggested for the above experiment, heat losses are inevitable. If, however, the liquid is first cooled to about 5 K below
room temperature and the heating continued until it is about 5 K above, we may assume that the heat gain from the atmosphere during the first half of the time will
be compensated by that lost to the atmosphere during the second half.
It is of interest to note that this simple cooling correction was first suggested by Count Rumford back in the eighteenth century. The last two experiments are based on a steady rate of supply of heat to the substances concerned. We shall now describe a different procedure known as the method of mixtures.