76 HEAT. 



extension with increase of internal pressure. These could be determined 

 and allowed for, and the heat needed to raise a known weight of air at 

 constant volume from about 15 to the temperature of steam was deter- 

 mined. We shall give some of Joly's results later. 



The Method of Electrical Heating. This method was first 



suggested and used by Joule * after he had discovered the law of heat 

 development in a wire carrying an electric current, viz., that the heat is 

 proportional to C 2 R, where C is the current, R the resistance, and t the 

 time for which the current runs. 



Joule passed the same current through two similar platinum spirals, 

 one immersed in a standard calorimeter containing water, and the other 

 in the calorimeter containing the substance to be experimented on. 

 The heats developed were proportional to the resistances of the spirals. 

 These were measured beforehand. The heats could also be expressed in 

 terms of the heat capacities of the calorimeters and their contents and 

 the observed changes of temperature, and the equation of the two sets 

 of measurement gave the specific heat. 



We may take as an example an experiment described by Joule to 

 show the possibility of the method. The standard calorimeter, with 

 stirrer and thermometer, had water equivalent 280 grains and it con- 

 tained 35,000 grains of water. The immersed spiral had resistance 

 taken as 100. The other calorimeter had water equivalent 260 grains ; 

 it contained 28,000 grains of water and 80,500 grains of lead. The 

 immersed spiral in it had resistance 106. A current was passed through 

 the two in series for 20 minutes, when the increase of temperature 

 was in the first 3'575 and in the second 4'35. Expressing the fact 

 that the heat developed is proportional to the resistance, and denoting 

 the specific heat of lead by s, we have 



(8050Qg + 28260)4-35 106 

 35280 x 3-575 ~ 100 

 whence s = 0'03073 



This method has since been used by many experimenters, and has 

 been brought to great perfection by Griffiths in his research on the 

 mechanical equivalent of heat, to be described hereafter. It is not 

 necessary to employ two calorimeters. For, under proper conditions, 

 both the current and the resistance can be measured with very great 

 accuracy, and the value of C 2 IW is thus known in different experiments, 

 which may be carried out successively with the same calorimeter. 

 Equating the ratio of the heats developed in two experiments as ex- 

 pressed in terms of heat capacity and rise of temperature to the ratio of 

 C 2 IU in the two experiments, we have the data for the determination 

 of specific heat. 



The Specific Heat Of Water. Nearly all determinations of specific 

 heat have been made in terms of a water unit, such as the heat req'vired 

 to raise 1 gramme of water through 1 0. at some named part ot the 

 scale. It is, therefore, of the utmost importance to determine whether 

 the choice of the particular degree affects the value of the unit, that is, 

 to determine whether the specific heat of water itself varies as the 

 temperature changes. 



* Scientific Papers, vol. i. p. li)2. 



