CHAP. VIL] ELECTRICITY AND MAGNETISM. 331 



to the heat it developes. He ascertained that the number of 

 heat-units evolved when 33 grammes (i equivalent) of zinc are 

 dissolved in dilute sulphuric acid (from which it causes hydrogen 

 to be given off) to be 18,682. This figure was arrived at by 

 conducting the operation in a vessel placed in a cavity of his 

 calorimeter, an instrument resembling a gigantic thermometer 

 filled with mercury, the expansion of which was proportional to 

 the heat imparted to it. When a Smee's cell was introduced 

 into the same instrument, the solution of the same amount of 

 zinc was observed to be accompanied by the evolution of 18,674 

 units of heat (i.e. an amount almost identical with that observed 

 before), and this amount was the same whether the evolution 

 took place in the battery-cell when the circuit was closed with a 

 short thick wire, or whether it took place in a long thin wire 

 placed in the external circuit. He then arranged 5 Smee's cells 

 in series, in cavities of the calorimeter, and sent their current 

 round a small electromagnetic engine. The amount of heat 

 evolved during the solution of 33 grammes of zinc was then 

 observed in three cases ; (i. ) when the engine was at rest ; (ii. ) 

 when the engine was running round and doing no work beyond 

 overcoming the friction of its pivots ; (iii. ) when the engine was 

 employed in doing 13,124,000 gramme-centimetres (= 12,874 

 x io 6 ergs) of work, by raising a weight by a cord running over 

 a pulley. The amounts of heat evolved in the circuit in the 

 three cases were respectively, 18,667, 18,657, and 18,374 units. 

 In the last case the work done accounts for the diminution in 

 the heat frittered down in the circuit. If we add the heat- 

 equivalent of the work done to the heat evolved in the latter 

 case, we ought to get the same value as before. Dividing the 

 12,874 x Io6 ergs of work by Joule's equivalent, expressed in 

 "absolute" measure (42 x io 6 ), we get as the heat- equivalent of 

 the work done 306 heat units. Now 18,374 + 306 = 18,680, 

 a quantity which is almost identical with that of the first 

 observation, and quite within the limits of unavoidable experi- 

 mental error. 



369. Rise of Temperature. The elevation of 

 temperature in a resisting wire depends on the nature of 

 the resistance. A very short length of a very thin wire may 

 resist just as much as a long length of stout wire. Each 

 will cause the same number of units of heat to be evolved, 

 but in the former case, as the heat is spent in warming a 



