RECENT PROGRESS IN PHYSICS. 359 



If we take for the unit of heat, as is usually done, that quantity 

 which raises the temperature of one kilogramme of water 1°, then it 

 follows from the above investigations that the unit of the force of cur- 

 irent_, in passing through the unit of resistance produces in it 0.001057 

 units of heat in one hour and 0.0000176 in one minute. 



i' § 55, Observations on the results obtained by Lenz. — After Lenz had 

 "determined the relation between the force of current and the production 

 of heat in a metallic circuit, the idea naturally occurs that we might 

 'compare the heat produced in the circuit wire with the quantity of 

 ^detonating gas produced by decomposition of water in the exciting 

 cells, but a more intimate study of the subject soon proves that such a 

 comparison cannot lead to a constant result. 



If there be a fixed relation between the quantity of detonating gas 

 and the production of heat for any given arrangement of the Voltaic 

 battery and a given closing wire, it will be changed as soon as — ceteris 

 paribus — either the specific resistance to conduction of the battery or 

 its electromotive force is changed ; for by either of these changes the 

 strength of current is altered, and the evolution of gas changes pro- 

 portionally with the force of current, while the production of heat 

 increases as the squares of this force; and the relation between the 

 quantity of gas and the heat developed must necessarily become dif- 

 ferent from what it was before. The heat produced in the closing 

 circuit, therefore, can by no means be considered as a thermal equiva- 

 lent of the detonating gas evolved in the exciting cells, and con- 

 sequently there can be no definite relation between the heat produced 

 by exploding a certain quantity of detonating gas, and that set free in 

 the closing circuit during the evolution of an equal quantity of gas 

 in the exciting cells of a galvanic battery. 



The attempt to compare, even with only approximate correctness, 

 the quantity of electricity of the electrical machine with that of Volta's 

 apparatus, has always, hitherto, been unsuccessful. After Lenz's 

 accurate researches on the production of heat by the galvanic current, 

 and Riess' reliable quantitative determinations of the heat set free in 

 a wire by the passage of the discharge of a Leyden jar, at first sight 

 it would seem that a basis was found for this comparison. But here 

 too the result on examination is a negative one. 



From the rather large quantity of heat produced in metallic wires 

 by the discharge of the jar, we should be disposed to infer that a rather 

 large quantity of electricity was brought into action thereby ; according 

 to the experiments before mentioned* the increase of heat in the pla- 



tinum wire of the air thermometer for -^ = 1 is equal to 0.3787, or 



in round numbers equal to 0.4. That wire had a diameter of 0.072'" 

 and a length of 59.7'" and therefore it weighed about 60 milligrammes ; 

 to raise the temperature of this wire 0.4° requires, as can easily be 

 computed, 0.000000768 units of heat. 



Let us now consider what quantity of heat would have been set free 

 in the same platinum wire by the unit of galvanic current. In the 



* See Report of 1856, p. 437. 



