Prof. Thomson on the Dynamical Theory of Heat. 287 



equivalent to that of a copper wire of about 100 feet long and 

 about one-eighth of an inch in diameter, and if the engine be 

 allowed to move at such a rate as by inductive reaction to dimi- 

 nish the strength of the current to the half of what it is when 

 the engine is at rest, would produce mechanical effect at the rate 

 of about one-fifth of a horse-power. The electromotive force of a 

 copper and bismuth element, with its two junctions atO° and 1°, 

 being found by Pouillet to be about -j-— of the electromotive 

 force when the junctions are at 0° and 100, must be about 163. 

 The value of ©^^ " [i. e. in terms of the notation now used, 

 n(273"7), or the value of n(/), for the freezing-point] " 'for 

 copper and bismuth, or the quantity of heat absorbed in a second 

 of time by a current of unit strength in passing from bismuth to 

 copper, when the temperature is kept at 0° C, must therefore 

 be [6oT6' ^'' ^^"T i2<^^i'ly equal to the quantity required to raise 

 the temperature of a grain of water from 0° to 1° C " 



119. Example 2. Copper and Iron. — "By directing the elec- 

 tromotive force of one copper and bismuth element against that 

 of a thermo-electric battery of a variable number of copper and 

 iron wire elements in one circuit, I have found, by a galvano- 

 meter included in the same circuit, that when the range of tem- 

 perature in all the thermo-electric elements is the same, and not 

 very far at either limit from the freezing-point of water, the cur- 

 rent passes in the direction of the copper-bismuth agency when 

 only three, and in the contrary direction when four or more of 

 the copper-iron elements are opposed to it. Hence the electro- 

 motive force of a copper-bismuth element is between three and 

 four times that of a copper-iron element with the same range of 

 temperature, a little above the freezing-point of water. The 

 electromotive force of a coppei'-iron element, with its two junc- 

 tions at 0° and 1° C. respectively, must therefore be something 

 greater than one-fourth of the number found above for copper- 

 bismuth with the same range of temperature, that is, something 

 more than forty British absolute units, and we may consequently 

 represent it by m x 40, where m > 1 . We have then by the 

 equation expressing the application of Carnot^s principle [equa- 

 tion (19) of § 116], 



whence* 



Ho=^m nearly (a). 



* The value of .1 now used being 32-2x 13yO=44,758, which is the 

 equivHlent of the unit of heat in " absolute units " of work. The " abso- 

 lute unit of force " on which this unit of work is founded, and which is 

 generally used in magnetic and electro-magnetic expressions, is the force 



