138 PROFESSOR W. THOMSON ON THE 
passing from bismuth to copper, when the temperature is kept at 0° Cent. must 
therefore be ,1°3-, or very nearly equal to the quantity required to raise the 
temperature of a grain of water from 0° to 1° Cent.’” 
119. Example 2. Copper and Iron.—* By directing the electro-motive 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 galvanometer included in the same circuit, that when the range of tempe- 
rature in all the thermo-electric elements is the same, and not very far at either’ 
limit from the freezing point of water, the current 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 electro-motive force of a copper-iron element, with its two junc- 
tions at 0° and 1° Cent. 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 40 British absolute units, and we may 
consequently represent it by mx 40, where m>1. We have then by the equation 
expressing the application of Carnot’s principle, [ equation (19) of § 116.], 
Q, [t= area =m x 40, 
whence* ©, =imnearly . : : 5 : : - (@): 
“Now, by the principle of mechanical effect, we have 
280 
ras (if Sdt—,) ; 
0 
if F,**° denote the electro-motive force of a copper-iron element, of which the two 
junctions are respectively 0° and 280° Cent., and 9. d7, the quantity of heat absorbed 
per second by a current of unit strength, in passing in copper from a locality at 
temperature ¢ to a locality at ¢+d¢, and in iron from a locality at t+d7 to a 
locality at ¢;+ since the PeLTier generation of heat between copper and iron at 
their neutral point, 280°, vanishes;{ and therefore the only absorption of heat is 
that due to the electric convection expressed by f 3 dt; while there is evolution of 
* The value of J now used being 32:2 x 1390=44,758, which is the equivalent of the unit of 
heat in “ absolute units” of work. The “absolute 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 which 
acting on the unit of matter (one grain) during the unit of time (one second), generates a unit of 
velocity (one foot per second). The “absolute unit of work” is the work done by the absolute unit 
of force in acting through the unit of space (one foot), 
+ That is, if 3 denote the algebraic excess of the specific heat of electricity in copper, above the 
specific heat of electricity in iron, according to the terms more recently introduced. 
t See § 123, below. Instead of 240°, conjectured from Reanavrt’s observation when these 
Here were first published, 280° is now taken as acloser approximation to the neutral point of copper 
and iron. 
