566 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
Some observations of Regnault’s having- appeared to indicate 240° Cent, as, more 
nearly than 300°, the temperature of the hot junction which gives the current its 
maximum strength, I concluded the following proposition : — 
17* “When a thermo-electric current passes through a piece of iron from one end 
kept at about 240° Cent.*, to the other end kept cold, in a circuit of which the 
remainder is copper, including a long resistance wire of uniform temperature through- 
out, or an electro-magnetic engine raising weights, there is heat evolved at the cold 
junction of the copper and iron, and (no heat being either absorbed or evolved at the 
hot junction) there must be a quantity of heat absorbed on the whole in the rest of 
the circuit. When there is no engine raising weights, in the circuit, the sum of the 
quantities evolved, at the cold junction, and generated in the ‘resistance wire,’ is 
equal to the quantity absorbed on the whole in the other parts of the circuit. When 
there is an engine in the circuit, the sum of the heat evolved at the cold junction 
and the thermal equivalent of the weights raised, is equal to the quantity of heat 
absorbed on the whole in all the circuit, except the cold junction -I-.” 
18. Hence, if the reversible part of the effect of a current from hot to cold in iron 
is an evolution of heat, the corresponding effect in copper must be a greater evolu- 
tion of heat. But if, on the other hand, a cooling effect be produced by a current 
from hot to cold in iron, there must be either a less effect of the same kind, or a 
reverse effect, in copper. It is left to experiment to determine which of the two 
hypotheses is true regarding iron ; and should it turn out to be the latter, to ascer- 
tain which of the two remaining alternatives regarding copper must be concluded. 
With this object I commenced the experimental researches which I now proceed to 
describe. 
§§ 19 to 77- Expej'imental Investigation of the Electric Convection of Heat in Copper, 
in Iron, and in some other Metals. 
§§19, 20. Unsuccessful attempts, and first result. 
19. I began, more than four years ago, by observing carefully the ignition pro- 
duced in short wires of copper, iron, and platinum by electric currents alternately 
in the two directions, thinking that some of the effects described by various experi- 
menters, as showing a superior heating power in the positive electrode, might possibly 
be dependent on the convective agency which I was endeavouring to discover. But 
I never observed the slightest variation in the position of the incandescent part of the 
* I have since ascertained (see Part II. belovr), by keeping the ends of an iron wire, with copper wires from 
the galvanometer soldered to them, in separate vessels of hot oil, and determining different temperatures of the 
two which give no current, that the neutral point for the particular specimens of iron and copper which I used 
must be about 284° Cent. I should therefore, at present, substitute 284° for 240° in the proposition quoted in 
the text ; without further research, however, it is impossible to ^ironounce upon the limits between which the 
neutral point of various specimens of copper and iron wires may be found to lie. 
f Proceedings of the Royal Society of Edinburgh, Dec. 15, 1851, republished in Phil. Mag., June 1852. 
