It has also been shown by Lord Kelvin (1) that an electromotive force occurs between differently tempered parts of the same metallic conductor, and this strives to conduct heat from the warmer to the cooler part.
This so-called Thomson eflfect is very inappreciable for metals; it occurs in liquids and probably also in gases. It has not yet been very thoroughly investigated, and may here be neglected. In galvanic elements and electrolytic decomposition cells, besides the Joule heat, a quantity of heat, w, is evolved for each equivalent of substance taking part in the chemical change, and this is partially used up in doing work to send the current through the circuit, which part ifl measured by the expression 23,070P. (P denotes the electromotive force of the element, or of the decomposition cell, see p. 205.)
The quantity of heat —
IF2 = oi - 23,070P
is termed local heat (or internal heat), and, like the Joule heat, remains in the element or vessel (voltameter) in which the electrolytic process takes place.
In elements to is generally positive, as also is F; in decomposition cells it is negative.
Arc Light. — The greatest development of heat takes place when the electricity passes through gases. The passage may be disruptive, as in the spark discharge and outflow of electricity from points, or it may be continuous with forma- tion of an arc light. In the former case the quantity of electricity transported is very small.
The arc light, or Volta arc, which is now so much used for illuminating purposes, was discovered by Volta in 1808, and afterwards thoroughly studied by several investi- gators.
Edlund (J) showed that the potential difference £ between two carbon points between which the arc is playing is given by the formula —
t =■ a-^hl
u
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