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# Thermoelectricity

## current, temperature, junctions, temperatures, junction and forces

THERMOELECTRICITY. It was observed by Seebeck in 1822 that, if the two junctions of a closed metallic circuit made up of two differ ent wires in series were at different temperatures. there was produced an electric current. As the difference of temperature at the junctions is in creased, the current increases. It was shown, however, by Cumming in 1823, that for any tem perature of one junction there is one for the other junction such that there is no current: this is known as the temperature of 'inversion.' If in general, then, the temperatures of the junc tions are made more and more different, the current increases, then decreases, becomes zero, and is finally reversed. The average of the temperatures of the junctions when the current is zero is called the 'neutral' temperature. and is a constant for any two bodies.

These thermo-electrie currents are due to molec ular actions at the junctions and also through out the conducting wires, owing to their non uniformity of temperature. Where the two dif ferent metals join there are electric forces called the 'Peltier electro-motive forces;' while the electric forces through the conductors themselves are called 'Thomson electro-motive forces.' The existence of these electro-motive forces is proved by forcing a current around a circuit by means of a battery or cell; at the junctions there is rise or fall of temperature. depending upon the direction of the current, showing a force oppos ing or helping on the current; similarly the conductor itself has its temperature raised or lowered, depending upon the direction of the cur rent—quite apart from the usual heating effect of a current.

The properties of thermo-electric currents are best studied by diagrams. Some one metal is chosen as a standard; lead is generally selected because it has no 'Thomson effect.' A circuit

is made part of lead and part of another metal; one junction is kept at 0° C., and while the tem perature of the other is varied continuously, both above and below zero the total electro motive force around the circuit is measured.

These values of the E.M.F. for different values of the temperature of the variable junction are plotted in a curve, having E.M.F. for ordinates and temperatures for abscissa'. These curves when drawn for different bodies—all with refer ence to lead—are found to be approximately parabolas. Another form of diagram is made as follows: The junctions of the two metals, one of which is lead, are kept at temperatures T and T AT, where AT is a small quantity; the resulting E.M.F. around the circuit is measured, AE call it E. The ratio is called the `thermo-electric power;' it is evidently a func tion of the temperature T, and when plotted on a diagram having temperatures as abscissa', the curves are practically straight lines.

A thermo-couple is one of the most sensitive instruments known for detecting differences in temperature. It can be improved by joining in a zigzag series several couples: first, a piece of bismuth, then one of antimony, then one of bis muth. etc., the first end of the first piece of bismuth and the last end of the last piece of antimony being joined by a wire passing around a galvanometer. Thus every other junction of bismuth and antimony is turned to form one face of the zigzag; and, if one face is at a higher tem perature than the other. each couple helps the others, and thus a considerable current may be produced through the galvanometer. Such a combination of couples forms a 'thermopile.' It was invented by Nobili in 1834.