Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/347

he found that the current produced opposite effects in copper and in iron^[1].

When a stream of a material fluid passes along a tube from a hot part to a cold part it heats the tube, and when it passes from cold to hot it cools the tube, and these effects depend on the specific capacity for heat of the fluid. If we supposed electricity, whether positive or negative, to be a material fluid, we might measure its specific heat by the thermal effect on an unequally heated conductor. Now Thomson's experiments shew that positive electricity in copper and negative electricity in iron carry heat with them from hot to cold. Hence, if we supposed either positive or negative electricity to be a fluid, capable of being heated and cooled, and of communicating heat to other bodies, we should find the supposition contradicted by iron for positive electricity and by copper for negative electricity, so that we should have to abandon both hypotheses.

This scientific prediction of the reversible effect of an electric current upon an unequally heated conductor of one metal is another instructive example of the application of the theory of Conservation of Energy to indicate new directions of scientific research. Thomson has also applied the Second Law of Thermodynamics to indicate relations between the quantities which we have denoted by $P$ and $Q$, and has investigated the possible thermoelectric properties of bodies whose structure is different in different directions. He has also investigated experimentally the conditions under which these properties are developed by pressure, magnetization, &c.

254.] Professor Tait^[2] has recently investigated the electromotive force of thermoelectric circuits of different metals, having their junctions at different temperatures. He finds that the electromotive force of a circuit may be expressed very accurately by the formula

$E=a(t_{1}-t_{2})[t_{0}-{\frac {1}{2}}(t_{1}+t_{2})],$

where $t_{1}$ is the absolute temperature of the hot junction, $t_{2}$ that of the cold junction, and $t_{0}$ the temperature at which the two metals are neutral to each other. The factor $a$ is a coefficient depending on the nature of the two metals composing the circuit. This law has been verified through considerable ranges of temperature by Professor Tait and his students, and he hopes to make the thermoelectric circuit available as a thermometric instrument in his

1. 'On the Electrodynamic Qualities of Metals,' Phil Trans., 1856.
2. Proc. R. S. Edin., Session 1870–71, p. 308, also Dec. 18, 1871.