practical electrician. With regard to the heat evolved by a metallic
conductor carrying an electric current, he established what was already
supposed to be the law, namely, that "the quantity of heat evolved by
it [in a given time] is always proportional to the resistance which it
presents, whatever may be the length, thickness, shape, or kind of the
metallic conductor," while he obtained the law, then unknown, that
the heat evolved is proportional to the _square_ of the quantity of
electricity passing in a given time. Corresponding laws were established
for the heat evolved by the current passing in the electrolytic cell,
and likewise for the heat developed in the cells of the battery itself.

In the year 1840 he was already speculating on the transformation of
chemical energy into heat. In the paper last referred to and in a short
abstract in the _Proceedings of the Royal Society_, December, 1840, he
points out that the heat generated in a wire conveying a current of
electricity is a part of the heat of chemical combination of the
materials used in the voltaic cell, and that the remainder, not the
whole heat of combination, is evolved within the cell in which the
chemical action takes place. In papers given in 1841 and 1842, he pushes
his investigations further, and shows that the sum of the heat produced
in all parts of the circuit during voltaic action is proportional to the
chemical action that goes on in the voltaic pile, and again, that the
quantities of heat which are evolved by the combustion of equivalents
of bodies are proportional to the intensities of their affinities for
oxygen. Having proceeded thus far, he carried on the same train of
reasoning and experiment till he was able to announce in January, 1843,
that the magneto-electric machine enables us to _convert mechanical
power into heat_. Most of his spare time in the early part of the year
1843 was devoted to making experiments necessary for the discovery of
the laws of the development of heat by magneto-electricity, and for the