Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/261

chemical energy of the system itself, but at the expense of the thermal energy of neighbouring bodies. Now in the case of the voltaic cell, the principle of Roget, Faraday, and Joule is expressed by the equation

$E=\lambda$,

where E denotes the available or electrical energy, which is measured by the electromotive force of the cell, and where λ denotes the heat of the chemical reaction which supplies this energy. In accordance with Thomson's principle, we must replace this equation by

$E=\lambda +T{\frac {dE}{dT}}$,

which is the correct relation between the electromotive force of a cell and the energy of the chemical reactions which occur in it. In general the term λ is much larger than the term TdE/dT; but in certain classes of cells—e.g., concentration-cells—λ is zero; in which case the whole of the electrical energy is procured at the expense of the thermal energy of the cells' surroundings.

Helmholtz's memoir of 1847, to which reference has already been made, bore the title, "On the Conservation of Force." It was originally read to the Physical Society of Berlin^[1]; but though the younger physicists of the Society received it with enthusiasm, the prejudices of the older generation prevented its acceptance for the Annalen der Physik; and it was eventually published as a separate treatise.^[2]

In this memoir it was asserted^[3] that the conservation of

1. On July 23rd, 1847.
2. Berlin, G. 2. Reimer. English Translation in Tyndall & Francis' Scientific Memoirs, p. 114. The publisher, to Helmholtz's "great surprise," gave him an honorarium. Cf. Hermann von Helinholtz, by Leo Koenigsberger; English translation by F. A. Welby.
3. Helmholtz had been partly anticipated by W. R. Grove, in his lectures on the Correlation of Physical Forces, which were delivered in 1843 and published in 1846. Grove, after asserting that heat is "purely dynamical" in its nature, and that the various "physical forces" may be transformed into each other, remarked: "The great problem which remains to be solved, in regard to the correlation of physical forces, is the establishment of their equivalent of power, or their measurable relation to a given standard."

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