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 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

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

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 ; 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.

In this memoir it was asserted that the conservation of