have a simple copper, zinc, and acid cell producing a steady current. There is probably a considerable sudden rise in passing from the zinc to the acid, the place where the chemical energy is given up, a fall through the acid depending on the resistance, a sudden fall on passing from the acid to the copper, where some energy is absorbed with evolution of hydrogen, and then a gradual fall through the wire of the circuit round to the zinc again. There will be a slight change of potential in passing from copper to zinc, but this we shall neglect for simplicity. The equipotential surfaces will probably then be somewhat as sketched in fig. 3,[1] all the surfaces starting from where the acid comes in contact with the zinc, some of the highest potential passing through the acid, others passing between the acid and copper, and crowding in there, the rest lower than these cutting the circuit at right angles in points at intervals representing equal falls of potential.
If this be the actual arrangement, then it is seen that the current, which travels round the circuit from zinc through acid to copper, is in opposition to the E.M.I. between the zinc and acid, while the M.I. is related to the current in the ordinary way. The energy will therefore pass outwards from there along the level surfaces. In fact, the medium between the zinc and acid behaves like the medium between the plates of the condenser in case No. (2), and it seems possible that the chemical action produces continually fresh "electric displacement" from acid towards zinc which yields as rapidly as it is formed, the energy of the displacement moving out sideways.
- ↑ In this and the succeeding cases the circuit is alone supposed to cause the distribution of potential. In actual cases the surfaces would probably be very much deflected from their normal positions in the dielectric through the presence of conductors, electrified matter, and so on.
between the air near two different metals in contact are, in this theory, to be accounted for by the supposition that the air acts in a similar manner to an oxidising electrolyte. A short statement of the theory is given in a letter by Professor Maxwell in the 'Electrician' for April 26th, 1879, quoted in a note on page 149 of his 'Elementary Treatise on Electricity'. (See also § 249, vol. i, Maxwell's 'Electricity and Magnetism'.—June 19, 1884.
