OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
353 
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 he 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,'“ 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. 
Fig. 3. 
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.l. 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. 
between tbe air near two different metals in contact, are in this theory to be accounted for by the suppo¬ 
sition that the air acts in a similar manner to an oxidising electrolyte. A sliort 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. 
* 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, 
MDCCCLXXXIV. 2 Z 
