Let us now imagine the metal rod flattened into a plate, and one face
stimulated by light, while the other is protected. Would there be a
difference of potential induced between the two faces of this same sheet
of metal?

Let two blocks of paraffin be taken and a large hole drilled through
both. Next, place a sheet of metal between the blocks, and pour melted
paraffin round the edge to seal up the junction, the two open ends being
also closed by panes of glass. We shall have then two compartments
separated by the sheet of metal, and these compartments may be filled
with water through the small apertures at the top (fig. 97, _a_).

[Illustration: FIG. 97 (_a_)
A, B are the two faces of a brominated sheet of silver. One face, say A,
is acted on by light. The current of response is from B to A, across
the plate.]

[Illustration: FIG. 97 (_b_).--RECORD OF RESPONSES OBTAINED FROM THE
ABOVE CELL
Ten seconds' exposure to light followed by fifty seconds' recovery in
the dark. Thick lines represent action in light, dotted lines
represent recovery.]

The two liquid masses in the separated chambers thus make perfect
electrolytic contacts with the two faces A and B of the sheet of metal.
These two faces may be put in connection with a galvanometer by means of
two non-polarisable electrodes, whose ends dip into the two chambers. If
the sheet of metal have been properly annealed, there will now be no
difference of potential between the two faces, and no current in the
galvanometer. If the two faces are not molecularly similar, however,
there will be a current, and the electrical effects to be subsequently
described will act additively, in an algebraical sense. Let one face now
be exposed to the stimulus of light. A responsive current will be found
to flow, from the less to the more stimulated face, in some cases, and
in others in an opposite direction.

It appears at first very curious that this difference of electric
potential should be maintained between opposite faces of a very thin and
highly conducting sheet of metal, the intervening distance between the
opposed surfaces being so extremely small, and the electrical resistance
quite infinitesimal. A homogeneous sheet of metal has become by the
unequal action of light, molecularly speaking, heterogeneous. The two
opposed surfaces are thrown into opposite kinds of electric condition,
the result of which is as if a certain thickness of the sheet,
electric