each was fixed into a tube passing through its upper end by
an air-tight joint, that it might be moveable, and yet that the gas evolved
at it might be collected. The tubes were filled with the acid, and one
immersed in each cell. Each platina pole was equal in surface to one side
of the dividing plate in the middle glass vessel, and the whole might be
considered as an arrangement between the poles of the battery of a humid
decomposable conductor divided in the middle by the interposed platina
diaphragm. It was easy, when required, to draw one of the poles further up
the tube, and then the platina diaphragm was no longer in the middle of the
humid conductor. But whether it were thus arranged at the middle, or
towards one side, it always evolved a quantity of oxygen and hydrogen equal
to that evolved by both the extreme plates[A].

[A] There are certain precautions, in this and such experiments, which
can only be understood and guarded against by a knowledge of the
phenomena to be described in the first part of the Sixth Series of
these Researches.

502. If the wires of a galvanometer be terminated by plates, and these be
immersed in dilute acid, contained in a regularly formed rectangular glass
trough, connected at each end with a voltaic battery by poles equal to the
section of the fluid, a part of the electricity will pass through the
instrument and cause a certain deflection. And if the plates are always
retained at the _same distance from each other_ and from the sides of the
trough, are always parallel to each other, and uniformly placed relative to
the fluid, then, whether they are immersed near the middle of the
decomposing solution, or at one end, still the instrument will indicate the
same deflection, and consequently the same electric influence.

503. It is very evident, that when the width of the decomposing conductor
varies, as is always the case when mere wires or plates, as poles, are
dipped into or are surrounded by solution, no constant expression can be
given as to the action upon a single particle placed in the course of the
current, nor any conclusion of use, relative to the supposed attractive or
repulsive force of the poles, be drawn. The force will vary as the distance
from the pole varies; as the particle is directly between the poles, or
more or less on one side; and even as it is nearer to or further from the
sides of the containing vessels, or as the shape of the vessel itself
varies; a