plates is consumed, and that on the other plate some element is
deposited, this element being sometimes a gas and sometimes a solid.
The plate which is consumed is always the negative plate, and the one
on which the element is deposited is always the positive, the current
through the connecting wire always being, therefore, from the
unconsumed to the consumed plate. Thus, in the simple copper-zinc cell
just considered, the zinc is consumed, the element hydrogen is
deposited on the copper, and the current flow through the external
circuit is from the copper to the zinc.

The positive charges, leaving the zinc, or consumed, plate, and
passing through the electrolyte to the copper, or unconsumed, plate,
constitute in effect a current of electricity flowing within the
electrolyte. The current within the cell passes, therefore, from the
zinc plate to the copper plate. The zinc is, therefore, said to be
positive with respect to the copper.

_Difference of Potential._ The amount of electromotive force, that is
generated between two dissimilar elements immersed in an electrolyte
is different for different pairs of elements and for different
electrolytes. For a given electrolyte each element bears a certain
relation to another; _i.e._, they are either electro-positive or
electro-negative relative to each other. In the following list a group
of elements are arranged with respect to the potentials which they
assume with respect to each other with dilute sulphuric acid as the
electrolyte. The most electro-positive elements are at the top and the
most electro-negative at the bottom.

+Sodium Lead Copper
Magnesium Iron Silver