Zinc Nickel Gold
Cadmium Bismuth Platinum
Tin Antimony -Graphite (Carbon)

Any two elements selected from this list and immersed in dilute
sulphuric acid will form a voltaic cell, the amount of difference of
potential, or electromotive force, depending on the distance apart in
this series of the two elements chosen. The current within the cell
will always flow from the one nearest the top of the list to the one
nearest the bottom, _i.e._, from the most electro-positive to the most
electro-negative; and, therefore, the current in the wire joining the
two plates will flow from the one lowest down in the list to the one
highest up.

From this series it is easy to see why zinc and copper, and also zinc
and carbon, are often chosen as elements of voltaic cells. They are
widely separated in the series and comparatively cheap.

This series may not be taken as correct for all electrolytes, for
different electrolytes alter somewhat the order of the elements in the
series. Thus, if two plates, one of iron and the other of copper, are
immersed in dilute sulphuric acid, a current is set up which proceeds
through the liquid from the iron to the copper; but, if the plates
after being carefully washed are placed in a solution of potassium
sulphide, a current is produced in the opposite direction. The copper
is now the positive element.

Table II shows the electrical deportment of the principal metals in
three different liquids. It is arranged like the preceding one, each
metal being electro-positive to any one lower in the list.