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
 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.

TABLE II

Behavior of Metals in Different Electrolytes