point of METALLIC contact." The generation of this current is
accompanied by chemical action in the cell. Experiment shows that
the mere CONTACT of dissimilar materials, such as copper and zinc,
electrifies them--zinc being positive and copper negative; but
contact alone does not yield a continuous current of electricity.
When we plunge the two metals, still in contact, either directly
or through a wire, into water preferably acidulated, a chemical
action is set up, the water is decomposed, and the zinc is
consumed. Water, as is well known, consists of oxygen and
hydrogen. The oxygen combines with the zinc to form oxide of zinc,
and the hydrogen is set free as gas at the surface of the copper
plate. So long as this process goes on, that is to say, as long as
there is zinc and water left, we get an electric current in the
circuit. The existence of such a current may be proved by a very
simple experiment. Place a penny above and a dime below the tip of
the tongue, then bring their edges into contact, and you will feel
an acid taste in the mouth.

Figure 12 illustrates the supposed chemical action in the cell. On
the left hand are the zinc and copper plates (Z C) disconnected in
the liquid. The atoms of zinc are shown by small circles; the
molecules of water, that is, oxygen, and hydrogen (H2O) by
lozenges of unequal size. On the right hand the plates are
connected by a wire outside the cell; the current starts, and the
chemical action begins. An atom of zinc unites with an atom of
oxygen, leaving two atoms of hydrogen thus set free to combine
with another atom of oxygen, which in turn frees two atoms of
hydrogen. This interchange of atoms goes on until the two atoms of
hydrogen which are freed last abide on the surface of the copper.
The "contact electricity" of the zinc and copper probably begins