at all.  The positive electricity of the zinc appears to traverse the liquid to the copper, from which it flows through the wire to the zinc.  The effect is that the end of the wire attached to the copper is positive (+), and called the positive “pole” or electrode, while the end attached to the zinc is negative (-), and called the negative pole or electrode.  “A simple and easy way to avoid confusion as to the direction of the current, is to remember that the positive current flows from the copper to the zinc at the 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 the process, and the chemical action keeps it up.  Oxygen, being an “electro-negative” element in chemistry, is attracted to the zinc, and hydrogen, being “electro-positive,” is attracted to the copper.

The difference of electrical condition or “potential” between the plates by which the current is started has been called the electromotive force, or force which puts the electricity in motion.  The obstruction or hindrance which the electricity overcomes in passing through its conductor is known as the resistance.  Obviously the higher the electromotive force and the lower the resistance, the stronger will be the current in the conductor.  Hence it is desirable to have a cell which will give a high electromotive force and a low internal resistance.