It is conventional to speak of the electric current as flowing from the carbon through the wire to the zinc, although every one has suspicions that it may flow in the other direction or even that it may not flow at all. It is conventional to designate any part of the circuit from which the current comes as positive (+) to any other part toward which it flows, this latter being considered negative to the former and designated (-). The current is conceived of as making a complete circuit, from carbon to zinc through the wire and from zinc to carbon through the liquid. Hence, the binding post of the carbon rod is called the + pole and that of the zinc is called the-pole, while the zinc rod or plate beneath the surface of the fluid is called the + plate and the carbon is called the-plate. The liquid is termed the electrolyte. The sodium bichromate, introduced to cause the hydrogen to unite with oxygen, is called an oxidizing agent or even a depolarizing agent, and hydrogen collecting upon the negative plate is said to polarize the cell.
Hydrogen may be made to collect upon the carbon or negative plate until the electric current reverses its direction. The hydrogen is said to be more - than the zinc. If we connect the zinc and carbon rods with the wires bringing an electric current from the dynamo we may make either one positive as we choose, according to which is connected with the positive wire. Hydrogen bubbles will collect upon whichever plate we make the negative one.
Fig. 151
When we send an electric current from the dynamo into this cell it is called an electrolytic cell, and when it is used to generate an electric current it is called a battery cell. In either case the electrolyte is decomposed and put through a chemical change, though the chemical action in one case is the reverse of that in the other, and the direction of the electric current in one case is the reverse of that in the other. For example let us consider the case of a zinc rod and a carbon rod immersed in sulphuric acid and the external circuit closed. The current passes as indicated by the arrows in [Fig. 151], and the chemical actions result in hydrogen leaving the sulphuric acid and zinc taking its place, forming zinc sulphate. This is a white salt and for purposes of this experiment must remain dissolved in water. So far we have been considering a generator of electricity—a battery cell. We may introduce something at m, say a motor, which will indicate that an electric current is flowing. At length the cell ceases to generate current and is, as we say, "run down." Suppose now we substitute a dynamo in place of the motor in this circuit, connecting it so that the carbon rod shall be its positive pole and the zinc its negative pole. We now call this an electrolytic cell, ([Fig. 152]). The current will decompose the zinc sulphate. The zinc will be coated upon the zinc rod and hydrogen will be procured from the water present, of which it is a constituent, to form again sulphuric acid as originally.
Fig. 152
We shall thus restore the conditions which prevailed in the first case as represented in [Fig. 151]. H2SO4 is the chemist's designation of sulphuric acid and ZnSO4 is his expression for zinc sulphate.