Voltaic Current.—The Voltaic current makes its appearance under the general laws of electrical action.
When a body charged with an excess of, or positive, electricity, is connected with the earth, electricity is transferred from the charged body to the earth; and similarly when a body is charged with a deficiency of, or negative, electricity, is connected with the earth, electricity is transferred from the earth to the body.
Generally whenever two conductors in different electrical conditions are put in contact, electricity will flow from one to the other. That which determines the direction of the transfer is the relative potential of the two conductors. Electricity always flows from a body at higher potential to one at lower potential, when the two are in contact, or connected by a conductor. When no transfer of electricity takes place under these conditions, the bodies are said to be at the same potential, which may be either high or low. The potential of the earth is assumed to be zero.
Definition of Potential.—"The potential of a body or point, is the difference between the potential of the body or point, and the potential of the earth."
Difference of potential for electricity is analogous to difference of level for water. Now, since, when a metal is placed in a vessel containing a liquid, electricity is produced, the liquid becomes of a different potential to the metal, each being electrified in an opposite way; and therefore, as above stated, there being a difference of potentials, electricity will tend to flow from one to the other.
This is evidence of a force being in action, for there can be no motion without some force to produce it.
Electro-motive Force.—Electro-motive force is the name given to a peculiar force to which is due the property of producing a difference of potentials. When it is said that zinc and water produce a definite electro-motive force, what is meant is, that by their contact a certain definite difference of potentials is produced.
The electro-motive force of a Voltaic element may be termed its working power, in the same way as the pressure of steam is the working power of a steam engine, though this is not to be considered as the real source of power, which, as will be seen, is uncertain. Due to the difference of potential of the metal and the liquid, a current of electricity will flow from one to the other, causing the chemical decomposition of the liquid, and the reaction may be taken as the origin of the power employed.
But while the expenditure of energy (which is necessary to produce a force) is accounted for by taking the chemical action as the source of power, the preceding cause of this chemical action, viz. the flowing of the current of electricity due to the difference of potential of the metal and the liquid, must also have first involved the expenditure of energy; thus the real source of power is very uncertain.
Electrolytes.—As before stated, a Voltaic cell consists of two plates of dissimilar metals, which must be immersed in a liquid composed of two or more chemical elements, one of which at least will combine with one or other of the metals, or both in a different degree. Those liquids which are thus decomposed by the passage of a current of electricity are termed electrolytes.