The elements, then, forming the electrolyte may have chemical affinity for both metals, though in a greater degree for one than the other.
"Oxygen" is the most important element of an electrolyte, and to the affinity for oxygen of the metals is the magnitude of the result and effect.
Terms Electro-positive and Electro-negative.—All metals have a definite relation to each other as to the potential which any one may have when brought into contact with another. Thus, when zinc is brought into contact with copper, the former has a potential positive to the latter, i.e. a current of electricity will tend to flow from the zinc to the copper. The metals may be so placed in a list that each one would be positive to any of those that follow it; it is then said to be electro-positive to them, and they are electro-negative to it. As those metals which are electro-positive to others have a greater affinity for oxygen, and those that are electro-negative to others a less affinity for this element, the terms electro-positive and electro-negative signify, in effect, greater or less affinity for this element. Conversely, oxygen will combine more readily with the former than with the latter.
The following list shows the commoner metals arranged in electro-chemical order.
| + | Zinc. |
| Lead. | |
| Tin. | |
| Iron. | |
| Antimony. | |
| Copper. | |
| Silver. | |
| - | Gold. |
Take the case of a Voltaic cell composed of zinc and copper plates immersed in water.
The passage of electricity through the water will decompose it into its elements hydrogen and oxygen, the latter having an affinity for both the plates, but considerably more so for the zinc plate.
Then, an electro-motive force will be generated at each metal, and these forces will act in opposition to each other, but the greater strength of the one will overcome the weaker, and the real power of the electric current will be the difference between the two.
Definition of "Elements."—The battery plates are termed the positive and negative elements. A Voltaic battery has two poles—a positive and a negative—which are the terminations of the plates.
Direction of Current.—The course of the current in a Voltaic cell is as follows:—Within it leaves the electro-positive plate (or element), and flows to the electro-negative plate, but outside the cell (or as it were on its return path) it flows from the positive pole to the negative pole. The current always leaves the battery by the positive pole, and thus the copper is the negative element, but the positive pole, because the current leaves the battery by it; and the zinc is the positive element because the current begins there, within the cell, and the negative pole because it ends there, outside.