The positive pole is the terminal of the negative plate, and vice versâ. There is but one current from a battery, viz. a positive one; what is called a negative current is merely the positive current passing in the reverse direction from the same pole, that is, the positive pole.
Single and Double Fluid Batteries.—Galvanic batteries may be divided into single fluid and double fluid batteries. The simplest form of galvanic cell practically in use is a single fluid cell, consisting of a zinc and a copper element, immersed in water slightly acidulated by the addition of a little sulphuric acid. In a battery of several cells, the zinc and copper plates are generally soldered together in pairs, and placed in a long stoneware or glass trough, divided into separate cells by means of partitions. By filling the cells with sand, this battery is made more portable, the plates being thus supported, and the liquid prevented from splashing about during transit.
In this form it is called the common sand battery.
Action in a Single Fluid Cell.—The following process goes on in the single fluid cell when the circuit is closed—that is, when the battery is set to work.
The water (composed of hydrogen and oxygen) is decomposed by the passage of the electric current, and oxide of zinc is formed. The oxygen of the water having greater affinity for the zinc, leaves the hydrogen. The zinc during the process is being consumed, as coal is consumed when it burns, while combining with the oxygen of the air. This oxide of zinc combines with the sulphuric acid, and forms sulphate of zinc; this salt is found to accumulate in solution in the liquid of the cell. At the same time the hydrogen of the water goes to the negative or copper plate, and gathers over it in bubbles.
The process will be better seen by the accompanying plan of the chemical decomposition and recombinations.
No single fluid cell can give a constant electro-motive force because of the polarisation of the plates.
Definition of the term Polarisation.—The word polarisation means that the plates become coated with the products of the decomposition of the electrolyte, producing a diminution of current. In the above described battery, the hydrogen gathers on the surface of the copper plate, and an electro-motive force is set up which counteracts the electro-motive force producing the current—the copper plate is said to be polarised. By the bubbles of hydrogen collecting on the face of the negative plate, the surface in contact with the liquid is gradually decreased; thus the plate becomes practically smaller, and a single fluid cell which at starting gave a good current soon shows that it is really weakened. The consequence is that the zinc is consumed extravagantly as well as the acid, and the cell working with poor results. Also the resistance of the cell is increased, due to the sulphuric acid, which is added to the water to increase its conductivity, being gradually used up, by combining with the oxide (see plan) and forming sulphate of zinc. Liquids are very bad conductors of electricity; the greater part of the ordinary internal resistance of a battery arises from this cause. The common sand battery is the worst of all batteries as regards constancy of electro-motive force, the polarisation being greater in this battery than any other because the gas cannot readily escape. The common copper and zinc cell is the next in order of demerit. The Smee single fluid cell, in which the negative plate is a platinum instead of a copper one, is better than the copper zinc cell, because the free hydrogen does not stick to the rough surface of the platinum plate so much as to the copper.
Double Fluid Batteries.—All the defects of the single fluid battery, which are as follows—
1. Diminution of electro-motive force,