_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_. 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. Sulphuric Acid } Zinc } }Sulphate of zinc found at { Oxygen } Oxide of Zinc } positive plate. Water{ Hydrogen Hydrogen found at negative plate. No _single fluid_ cell can give a constant electro-motive force because of the _polarisation_ of the plates.