Batteries are of two kinds, primary and secondary. In the primary battery we choose materials that are readily obtainable in the required condition to react on one another. Thus, in the simple Volta cell we have a stick of zinc dipping into a vessel containing dilute sulphuric acid. Zinc has an affinity for sulphuric acid, and when impure will readily dissolve therein without the use of a separate piece of other metal. If pure the zinc will not dissolve unless a conducting circuit be formed. This is effected by placing a stick of copper in the liquid, and by joining the two metals outside the cell with wire or some other conductor. In this case, as soon as the circuit is closed the zinc stick begins to waste away, zinc sulphate is formed in the acid solution, and hydrogen bubbles appear on the copper stick that dips into the liquid; moreover, the circuit acquires properties that we understand to be due to the flow of an electric current through it. The E.M.F. of the battery depends on the substances used, and may be approximately calculated with a knowledge of the energy with which the two poles become oxidised. It is conventional to regard the current as flowing from copper to zinc in the outside circuit, which is the direction of the apparent passage of the hydrogen through the liquid. Descriptions of the more important batteries are given separately. Grove's cell is useful for its high E.M.F., about 1-9 volts, its fair constancy and low resistance; Leclanche's for its applicability to intermittent easy duty; and Latimer Clark's Standard cell for its constancy. The deposition of hydrogen on the copper pole diminishes the efficacy of the battery by setting up a counter E.M.F. The means adopted to remedy this are discussed under Polarisation, as this deposition is termed.
Secondary batteries do not differ intrinsically from primary batteries. They are simply brought to the condition of being able to drive a current in one direction by the previous passage of a suitable current in the reverse direction. This effects certain changes in the materials of the battery at the expense of electrical energy; which, however, is recovered when the battery reproduces an electric current. A secondary battery may therefore be regarded as an arrangement for the convenient storage of electrical energy, which may be taken out when desired. It is extremely important practically, on account of its high E.M.F., its very low resistance, and its portability.