ELECTRIC BATTERIES. The electric battery is a device by which electric energy is derived directly from chemical action. There are two types of electric batteries: (1) primary, and (2) secondary. Secondary batteries are usually called °storage batteries° or °accumu lators° and are discussed in another article.
The battery unit is called a well.* The simple primary cell, or Voltaic cell, as it is often called, from its inventor, Volta, consists of two different metals immersed in a weak water solution of some acid which will act with unequal intensity upon the two metals. The greater this inequality of action, the larger will be the difference of electric potential be tween the two metals; and, as the current ex cited in the cell depends upon this difference of potential, the greater will be the strength of the current. The two metals form the elec trodes of the battery cell, and the solution is the electrolyte.
The chemistry of the primary cell is thus explained: When a piece of metallic zinc is placed in sulphuric acid diluted with water, a chemical union takes place, the acid and the zinc combining to form the new substance, zinc sulphate. In order that this may be brought about a certain amount of oxygen must be obtained to complete the combination, and as neither the acid nor the zinc can supply it, it is taken from the water lying next to the zinc, and which is thus decomposed— the hydrogen fortnerly in combination with the oxygen being set free in little bubbles which cling to the zinc. These bubbles eventually cover the zinc and slow down the formation of zinc sulphate until it nearly ceases. If now a strip of copper be placed in the same vessel, but not in contact with the zinc, .the conditions re main as they were; but if the ends of the pieces of zinc and copper above the level of the water are leaned together so as to touch above the water, the chemical action is vigorously re newed, but the hydrogen bubbles now appear on the copper. The action of the acid upon the zinc so reduces its electric potential that when contact is made with the copper an electric current immediately. moves to restore the elec tr:cal equilibrium. This being restored, the chemical action—the formation of zinc sul phate —is free to go on, and thus the cycle continues until the zinc has been entirely trans formed into sulphate. And all the time the
action is proceeding the electric current is con tinually flowing to preserve the equilibrium. If, instead of tipping the two metals in the cell until they touch, a metallic conductor is placed so that one end touches the zinc and the other the copper, the current will traverse the whole length of the conductor; and this conductor may be cut and a machine inserted, so that the passing current may be made to expend part of its energy in work. The current is observed to flow from the zinc anode to the copper cath ode w:thin the battery. Outside of the bat tery the current flows through the conductor from the copper toward the zinc. From this external movement the copper has received the title of the positive poie and the zinc of the negative pole.
The force or pressure which causes the current to flow is called the electromotive force (commonly abbreviated to E.M.F.). It should be understood that the words °current° and *flow° are not used in the same sense as with a liquid like water. A better understanding is obtained from the illustration of a steam boiler carrying a high pressure of steam. When a valve into an empty pipe is opened, the pres sure of steam in the boiler is transmitted to the further end of the pipe. In this case the steam fills the pipe carrying the pressure with it. In the case of the electric current passing along a conductor only the pressure (E.M.F.) is transmitted, there being no flow of any known material substance.
The electromotive force of a cell is depend ent to a large degree upon the kind of acid used to dissolve the znic. When the two plates are immersed in other acids than sulphuric, a con siderable variation is found in the difference of electric potential set up in the cell, and it is to be borne in mind that it is upon this differ ence that the strength of the current depends. The electric energy produced or released by a cell depends on the number of pounds of zinc and acid consumed in the formation of zinc sul phate. The zinc is the battery fuel and is oxidized just as coal is oxidized in a furnace. The sulphuric add does not dissolve the zinc itself, but dissolves the oxide as fast as it forms, thus malting the action of the cell continuous.