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Read the text. An electric battery is a device for producing an e.m.f
An electric battery is a device for producing an e.m.f. by chemical means. When such a source of an e.m.f. is connected to a closed electric circuit, chemical energy is transformed into electrical energy. An e.m.f. will be produced by chemical means whenever two dissimilar solid conductors are immersed into a conducting liquid. The solid conductors are called electrodes, and the conducting liquid is called the electrolyte. Such a combination of chemicals resulting in the production of an e.m.f. is called a voltage cell. A battery may consist of a single cell or a combination of cells. The voltage of a cell depends upon the material for the electrodes and the electrolyte and is independent of the dimensions of the cell. The current and power capacity of a cell are, however, directly dependent upon the dimensions of the cell and the weight of active material in the electrodes. Although there is an infinite number of different combinations of electrodes and electrolytes which will produce a voltaic cell, there are only a limited number of combinations which are practicable.
For practical purposes, batteries may be classified as primary and secondary. A primary battery is used only for discharge (conversion of chemical energy into electrical energy). As such battery is discharged; the material of one of the electrodes goes into solution in the electrolyte. The electrode is thus consumed, and the character of the electrolyte altered so that with primary batteries it is necessary to renew from time to time both the electrode which goes into solution and the electrolyte. A secondary battery is alternately discharged and charged. As a battery discharges, the electrodes and electrolyte undergo chemical changes. After a secondary battery has been discharged, the electrodes and electrolyte can be restored to their original charged condition by passing a current through a battery in the reverse direction from of discharge. In charging a battery, electrical energy is transformed into chemical energy. Secondary batteries are generally called storage batteries. The internal resistance of batteries is the resistance offered to the flow of current inside the battery, due to its electrodes and electrolyte. Owing to its internal resistance, the voltage at the terminals of a battery is less when it is discharging than when it is on open circuit. The internal resistance of a battery will change with the condition of discharge. As the battery discharges, the internal resistance increases, so that the terminal voltage decreases as the battery discharges. The terminal voltage of a battery is thus dependent upon the rate of discharge. The voltage of a battery will decrease rather slowly as the battery discharges until nearly all the active material of the electrode has gone into solution. The battery is then discharged, and the voltage will drop rapidly to a very low value with any further attempt to discharge.
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