Nature is the storehouse of potential electricity. Electricity is stored in fuels like coal, gas, oil, etc., in the potential and kinetic energy of water and wind and so on. All we need is to find means of obtaining electricity from such stores. If any two dissimilar conducting substances are kept immersed in a liquid which acts chemically on one of them, then it becomes a store of electric energy in the form of chemical energy. When these two substances are externally connected with an electric wire, electricity flows through the wire and continues till the chemical reaction lasts i.e. Till entire reacting substance becomes dissolved in the liquid. Such arrangements are called electric cells which are also known as primary cells and the cells are called fully discharged when the entire reaction ceases. However there are some cells in which the original chemical conditions could be brought back by passing electricity through them.