About 75 per cent. of the energy put into the storage cell in charging can be obtained upon _discharging_. Therefore the _efficiency_ of a good storage cell is about 75 per cent. Fig. 268 represents a storage battery connected to charging and discharging circuits. The lower is the charging circuit. It contains a dynamo and a resistance (neither of which are shown in the figure) to control the current sent into the cell. The charging current enters the positive pole and leaves by the negative pole. The current produced by the cell, however, flows in the _opposite_ direction through it, that is, out from the positive and in at the negative pole. This current may be controlled by a suitable resistance and measured by an ammeter. Storage cells have several advantages: (a) They can be charged and discharged a great many times before the material placed in the perforations in the plates falls out. (b) The electrical energy used in charging the plates _costs less_ than the plates and electrolyte of voltaic cells. (c) Charging storage cells takes much _less labor_ than replacing the electrolyte and plates of voltaic cells. (d) Storage cells produce _larger currents_ than voltaic cells. The two principal _disadvantages_ of storage cells are that (a) they are _very heavy_, and (b) their initial _cost_ is _considerable_. [Illustration: FIG. 269.--The Edison storage cell.] [Illustration: FIG. 270.--The plates of the Edison storage cell.] =289. The Edison storage cell= (Figs. 269 and 270) has plates of iron and nickel oxide. The electrolyte is a strong solution of potassium hydroxide. These cells are lighter than lead cells of the same capacity and they are claimed to have a longer life.