Ordinary commercial or technical grades of sulphuric acid and ordinary water are satisfactory for primary batteries, but the acid and water used in making the electrolyte for storage cells must be *chemically pure* if you wish to obtain good results and desire your batteries to hold their charge while standing. Storage cells can be recharged with DIRECT current only. A dynamo for recharging storage cells must be SHUNT wound. The voltage of the charging current must be greater than that of the storage cells. About three volts of charging current will be required for each cell of the storage battery. Cells may be connected in multiple when recharging so as to bring the voltage of the cells below that of the charging current. It will of course, however, take much longer to recharge cells connected in multiple than the same cells connected in series, provided that the amperage of the current is the same. Storage cells must not be recharged too rapidly. It is better to recharge them slowly rather than too rapidly. Two amperes is plenty for small cells of 10-15 ampere hours capacity. Three amperes is sufficient for cells of 15-25 ampere hours capacity. Five to six amperes is the right charging rate for a 40 ampere hour battery and 8 amperes in the case of a 60 ampere hour battery. [Illustration: FIG. 46.—A Hydrometer for preparing and testing the Acid Solution for Storage Batteries.] Storage cells should never be allowed to stand discharged for any length of time or the plates are liable to become hardened and "sulphated." They turn white when they are sulphated. It will take a great deal of charging and recharging to get them back in shape when once they get in that condition. Never short circuit a storage cell or discharge it too rapidly. Whenever any great amount of sediment collects in the bottom of the jars, pour out the acid solution and wash the cell out thoroughly with some pure water. The plates of a storage cell should always be raised up off the bottom of the jar, so that any sediment which collects will fall below. [Illustration: FIG. 47.—The proper way of Recharging Storage Cells from the 110 Volts D. C. Supply in series with a set of Lamps.] It is a very good plan to keep the terminals of a storage cell or battery smeared with vaseline so that they will not become corroded. You can tell when a storage cell or battery is fully recharged by the color of the plates. The positives will in that case be a dark chocolate brown and the negatives a light slate gray color. A cell which is fully recharged will indicate 2 1/2 volts on a voltmeter connected across its terminals while the charging current is still on. CHAPTER III. HOW TO REDUCE THE 110 V. D.C. OR A.C. TO A LOWER VOLTAGE FOR EXPERIMENTAL PURPOSES.