The specific gravity of sulphuric acid of the concentration used in batteries varies with temperature, a rise of 10° F. causing a drop of .003 specific gravity; and as a matter of refer ence it is usual to correct all readings to 70° F.
Applications of the Storage Battery.— The field of the storage battery to-day is so broad that a few of the most important appli cations only may be enumerated, as follows: Propulsion of Automobiles and Commercial Trucks.— Usually 40 or 42 cells, capacity of 100-250 ampere hours, type of cell as per Fig. 24.
Propulsion of Mine Locomotives.— 40-88 cells, 200•300 ampere hours capacity. Fig. 36.
Propulsion of Small Industrial Trucks.— Usually 12-16 cells, 100-200 ampere hours capacity, type of cell as per Fig. 24.
Propulsion of Submarine Torpedo Boats, while running submerged.— Usually 120 cells, 4,000-14,000 ampere hours capacity.
Electric Lighting of Railway Trains.— Each car equipped with a battery, usually 16 cells, 150-350 ampere hours capacity. Fig. 35.
Starting and Lighting of Automobile.— Usually 3 cells, designed to give 150-250 am peres for short intervals of a few minutes du ration. Fig. 26.
Airplane Motor Ignition.— Usually 4 small cells, to insure motor reliability.
Mine Lamps.— One or two cells, 8-12 am pere hours capacity.
Railway Signal Service, for operating the signals which control the movements of trains.
Wireless Telegraphy as the source of power, both ashore and afloat, in army and navy, as well as commercial service.
Telephone Stations furnishing power for the telephone systems. Practically every central is provided with a battery, charged from a small dynamo, and for the purpose of assuring con tinuity of operation. Batteries from 15 to 2,400 ampere hours capacity; Fig. 25 shows a typical cell.
Light and Power Plants, in great variety. Fig. 37 shows a small 16 cell battery, charged from a low power gasoline engine and dynamo, and suitable for lighting the buildings on a form. Fig. 30 shows a mammoth size central station battery, used as a reserve or stand-by in connection with a large central power station.
Between these extremes, batteries of all kinds and sizes are employed, in steam, water and oil operated power plants.
As an indication of the importance of the storage battery in the United States, it may be stated that their manufacture gives direct em ployment to some 14,000 persons, with many more indirectly employed; that it is among the three largest consumers of lead in the country; and that the annual output approximates 2, 000,000 horse power hours capacity.
Bibliography.— Dolezalek, 'Theory of the Lead Accumulators (tr. by Van Ende, 1904); Lyndon, Lamar, 'Storage Battery Engineering> (1911) ; Morse, Harry W., 'Storage Batteries) (1916); Wade, E. J., 'Secondary Batteries) (London 1902).