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448 RICHARDSON'S BLUEBOOK OF PROJECTION
loosens active material from the plates and causes it to fall to the bottom of the container as sediment, thus shortening the life of the battery. Gassing, and therefore charging, should consequently be limited to just so much as is necessary to prevent accumulation of excess lead sulphate, and no more. Excessive gassing may also cause some of the sediment to boil up from the bottom and deposit on the plates or the porous separators between the plates, thereby forming an internal conducting path that partly short-circuits the cell. (22) Manufacturers' specifications as to charging should be followed carefully and intelligently. Otherwise the cell needs only external cleanliness, and periodic replacement of the water that has evaporated.
(23) The acid does not evaporate but some spray of acid will be blown out of the ventilation openings by gassing during charge. Generally, the amount of acid lost in this way is so small that new acid will never be needed during the life of the battery. Sulphuric acid is not added to batteries. If it is needed the cell is emptied and a new solution of acid and water poured in; which is not, in general, a job for the projectionist, but for a battery expert. (24) The acid lost through gassing, though not enough to make much difference inside the cell, becomes, however, a very serious matter when it collects on the outer surface. It corrodes the terminals and the connecting wires to an extent that may result in imperfect contact and therefore in noisy sound.
Trouble from this source is prevented by keeping the terminals of the battery carefully greased. This is a very important precaution.
(25) Spray of acid collecting upon the top of the battery forms a conducting path of high and often of variable resistance. Noisy sound may be caused by unclean battery tops. Acid spray should be carefully washed off with excess of pure water. Ammonia will neutralize the acid and a weak solution of it is sometimes used for washing the tops of storage batteries.
(26) The only safe water to use in a storage battery, to make up for evaporation, is distilled water. Hydrant