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454 Transactions of S.M.P.E., Vol. XIII, No. 38, 1929
impossible to establish any rule by which battery size can be determined, as there are so many factors that affect the choice. For example, the space or weight limitations, the current necessary and the cost of installation.
This discussion will attempt to touch on some of these matters ; and from the experience of the dry cell manufacturers, call attention to those factors that the motion picture engineers should consider in choosing a dry cell battery for their unit.
General Characteristics
The characteristics of dry cell batteries, most generally discussed, are capacity, shelf -life, and recuperation.
Capacity is the electrical energy that can be obtained from the cells or battery. It can be measured as service hours or ampere hours when discharged under standard conditions to a definite voltage. In comparing two batteries, therefore, one must be sure that all of the test conditions are standard or at least comparable. For every size of dry cell there is a value of current where its capacity is the greatest.
Shelf -life is that quality of a dry cell which permits it to stand on a shelf for a long time without losing an appreciable quantity of its electrical capacity. In general a large cell has a better shelf -life than a small one, but it is much affected by the purity of materials and the methods of manufacture.
Recuperation is the measure of the ability which the cell has to return to its previous state of activity during an interval of rest following a period of work. As a cell is discharged, its working voltage gradually lowers. Periods of rest allow this voltage to regain a large portion of the previous value. Naturally a good recuperation is desired in a dry cell battery.
Testing
The only way by which the capacity of a battery can be measured is to discharge it. This of course ruins the battery for further use. Voltage and amperage measurements can be made on a battery, but they must be interpreted and weighed against the values obtained from similar batteries of the same manufacture.
Six inch dry cells, when sold over the counter, are usually flashed on a pocket ammeter which merely indicates that the cell