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458 Transactions of 8M.P.E., Vol. XIII, No, 38, 1929
Photo-electric cells require higher voltages and photo engineers began combining units of 45 volt ''B" batteries to supply their high voltage cells. As the currents that these cells draw are small, the smallest ''B" batteries could be used. Battery makers thought they had gained enough information during the past years to enable them to assemble a satisfactory higher voltage unit of small dry cells. This was done, but it was felt that there was a danger in calling such a high voltage battery a ' ' B " battery, and they were therefore termed ' ' Potential " or " PL ' ' batteries. These batteries, as they contain the smallest sizes of dry cells, should be considered as being a source of potential rather than as a generator of current. In exceptional cases, medium currents can be drawn from them, but it should always be under 5 mils.
These batteries have been used in a number of places in the moving picture field, because perhaps of their comparatively light weight and convenience.
The cells are cylindrical cells that have been double tested after aging. They are assembled in tubes of insulating paper and soldered together in series. To minimize current losses between adjacent cells, the connections are made to give the smallest voltage between them and a spacing of air insulation. As long as the batteries are kept dry and there is no moisture in them at the start, the air spacings between sticks gives the best possible electrical insulation.
These single unit, high potential batteries are convenient for use in portable amplifier units, especially with condenser microphones. For portable recorders, where compactness and a minimum of electrical connections are desired, they will supply the various high voltages.
In film sound projecting equipment, standard 45 volt large sized ^'B" batteries are being made use of, because the current drains are comparatively high. These batteries, furthermore, are easily obtainable at reasonable prices; as they are not moved after installation, there is not much danger from the many connections necessary to wire them into the circuits.
On truck recorders the battery requirements are varied. Potential batteries can be used provided the current drain is not too high. At present, the tendency has been toward the use of a number of 45 volt ''B" battery units, but these have their, objections because