In
electronics, an A battery is any
battery used to provide power to the
filament of a
vacuum tube. It is sometimes colloquially referred to as a "
wet battery" (although there's no reason why a "dry" battery of suitable voltage couldn't be utilised for the purpose; the A battery in the photo is a
dry battery.) The term comes from the days of valve (tube) radios when it was common practice to use a dry battery for the plate (Anode) voltage and a rechargeable lead/acid "wet" battery for the filament voltage.
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