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 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. (The filaments in vacuum tubes consumed much more current than the anodes, and so the "A" battery would drain much more rapidly than the "B" battery; therefore, using a rechargeable "A" battery in this role reduced the need for battery replacement. In contrast, a non-rechargeable "B" battery would need to be replaced relatively infrequently.)
A batteries were initially 2 volts, being lead acid accumulators, but with the introduction of all dry battery radios, 1.4 volts became more common. Other voltages can be encountered. For example 7.5 volt batteries were sometimes used to power a series connected set of 1.4 volt valves (tubes).
In Britain and in some other countries, the "A" battery was known as the "LT" (low tension) battery.
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