In the pre-transistor era, all radios (and computers!) were constructed with vacuum tubes: Simply put, a negatively charged cathode(-) when heated, emits electrons which travelled to the anode(+) thru a grid(-) that regulated the flow--a triode.

Battery-powered radios had a power supply made of three different types of batteries: Battery 'A' provided low voltage, high current power for the heating elements inside the vacuum tubes, Battery 'B' provided the power between cathodes and anodes and battery 'C' powered the grid circuitry.

Thus, everybody simply adopted the convention that 'A' battery referred to ones suitable to provide the glow current for vacuum tube heating elements. In this case, it is not a form factor like today's AA,AAA,C,D but voltage and mAh specs.

A power source used in older vacuum tube-based radios to provide power for the tube filaments, in situations where alternating current (AC) power was either unavailable or undesirable. Its use was the origin of the terms "A+" and "A-", to denote the filament voltage in a receiver.

The first batteries used for this purpose were 6-volt automobile storage batteries, since the tubes of the time (up to roughly 1929) contained filaments that required 5 volts. A new series of tubes that required only 2 volts came into use in late 1929, and their filaments could be lit by using standard 1.5-volt cells wired in series/parallel, with a rheostat to reduce the voltage; or with a 2-volt "Air Cell", a type of rechargeable storage battery.

Just before World War II, a new series of battery-powered vacuum tubes appeared on the market. These types required only 1.5 volts to light their filaments, thus a single 1.5-volt cell (or group of cells wired in parallel) could be used as a filament power source. These more-efficient tubes also required less filament current, so A batteries tended to last longer than in previous receivers.

In the 1950s, as vacuum tubes began to be replaced by transistors that required far less power, the use of the A battery as a filament voltage source passed into radio history. Smaller-voltage batteries became the radio’s sole source of power.


SOURCES

Ghirardi, Alfred A., Modern Radio Servicing. New York: Radio & Technical Publishing Co., 1935.
Kendall, Lewis F, Jr., and Koehler, Robert Philip, Radio Simplified. Philadelphia: The John C. Winston Company, 1925.

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