Imagine buying a portable radio in 1925. You bring it home, plug in... nothing. Instead, you haul out a small arsenal of three separate batteries, each with its own voltage, its own purpose, and its own alphabetical letter. Welcome to the strange, expensive, leaky world of the vacuum tube battery — an era when listening to the news literally cost you a dollar a day.

Every vacuum tube needed three different power supplies simultaneously, and engineers labeled them with disarming simplicity:

• The A battery heated the filament — a constant drain of amps that turned the cathode hot enough to boil off electrons. Usually a wet lead-acid cell, like a tiny car battery sitting in your living room. You'd unbolt it weekly and lug it to the drugstore for recharging.
• The B battery provided the high plate voltage — typically 45, 90, or even 135 volts of dry cells stacked in series. This is the origin of the now-archaic term "B+" that still appears in vintage schematics, and the reason a 9V battery is technically a miniature descendant of the B.
• The C battery biased the control grid with a small negative voltage. It barely drained at all, but you needed it anyway — three batteries to hear Amos 'n' Andy.

The economics were brutal. A typical 1920s family spent more on batteries per year than on the radio itself. The A battery alone could cost the equivalent of $15 a week in today's money, and rural households without electricity had no alternative. This is why "battery eliminators" — bulky transformers that converted household AC into all three required voltages — became one of the hottest consumer products of the late 1920s, and why the development of AC-powered tubes around 1927 essentially killed an entire industry overnight.

If you've ever wondered why old radio chassis have that distinctive smell, it's partly the residue of leaked B-battery electrolyte that ate through cabinet wood for decades. And here's where it connects to something you probably know: the iconic 9-volt rectangular battery in your smoke detector? It's a direct vestige of B-battery design — six 1.5V cells stacked exactly the way Eveready stacked 60 of them to make 90V transistor radios in 1956 needed only a tiny version, and the form factor never died.

Even more delightfully obscure: portable tube radios in the 1940s used hearing-aid-style "AB packs" that combined both batteries into a single sealed unit, but the filaments were so power-hungry that a "portable" radio would die after about 20 hours of listening. Compare that to a modern smartphone playing streaming audio for days on a battery a fraction the size — a roughly 10,000-fold improvement in energy efficiency per minute of audio.