Battery (vacuum tube)

In the early days of electronics, vacuum tube devices (such as radios) were powered by batteries. Each battery had a different designation depending on which vacuum tube element it was associated with.

Initially, the only such device was a diode with only a filament (cathode) and a plate (anode). Following the direction of electron flow, these electrodes were identified as "A" and "B", respectively and thus the associated batteries were referred to as the "A" battery and "B" battery, respectively. Later, when the control grid element was added to create the triode tube, it was logically assigned the letter "C" and supplied from a "C" battery. Subsequent addition of further internal elements to improve the performance of the triode did not require an extension to this series of batteries – these elements were either resistively-biased from the existing batteries, connected to ground or to the cathode.