It resembled a small incandescent light bulb, yet instead of a hot wire filament strung between two supporting wires it had three elements—a metal filament that would get hot and emit electrons (called a cathode); a metal plate that would stay cool and attract electrons (called an anode); and between them a wire mesh, or “grid.” A small electrical current, or signal, that was applied to the audion’s grid could be greatly amplified by another electrical current that was traveling from the hot cathode to the cool anode.
