An early hearing aid made with vacuum tubes from a 1933 electronics magazine. The invention of the amplifying Audion (triode) vacuum tube by Lee De Forest in 1906 made previous crude distorting hearing aids which used the amplifying ability of carbon microphones obsolete. The device consists of a box containing a built-in microphone and a simple one-tube triode amplifier circuit, with a jack to plug in the headphone shown. It was heavy because it required three batteries: two 1.5V "D" flashlight cells for the tube's filament, and a bulky 22V "B" battery for the plate, as well as an audio transformer. The door on the front may be the battery compartment. The source article says commercial vacuum tube hearing aids had only come on the market a few years previously.