File:Woman tuning radio 1923.jpg

English: Woman tuning early radio, from 1923 radio magazine. Broadcasting had just begun in 1920 and vacuum tube radio receivers like this had just come on the market.. They ran off batteries, usually a 3-6V storage battery to power the vacuum tubes' filaments and a 30-90V "B" battery to provide the plate voltage. This woman kept forgetting to turn off the set, running the battery down, so she has wired a small indicator lamp (on top of radio) in the filament circuit to remind her. The early vacuum tubes could not produce much audio power, so early radios used horn loudspeakers, like the one on top of the set. The horn coupled sound from the speaker's diaphragm to the outside air better, and so could produce 10 times (10 dB) more sound power from a given audio signal than a cone speaker.

Caption: A fair fan's idea that saves a lot of worry when she shuts off for the evening. Miss Margie O'Neil found that she sometimes went to her downy and forgot to turn the current off her Magnavox, with the result that her battery was run down the next morning. So she placed a small 6V lamp in series with the battery line, which warns her to "turn off".

Alterations to image: Removed aliasing artifacts (crosshatched lines) introduced when the original halftone photo was scanned, using Gimp FFT filter.