Glow-pot

The glow-pot faders were invented at the BBC Radiophonic Workshop to overcome the limitations of the lumpy faders that they had:

“Sine waves (theoretically the purest known form of audio) were completely unhandleable if you used the conventional mixing desks in those days because they were mixing desks with knobs that had a row of studs and a traveller on the back of them. Every time you faded them up, with a sine wave as the sound, you'd hear it go 'click, click, click' round the studs. So that was very difficult. It meant that you couldn't necessarily mix them. You had to have an accompanying atmosphere with theme. Later on, we developed what we called the 'glow-pot' fader — like it sounds, it's something to do with light. They were quadrant faders on the desk, but when you did that [sliding the slider forward] you didn't actually fade the sound directly, you actually altered the brilliance of a lamp which was coupled to a photoresistive cell that did fade the sound. Any 'click, click, click' in the studs on the fader was absorbed by the filament of the bulb, which was kept on a minimum glow all the time, not enough to let any sound through but just absorb the 'click, click, click', and then you could fade as pure a sound as you wanted to without any trouble.”