[analog31]

The famous HP 200A oscillator, which was their first product, used a light bulb in the feedback loop of a Wien bridge oscillator to stabilize its amplitude. Not only was it non-ohmic, but it had a nice dynamic response: Fast enough to keep up with minor amplitude drift, but not so fast as to cause harmonic distortion.

https://en.wikipedia.org/wiki/HP200A

[cjsuk]

Indeed. Works better with tubes. I built a slightly more modern version with an op amp and a small 28v bulb in the feedback circuit and it didn’t quite perform as well as I’d hoped. Was interesting however.