[cjsuk]

This is a very common little device. The V-I characteristics are non-ohmic i.e. non-linear which make it very useful for feedback. Some notable uses I've seen it for floating around:

1. One of my DMM's, a Fluke 8600A, has a light bulb in it as part of the charge circuit: https://i.imgur.com/TMvRmKF.jpg

2. My Tektronix DM44 strapped to the top of one of my scopes has one as part of the ohmmeter input circuit.

3. Also used for amplitude stabilisation in feedback networks in the negative feedback path.

Very common tricks and very useful little things.

Only problem is they get hot, eat a lot of current and snuff it occasionally. They were replaced by JFETs for typical feedback applications.

[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.