[winthrowe]

The traditional 5 dollar analog hygrometer uses a paper/metallic strip coil:

https://www.youtube.com/watch?v=jCUHDFYdG44

[bumby]

I guess I was dancing around the point a bit. I meant the definition of relative humidity uses dry/wet-bulb comparisons but we use all sorts of more pragmatic calibrated proxies to measure it in practice.