Analog Hygrometers - how?

It's a simple one this time. Simple! Simple!

So simple, even, that this descriptions is mostly blank. But here's the usual links 'n stuff.
Technology Connections on Twitter:
Twitter: TechConnectify
The TC Subreddit:
Reddit: technologyconnections
Technology Connextras (the second channel that stuff goes on sometimes):
@technologyconnextras
Patreon:
Patreon: technologyconnections