Humidity stick?

[abernat]

Sounds like a bimetallic strip - the theory is definitely sound. 

I'd be interested to see if there's a difference between gluing the entire length of the two woods or just gluing the ends to provide maximum independent movement.

[Tony Z]

Seen this in the past, in several variations. First time was around 60 years ago, while in elementary school, in a book (IIRC) "Scientific American Boy". Calibration may be problematic.

[MauleSkinner]

Here’s another version...the cross grain expansion pushes the needle.
https://m.youtube.com/watch?v=112v0QJz8EI

[Pirate]

Going with Arlin's idea, why bother having a woodshop, when you can just buy items, instead of making them.
No thanks.
I already have store bought hygrometer in the house.

[Alan S]

I already have store bought  hygrometer in the house.

That should help with calibration of the one you make.

I used to have a 3' long crossgrain piece from trimming the end of a panel that I used to measure every now and then. It worked.

Any mechanism that amplifies the motion to make it easier to read would be useful, like a long grain strip attached alongside, possible glued only at the ends to make sure the glue did not interfere.  The longer the better in terms of sensitivity.  Response time is still slow.

[ianab]

  Response time is still slow.

Make sure you keep the end grain unfinished. Then on a small piece of end grain the response will be hours, not weeks.

[AHill]

This looks close to your description:
https://www.youtube.com/watch?v=B9FPBQzlhbc

OK for a Cub Scout project, but Arlin's suggestions would be more practical.

I'm surprised he still has all his fingers intact.  His table saw technique leaves a bit to be desired - even with the push sticks.

[Pirate]

Made a stick with long grain mahogany and cross grain oak.
Made in metal shop with no a/c. Est temp 95. Outside est RH 85%
Brought in house to dry in clamps. Temp 76 RH 40%
Next day took clamps off and at end of day it looked like picture.
Left outside (RH est 85% temp 84°} next day and it bent 1/2" more.
Glad it wasn't made as a part if a project.
It was quarter sawn oak.
I'm making another from mahogany and cut up cross grain 2x4 today to see difference.

[Rick LoDico]

Just buy one of these magical thingies.
https://www.newenglandtradingpost.com/th...P_EALw_wcB