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I recently cannibalized an old thermostat for parts, and found a strange looking component with no markings at all.

Here's a picture (since I can't figure out how to describe it):

photo of part

My first guess was that this was some sort of resistor, but I can't seem to measure any resistance across it with my multimeter...

I'm very much an electronics newbie so please excuse me if the answer is obvious :)

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2 Answers 2

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It's a resistive hygrometer (or simply put: a humidity sensor). The resistance across the contacts varies depending on the relative humidity of the air it is suspended in.

Example from TDK

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    \$\begingroup\$ Can you please share the part number for this component and any manufacturer? \$\endgroup\$
    – AKR
    Commented Jul 23, 2013 at 16:19
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    \$\begingroup\$ Thank you very much! I had no idea such a thing existed. I can confirm that this indeed works. To test it, I hooked it up to an LED and breathed on it... As I breathed, the LED lit up brighter. I'm sure I will find an excuse to use this in a project ;) \$\endgroup\$
    – hairyhenderson
    Commented Jul 23, 2013 at 16:50
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    \$\begingroup\$ @dhenderson This could be used to automatically adjust the EQ to compensate for humidity's influence on guitar speakers. \$\endgroup\$
    – Kaz
    Commented Jul 23, 2013 at 20:59
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    \$\begingroup\$ @AnandKumarRai: I found the image by doing a Google Images search for "resistive humidity sensor" when I was trying to confirm my answer. The original page: tdk.co.jp/tfl_e/sensor_actuator/CHS \$\endgroup\$
    – Mels
    Commented Jul 24, 2013 at 9:57
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    \$\begingroup\$ Does it break if you get it wet? \$\endgroup\$
    – user10256
    Commented Aug 1, 2013 at 17:41
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It's a Dew sensor, I've also found one in a Camcorder, This sensor was faulty and it generates an error code because the thin polymer on it is erased.

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    \$\begingroup\$ Removing the question that this poster was trying to ask in the answer form doesn't leave a useful answer, but simply a weaker repetition of what was said seven years ago. This should simply be deleted. \$\endgroup\$
    – Chris Stratton
    Commented Sep 11, 2020 at 13:42

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