declinometer

Theme week: measuring devices from the alt-text of this XKCD. Because, dude, some of them are pretty cool, starting with:


declinometer (dek-luh-NOM-i-ter) - n., an instrument for measuring magnetic declination.


That is, if you don't want to bother clicking the link, the local angle between magnetic and true north. This varies over the earth's surface and over time, as its magnetic field is not stable. Here's a pic of an antique declinometer:


Thanks, WikiMedia!

---L.