A friend broke a string on her bowed psaltery (a small zither-like instrument) and needs to replace it. Problem: what gauge? The source of the instrument is unknown and it didn't come with spare wire or documentation.
Will she have to take the broken string to a music shop and ask for help with identification (maybe compare by visual inspection with other strings they have)? Is there some measurement that someone with ordinary household tools can take to figure it out?
Note that unlike guitar strings, there's nothing particularly special about the strings for a psaltery -- her best bet, once she knows the gauge, will be to just buy a spool of wire. (I doubt anybody even sells "psaltery strings".)