Interesting. My guess is though it's only going to work where there is metal driven into the stud...so you will have to find somewhere where there is a drywall nail or screw. If you're in a newer house, modern drywall (1960 or so & later), look along the 4' height along the wall, where the joint is. If you have plastered walls good luck, as all the metal mesh underneath, won't help you find a stud with a magnet.
Thanks for sharing "this old tool"; I love things like this.