To use, a watch is placed inside of the core; the slide is then moved in and out to change the polarity of the magnet, as described in patent number 699863:
The slide and comb thereon constitute a pole changer, and in practice the same is actuated to cause frequent reverses of the direction of current that goes through the electromagnet. A magnetized watch, tool, or other device is placed in the hollow core of the electromagnet… the pole changer is operated to effect its function… the result being demagnetization of said device.
Oddly enough, or maybe not oddly at all given the circumstances, Dennis had a collection of demagnetizers. This was my by far my favorite though.
Here are some other goodies from today’s work: