The Picard Loop
The basic operating principle is that an excitation signal is applied to the red coil of wire to create an alternating magnetic field to which the blue figure-eight coil of wire does not respond because with perfectly balanced construction, the pickups of the two halves of the blue wire cancel each other out. However, if someone comes near with a magnetically active tag, something that can magnetically saturate at a pretty low flux density, that tag will disrupt the "blue" winding's balance and cause an output signal to be generated.
These things have been deployed as a part of anti-shoplifting systems for many years. In theory, if you carried that tag exactly along the right path, you could preserve cancellation and the balanced coupling, but unless you are one of The Flying Wallendas, the odds that you could actually manage to do that are virtually zero.
Recently and to my great frustration, I went looking again for that 1938 patent document but I couldn't find it. What I remember about having once read it though was that coil excitation was shown as being applied to the blue figure-eight wiring and that output was taken from the red wire, the reverse of what I have seen as modern practice. Frustration aside though, I did find a collection of more recent patents that seem to be related to this. If you feel like it, these could be interesting to read. For a list of the patents try starting here.