A reed relay turned out to be sensitive to Earth's field if the coil current was raised slowly. Just before the trip-point, the relay contacts close early/late depending on tiny magnetic fields. Measuring relay timing, I managed to detect a 1" alnico magnet at 3ft distance. I now realize should have added long iron rods to each side, as a "DC antenna." Mechanical magnetometer. But the thing was even more sensitive to small motions, or to the direction of gravity. I wonder if it might also act as a "coherer," and detect radio signals? (Tried it, it does work, chopping RF to give an audible beat signal! Holy frikk'n Tesla!)

Go try this: power the relay coil with variable DC with some added high freq AC (a lab signal generator with dc-bias knob), then listen to the buzzing relay contacts using an AA-cell and headphones. Tune the AC drive freq, and easily we find the fundamental reed resonance, plus many overtones up to 10KHz, also some nonlinear hissing/grinding "bands of chaos." And all of it depending on DC bias or position of nearby magnets. Any relay is like an entire steampunk electronics lab! Cool, eh?