It had long been known that the discharges from a Leyden jar could magnetize or demagnetize steel needles.
Rutherford invented a magnetic detector depending on the power of electric oscillations to demagnetize iron or steel.
Rutherford examined it very carefully, and produced a magnetic detector for electric waves depending upon the power of electric oscillations in a coil to demagnetize a saturated bundle of steel wires placed in it.