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  These photos were grabbed from an eBay listing. This detector has been modified.  Originally it would have been built on polished lacquered wood or Bakelite and have manufacturer's nameplate affixed.  There should be two horseshoe magnets instead of the single one shown.  The continuous loop of wire wound around the black disks has been replaced by a cotton string for appearances sake.

   The loop originally consisted of a bundle of fine soft insulated iron wires. The clockwork mechanism would move the wire at a continuous steady rate of anywhere from 1 to 7 cm/sec. There are two coils, one wound on top of the other. The antenna and ground would be connected to one coil and the earphones would be connected across the second. The magnets served to 'erase' the wire as it entered the coils.  Any electromagnetic disturbance impinging on the antenna wire would induce a current into the coil, which in turn magnetizes the wire.  This flip of the iron molecules would induce a current into the earphone coil.

    The antenna coil is wound on a glass tube and is closest to the wire. The earphone coil is larger is diameter and is centered on the antenna coil--easily seen in the photos. The buzz of the spark transmitter would be clearly heard. There are several YouTube videos demonstrating this detecor.

   Later in life, this detector would have a Marconi Tuner placed between it and the antenna. This would improve the receiving station by providing some selectivity and antenna resonating. Sensitivity of the whole apparatus depended on the quality of the ear-phones used. 

   Magnetic detectors had a very short life, probably months, on the Canadian coast stations, being supplanted by simpler and more efficient crystal detectors.