Magnetic Detectors – By John Heys G3BDQ

Magnetic detectors of wireless transmissions were developed in the research section of the Marconi factory in 1902. They represented a huge leap in detector reliability, being especially suited to the movements of ships at sea. The Marconi Wireless Telegraph Company’s original premises were situated in Hall Street Chelmsford and were opened in 1898. This factory was in an old disused silk mill and was replaced by new buildings in 1912. Inventors seldom have sudden flashes of inspiration which lead to novel ideas but more often rely upon the work of earlier experimenters. This latter approach applied to the concept of a magnetic detector.

The movement of magnetised steel needles when close to discharging Leyden jars was first recorded by Joseph Henry of Princeton University in 1842. In 1895 Rutherford could detect ‘Hertzian Waves’ at a distance of three quarters of a mile when his receptor was a very sensitive magnetometer. In 1897 Professor Ernest Wilson worked upon this concept and finally the Marconi Company patented a wireless Magnetic detector in 1902. This device, a ‘Maggie’ as it became called by ships’ wireless telegraphists rapidly replaced coherer circuits through 1903. Headphones were used with the new detectors, and although rather insensitive the new detectors were robust and reliable. They could also detect Morse sent at high speeds.

The heart of a magnetic detector is shown in Fig.1.The low impedance output of a suitable aerial tuner is connected to coil L1 which is wound on a glass tube. The other end of this coil goes to earth. A second coil L2 has a high impedance winding and many more turns than L1. This second coil connects to medium impedance headphones (about 140 ohms). A soft iron band moves through the glass tube that is the former for L1 and a pair of permanent magnets are arranged close to the coils. When a signal from the aerial tuner reaches L1 there is a sudden change of magnetism on the iron band which induces a current in L2 which then energises the phones. The permanent magnets contribute to the detection operation and their positioning relative to the coils is adjustable.

The apparatus was housed in a wooden box and the crucial soft iron belt was made up from 70 strands of No. 40 sws (single wound silk) iron wire which was rotated by clockwork. The belt moved at a speed of approximately 1 inch per second. The primary coil (L1) was wound with No. 36 DWS (double wound silk) copper wire over 2 cms of tubing and it had a resistance of between 2 and 3 ohms with an inductance of 30 mH. The secondary coil L2 was wound on an ebonite bobbin and had a resistance of around 140 ohms. Each magnetic detector had two identical sets of coils and magnets which meant that there was a spare detector unit available. The magnets were adjusted by the operator to remove the slight hissing that could be present when the belt was moving. Oddly, some operators liked to have the hiss at all times for it reassured them that the detector was working. The clockwork drive motor would need rewinding every one hour and three quarters and could be stopped at any time. There was a knob which could be turned to change the tension on the belt.

magnetic-detectors-g3bdq-1 magnetic-detectors-g3bdq-2
Magnetic detectors could not receive unmodulated CW but were ideal for receiving spark (or later) ICW transmissions and telephony. The ‘Maggies’ proved to be very reliable and many remained in use right into the thermionic valve era, only being discarded during and just after WW1. I have been fortunate in meeting, in the late 1940’s and 1950’s several elderly radio amateurs who had owned and operated magnetic detectors. One of these was the late Hugh Ryan G5BV who said that they were prohibitively expensive for amateur use.

By John Heys G3BDQ – Vital Spark March 2012.

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