"Maggie" Magnetic Detector


SPECIFICATIONS

Type: Magnetic Detector, Maggie for slang
Frequency Range: 100 to 270 metres?
Circa: First patented in 1902

Marconi's Magnetic Detector was one of the first practical devices able to make radio signals audible through a pair of earphones and was first patented in 1902. It was an important advance being more sensitive than the coherer. In conjunction with the Multiple Tuner, the Magnetic Detector was the standard form of telegraphic detecting device in both ship and shore installations until it was gradually superseded by the crystal detector and later by the vacuum tube.

INITIAL USES

In the early 1900's Guglielmo  Marconi, introduced two important developments into the world of radio. The first of these was a transmitter with a separate oscillating circuit coupled by a transformer to the aerial circuit. His aim was to produce higher power than was possible with the old 'plain aerial'. Not only was he successful in this but the resulting transmissions were found to be on two separate wavelengths. These were known as Tune A and Tune B, the first being on a wavelength of about 100 m. with a range of 50-70 miles and the second on 270 m. with a range of 80-150 miles. Separate sets were at first produced for Tune A and Tune B so they could be used at the same time without interfering with each other. Later, combined sets were developed which could work on either Tune A or Tune B.

The second advancement made by Marconi was his magnetic detector or 'Maggie' which replaced the coherer as a receiver and could be used with headphones. It proved much more sensitive and it was found possible to raise the speed of communication from ten to twenty words a minute. The British Admiralty at first turned down the 'Maggie' as they were reluctant to rely on the ears of a single young signalman and preferred the 'inker' which recorded a message which could be checked by others later. It is assumed that the Maggie detector was used to copy the telegraphic signals sent on Tune A and Tune B.

maggie.jpg
Photo 1 - The Maggie detector principle was based upon the effect of high frequencies on the magnetic characteristics of iron. (Photo via Le Musée Québecois de la Radio)
By the end of 1906 the British Admiralty decided to have two types of wireless sets for the larger warships. In 1907, 'Service Installation Mark 1' grew out of the 'Standard Installation 1905' and was of moderate power (1.5 kw.) and able to transmit on five tunes from Q to U. It was powered from the ship's mains with a rotary converter. The operator sat in  a silent cabinet and received the signals using headphones and a receiver which incorporated the Marconi magnetic detector.

Advancements in receiver technology saw the debut of the crystal detector. This was much more sensitive and less prone to interference by atmospherics than earlier systems but, at first, was less robust than Marconi's magnetic detector which continued to be used for many more years as a standby device.

PRINCIPLE OF OPERATION

The World of Wireless web page provides this excellent explanation on the operation of the magnetic detector.

"The Marconi magnetic detector consists of an endless band built up of 70 strands of number 40 silk-covered iron wire. The band passes over two grooved pulleys which are kept in rotation by a clockwork motor and at a certain point in its journey passes through a small glass tube wound for a length of about two centimetres with a layer of number 36 silk-covered copper wire the ends of this wire which form the primary winding being brought out to terminals. Over this winding is a small bobbin wound with wire of the same gauge to a resistance of about 140 ohms, this forms the secondary winding and the ends are taken to terminals to which the telephone receivers are also connected. Above the coils are arranged two permanent horseshoe magnets, with like poles together as shown in Photo 1. The detector depends for its action on the fact that electrical oscillations have the ability to annul the magnetic hysteresis of iron.

Consider now the magnetic detector itself. We have here a soft iron band passing before the poles of two permanent magnets, as each portion of the band passes the poles it becomes magnetized and by the action of the clockwork motor this magnetized portion is carried forward. If now electrical oscillations pass through the primary windings the hysteresis of the band is annulled and the magnetized portion which has moved out of the field of the magnet has its magnetism destroyed and a redistribution of the lines of the force through the secondary winding takes place, which sets up a current in it and the telephone receivers which are connected to it and a sound is thus produced.

Photo 1 shows the instrument as manufactured by the Canadian Marconi Company; it will be seen that there are two sets of coils and magnets. The clockwork and moving iron band being common to both. In the event of one side breaking down all that is necessary is to change over to the other side. On the left hand of the instrument is the winding key and the key to start or stop the clockwork, the adjusting screw at the right is to regulate the tension of the moving iron band.

Figure 51 is a diagram of the detector and shows the magnets in the most sensitive position -i.e. with like poles together. In this position, although very sensitive, a breathing noise is sometimes produced in the telephone which is very disturbing when reading weak signals. 
maggie_fig52.gif This can be overcome by placing the magnets, as in (Fig.52), with unlike poles together, the pole of one being some little distance up the limb of the other, or moving the magnets away from the band, the best position being found by experiment. The magnets used on this detector have one face brightly polished and the other blackened. When both bright faces or both black faces are to the front like poles will be together, when one bright and one dull face to the front unlike poles will be together. In practical use this detector has proved itself to possess the great merit of reliability. It is also sensitive and needs practically no attention beyond occasional winding of the clockwork."


Contributors and Credits:

1) Le Musée Québecois de la Radio - Jacques Hamel, VE2DJQ
2) Electronics and Sea Power. V-Adm Arthur Hezlet. 1975  Stein and Day Publishers, NY.NY
3) World of Wireless   http://home.luna.nl/~arjan-muil/radio/Magnetic.html

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Dec 6/08