The Marconi magnetic detector.
From the book "A Handbook of Wireless Telegraphy" (1913) by J. Erskine-Murray. D.Sc.
James Erskine-Murray was a British
scientist, born at Edinburgh, October 24th, 1868. Erskine-Murray was for six
years associated in study and research with Lord Kelvin at Glasgow University.
He afterwards went to Trinity College, Cambridge, as a research student, and
from 1896 to 1898 was assistant professor of physics and electrical engineering
in the Heriot-Watt College, Edinburgh. In 1898 he became experimental assistant
to Marconi, and in 1900 lecturer and demonstrator in physics and electrical at
University College, Nottingham. In 1905 Erskine-Murray became lecturer in
electrical engineering at the George Coates Technical College, Paisley, and the
same year became a consultant on radiotelegraphy. From 1907 to 1911 he was
lecturer in that subject at the Northampton Institute, London.
Erskine-Murray is a past president of the Wireless Society of
London (1921), and the author of several books on wireless. During the First World War he was
in charge of the wireless instruments and experimental work in the Royal Air
Force.
James Erskine-Murray died in 1927.
DETECTION OF OSCILLATORY CURRENT OF HIGH FREQUENCY BY THEIR EFFECTS ON MAGNETISED IRON.
THE effect of an oscillatory magnetic field on magnetised steel or iron differs considerably in different circumstances. A permanent change, however, of the state of magnetisation of the iron is almost always apparent. The results depend on the qualities and magnetisation of the material, on the intensity and character of the oscillations, and on the relative directions of the magnetisation and the oscillatory field. It has been known since the earlier part of last century that the discharge of a Leyden jar will magnetise or demagnetise steel needles. Professor Joseph Henry made a large number of observations on the phenomena, and threw much light on the nature of the discharge of a condenser, but it was not until 1895 that the possibility of detecting, by this means, electric waves which had travelled considerable distances was proved by Mr Rutherford. In that year, by using a large Hertzian oscillator with plates about 1 by 2 m. and a resonator consisting of two copper rods about 30 cm. long, he was able to detect the radiations at distances up to three-quarters of a mile. The rods of the resonator were connected by a small helix of fine wire, in the axis of which was placed a magnetised needle or bunch of very fine steel wires. A magnetometer indicated the amount of magnetism in the needle, and decrease of its deflection showed the arrival of the waves. After each observation it was necessary to remagnetise the needle, no automatic arrangement for this purpose being provided. Professor Rutherford suggested to me in 1900 that the oscillations might be made to record themselves on a steel band, as in the telegraphone. I was, however, too fully employed with other work at the time to undertake the experimental work necessary to produce a practical wireless telegraph receiver. In 1900 the author constructed an automatic magnetic receiver. It was similar to Rutherford's first apparatus, but the remagnetisation of the needle was effected by a local battery whose circuit was closed by the motion of the magnetometer. This appears to be the first telegraphic receiver in which the magnetic detector was employed. Three years later, in 1903, Mr Marconi patented two forms of magnetic detector somewhat different in principle from those already described. Instead of allowing the steel to remain demagnetised for an appreciable time and then remagnetising it by closing a local circuit, he applied a slowly alternating field to the iron core.
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As the core goes through its cycle of magnetisation, the latter lags behind the applied force. If, however, an oscillatory field of high frequency is suddenly superimposed on the slowly alternating one, the molecules of the iron are shaken up and the lag instantaneously disappears, the magnetisation jumping at once into the condition corresponding to the applied field. A similar effect is produced by a mechanical shock. The sudden change in induction through the core is observed by means of a telephone receiver in circuit with a coil of wire round the core of the detector. On the arrival of oscillatory currents at the detector, the momentary current induced by the change of magnetic induction causes a click in the telephone. Mr Marconi's second magnetic detector attains a similar end in a somewhat different way. An endless band of fine insulated iron wires is kept moving at about 8 cm. per second in the direction of its length round two wooden pulleys. Near to one part of the band two horse-shoe magnets are fixed, two similar poles being together, and the opposite ones farther along the band on either side. A large number of lines of force pass into the band under the central poles, and after travelling some distance in either direction along it reach the outer poles of the horseshoes. Wound on a glass tube, through which the band passes at this point, are two coils of fine wire. The inner one is connected to the aerial and earth wires, and the outer to the terminals of a telephone receiver. As the band travels slowly along, the lines of force are drawn out in the direction of its motion owing to the retentive property of the iron; a proportion of those which cut one side of the telephone circuit when the band is at rest are, therefore, drawn away from it. When the oscillatory currents arrive from the aerial circuit these lines are freed and fly back, cutting one side of the coil as they go. The result is a momentary current which produces a click in the telephone. Dr L. W. Austin, of the United States Naval Wireless Department, has investigated the properties of this detector. The comparisons were made with the sensibility of a zincite-chalco-pyrite (or "perikon") detector which had been standardised by means of a thermo junction. Used in the ordinary way, it was found that the magnetic required 2.1 times as much energy to give an audible sound as the perikon used without external E.M.F. Further experiments showed, however, that by putting a condenser of 0.05 mfd. in series with the telephone, thus roughly tuning this circuit to the spark rate of the signals, the sensibility of the magnetic was nearly doubled. These experiments were made with a wavelength of 900 m. Other experiments showed that the magnetic, with tuned telephone, was approximately one and a half times as sensitive as the perikon for a wave-length of 3,000 m. For a wave-length of 2,000 m. they were nearly equal, and for a wave-length of 350 m. the perikon was nearly five times as sensitive as the magnetic. By experiment and calculation Dr Austin also showed that these differences were mainly due to the difference of the effective resistance of the magnetic at different frequences. For instance for A = 350 m., R1 = 110 ohms, while for A. = 3,000 m., R1=10 ohms. It was found that the loudness of the signals in the telephone attached to the magnetic is not proportional to the square of the current, but more nearly to the 1.4 power. Each spark at the transmitter thus makes an audible sound in the receiver. As in other forms of wireless telegraph a short succession of sparks constitute a dot, and a long one a dash. Messages in the Morse code are, therefore, transmissible. This receiver is more sensitive than the coherer, and is also more easily adjusted. It has come into very general use on account of these advantages, but its inability to actuate a recording instrument is in certain circumstances a drawback, particularly if the operator be not extremely expert and the message be in cypher. In Post Office work over the ordinary wires the difficulty is obviated by the use of an automatic type - printing telegraph where possible, thus eliminating errors which might occur at the receiving end. In other cases the repetition of the message, back from office to office, is insisted on by the Post Office as essential to accuracy, when cypher or code words occur in it.
The rest of the chapter deals with other magnetic detectors.
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