Detectors 165
Detectors 165 : Particle Detectors: (4.18) Magnetic Detector: (1) History: During his transatlantic radio communication experiments in December 1902 Marconi found the coherer to be too unreliable/insensitive for detecting the very weak signals inherent in long distance transmissions. It was this need that drove him to develop the "Maggie" or magnetic detector. The earliest development models and earliest patent of the magnetic detector had a rotating magnet above a stationary segment of iron band with coils on it. It was based on the theory of operation proposed by Rutherford in 1896 (Phil. Trans. Roy. Soc. London A V. 189, pp. 124). Further developments by Marconi, et al. resulted in a more effective configuration with the moving iron band driven by a clockwork motor and stationary coils. Sir John Ambrose Fleming writes in The principles of electric wave telegraphy and telephony: It was well known long before the middle of the last century that the discharge of a Leyden jar had a magnetizing power. Sir Humphry Davy magnetized sewing-needles with Leyden jar discharges in 1821. Joseph Henry, in the United States, between 1842 and 1850, explored many of the puzzling facts connected with this subject, and only obtained a clue to the anomalies when he realized that the discharge of a condenser through a low resistance circuit is oscillatory in nature. Amongst other things, Henry noticed the power of condenser discharges to induce secondary currents which could magnetize steel needles even when a great distance separated the primary and secondary circuits. He employed this magnetization to test the direction of the secondary currents, and he was followed in the same field of research by Abria, Marianini, Riess, and Matteucci. In 1870 Lord Rayleigh, in discussing some electromagnetic phenomena, pointed out that the resultant magnetic effect of an oscillatory discharge depends upon the direction of the maximum value of the current during the oscillation, and also that there may be superimposed magnetic effects in the same needle. In 1895 the subject was again taken up by Professor E. Rutherford, and in a very able paper, published in 1896, he described experiments he had made on the subject
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