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. 1—24 ). 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.
Read more about this topic: Magnetic Detector
Famous quotes containing the word history:
“... in a history of spiritual rupture, a social compact built on fantasy and collective secrets, poetry becomes more necessary than ever: it keeps the underground aquifers flowing; it is the liquid voice that can wear through stone.”
—Adrienne Rich (b. 1929)
“Let us not underrate the value of a fact; it will one day flower in a truth. It is astonishing how few facts of importance are added in a century to the natural history of any animal. The natural history of man himself is still being gradually written.”
—Henry David Thoreau (1817–1862)
“There has never been in history another such culture as the Western civilization M a culture which has practiced the belief that the physical and social environment of man is subject to rational manipulation and that history is subject to the will and action of man; whereas central to the traditional cultures of the rivals of Western civilization, those of Africa and Asia, is a belief that it is environment that dominates man.”
—Ishmael Reed (b. 1938)