Illustration of Magnetic detector

1896,1902 Magnetic detector

The book of science

Tom Sharp

Ernest Rutherford, Guglielmo Marconi electromagnetism Illustration of Magnetic detector

Magnetic detector

Ernest Rutherford made a device that detected Herzian waves based on the principle that an alternating current of decreasing amplitude demagnetizes iron. He placed a bundle of magnetized iron wires in a coil attached to a Hertzian receiver. He determined loss of magnetization with a small magnet suspended near the bundle. * Many others showed ways to improve the device. * Guglielmo Marconi knew he needed a more sensitive detector than a coherer. He created two magnetic detectors. In the first, a horseshoe magnet spun about the aerial coil. In the second, a loop of wires revolved though coils. A wind-up clockwork motor drove a loop of iron wires through the aerial coil. Two horseshoe magnets, one on each side, remagnetized the wires, first one way, then the other. A pickup coil attached to a telephone receiver detected a change of magnetization. Because of hysteresis, the position where the magnetization flips was away from the center in the direction that the wire moves, but a signal erased the magnetization, so that the flipping point moved to the center, which crosses the pickup coil and produced a sound. Unfortunately, the operator had to distinguish the signal from random noise caused by magnetic domains flipping orientations, which Heinrich Barkhausen explained later.

Magnetic detectors

E. Wilson showed that the antenna coil in Rutherford’s simple detector could have additional wires attached to a battery to remagnetize the bundle of wires. J. A. Fleming showed that a rotating cam could control when the wires were remagnetized and a coil attached to a galvanometer could indicate when the wires were demagnetized. Shoemaker showed that a loop of iron wire could pass by a permanent magnet and through two coils, an aerial coil and a pickup coil. In another embodiment, J. A. Fleming showed that a loop of iron wire could pass through four coils in series. The first magnetized the wire, the second imposed a tone on the wire, the third demagnetized the wire when a signal was received, the fourth connected to a telephone receiver so that the absence of the signal could be heard. C. Tissot showed two magnetic detectors in antiphase could cancel out low-frequency oscillations but not the Barkhausen noise from magnetic domains snapping into new orientations. J. G. Balsillie used several small aerial coils rotated in a fixed magnetic field and overwound with a secondary coil connected to a telephone receiver. Lee de Forest strengthened the effect of the aerial coil with windings both inside and outside a metallic cylinder. Reginald Fessenden’s magnetic detector used an iron disk and two alternating current sources to produce a rotating magnetic field. Windings on the disk carry the aerial current and connect to a speaker. E. Wilson built a hysteresis detector inside a telephone earpiece or armature and contact so that the signal could either be heard or connected to a recording device. L. H. Walter and J. A Ewing built a detector in which a spool with an aerial coil is allowed to rotate while immersed in oil and surrounded by a rotating electromagnet. They found that the superimposed fields increased the strength of the magnetization and therefore increased the hysteresis effect.

Hysteresis of iron

Iron resists change. An extra force is needed to overcome its history, especially if you rush it.

This field of research is richer with branches than the final results might have suggested. These magnetic detectors, like the coherer, worked only for detecting the presence of damped-wave Morse-code transmissions (where the transmitter was turned on briefly for sending a dot and a bit longer for sending a dash), and were made obsolete by continuous-wave transmission for AM radio.

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