Detectors 166
Detectors 166 : Particle Detectors: (4.18) Magnetic Detector: (2) How it works: A, Antenna Wire; E, Ground-plate; B B. . . , Iron band around pulleys; S N. . . , Permanent magnets; C C, RF excitation winding on glass tube through which the iron band travels; D, Audio pickup winding; T, Telephone receiver. The Marconi version consisted of an endless iron band built up of 70 strands of number 40 gage silk-covered iron wire. In operation, the band passes over two grooved pulleys rotated by a wind-up clockwork motor. The iron band passes through the center of glass tube which is close wound with a single layer along several millimeters with number 36 gage silk-covered copper wire. This coil functions as the radio frequencyexcitation coil. Over this winding is a small bobbin wound with wire of the same gauge to a resistance of about 140 ohms. This coil functions as the audio pickup coil. Around these coils two weak permanent horseshoe magnets are arranged to magnetize the iron band as it passes through the glass tube. The magnet poles are arranged to create two opposite magnetic fields each directed toward (or away) from the center of the coils in opposite directions along the wire. This functions to magnetize the iron band first in one direction as it approaches the center of the coils, then reverse its magnetism to the opposite direction as it leaves from the other side of the coil. This causes a reversal of the iron band's magnetism just as it passes through the coils. This continuous reversal in magnetism induces a very weak DC current in the audio pickup coil. The radio signal from the antenna is tuned and passed through the excitation coil, the other end of which is connected to ground
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