Magnetic detector

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Marconi's wireless magnetic detector (London)

The magnetic detector or Marconi magnetic detector, sometimes called the "Maggie", was an early

RMS Titanic which was used to summon help during its famous 15 April 1912 sinking.[6]

History

One of the first prototype magnetic detectors built by Marconi in 1902, in Milan museum. The sensing coils on this instrument are removed.
Recreation of a Marconi ship radio room at the Aalborg Maritime Museum, Aalborg, Denmark. A magnetic detector is on the desk to right of the Marconi tuner receiver, which provided the signal for the magnetic detector.

The primitive

spark gap radio transmitters used during the first three decades of radio (1886-1916) could not transmit audio (sound) and instead transmitted information by wireless telegraphy; the operator switched the transmitter on and off with a telegraph key, creating pulses of radio waves to spell out text messages in Morse code. So the radio receiving equipment of the time did not have to convert the radio waves into sound like modern receivers, but merely detect the presence or absence of the radio signal. The device that did this was called a detector. The first widely used detector was the coherer
, invented in 1890. The coherer was a very poor detector, insensitive and prone to false triggering due to impulsive noise, which motivated much research to find better radio wave detectors.

Lee De Forest, J.C. Balsillie, and L. Tieri had subsequently devised detectors based on hysteresis, but none had become widely used due to various drawbacks.[7] Many earlier versions had a rotating magnet above a stationary iron band with coils on it.[8]
This type was only periodically sensitive, when the magnetic field was changing, which occurred as the magnetic poles passed the iron.

During his transatlantic radio communication experiments in December 1902 Marconi found the coherer to be too unreliable and insensitive for detecting the very weak radio signals from long-distance transmissions. It was this need that drove him to develop his magnetic detector. Marconi devised a more effective configuration with a moving iron band driven by a clockwork motor passing by stationary magnets and coils, resulting in a continuous supply of iron that was changing magnetization, and thus continuous sensitivity (Rutherford claimed he had also invented this configuration).[8] The Marconi magnetic detector was the "official" detector used by the Marconi Company from 1902 through 1912, when the company began converting to the Fleming valve and Audion-type vacuum tubes. It was used through 1918.

Description

(A) Antenna wire, (B,B) Iron band around pulleys, (C, C) RF excitation winding on glass tube through which the iron band travels, (D) Audio pickup winding, (E) Ground-plate, (S, N) Permanent magnets, (T) Telephone receiver.

See drawing at right. The Marconi version consisted of an endless iron band (B) built up of 70 strands of number 40 gage silk-covered

resistance of about 140 ohms. This coil (D) functions as the audio pickup coil. Around these coils two permanent horseshoe magnets are arranged to magnetize the iron band as it passes through the glass tube.[1]

How it works

The device works by

coercive field, Hc) is required to reverse the magnetization. So the magnetization in the moving wires does not reverse in the center of the device where the field reverses, but some way toward the departing side of the wires, when the field of the second magnet reaches Hc.[1][2]
Although the wire itself is moving through the coil, in the absence of a radio signal the location where the magnetization "flips" is stationary with respect to the pickup coil, so there is no flux change and no voltage is induced in the pickup coil.

The radio signal from the

earphone) (T) which converts the current pulse to sound.[2]

The radio signal from a spark gap transmitter consisted of pulses of radio waves (

damped waves) which repeated at an audio rate, around several hundred per second. Each pulse of radio waves produced a pulse of current in the earphone,[1]
so the signal sounded like a musical tone or buzz in the earphone.

Technical details

Magnetic detector in use

The iron band was turned by a mainspring and clockwork mechanism inside the case. Differing values have been given for the speed of the band, from 1.6 to 7.5 cm per second; the device could probably function over a wide range of band speeds.[8] The operator had to keep the mainspring wound up, using a crank on the side. Operators would sometimes forget to wind it, so the band would stop turning and the detector stop working, sometimes in the middle of a radio message.

The detector produced

Barkhausen noise due to the Barkhausen effect in the iron.[9] As the magnetic field in a given area of the iron wire changed as it moved through the detector, the microscopic domain walls between magnetic domains
in the iron moved in a series of jerks, as they got hung up on defects in the iron crystal lattice, then pulled free. Each jerk produced a tiny change in the magnetic field through the coil, and induced a pulse of noise.

Because the output was an audio alternating current and not a direct current, the detector could only be used with earphones and not with the common recording instrument used in coherer radiotelegraphy receivers, the siphon paper tape recorder.[10]

From a technical standpoint, several subtle prerequisites are necessary for operation. The strength of the magnetic field of the permanent magnets at the iron band must be of the same order of magnitude as the strength of the field generated by the radio frequency excitation coil, allowing the radio frequency signal to exceed the threshold hysteresis (coercivity) of the iron. Also, the

match the low impedance of the excitation coil, requiring special tuner design considerations. The impedance of the telephone earphone must roughly match the impedance of the audio pickup coil, which is a few hundred ohms. The iron band moves a few millimeters per second. The magnetic detector was much more sensitive than the coherers commonly in use at the time,[1] although not as sensitive as the Fleming valve, which began to replace it around 1912.[5]

In the Handbook Of Technical Instruction For Wireless Telegraphists by: J. C. Hawkhead (Second Edition Revised by H. M. Dowsett) on pp 175 are detailed instructions and specifications for operation and maintenance of Marconi's magnetic detector.

References

  1. ^ a b c d e f g h i Fleming, John Ambrose (1911). "Telegraph" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 26 (11th ed.). Cambridge University Press. pp. 510–541, see page 536, second para, lines 8 & 9 and figure 45. In 1902 Marconi invented two forms of magnetic detector, one of which he developed into an electric wave detector of extraordinary delicacy and utility
  2. ^ a b c d e f g h i Fleming, John Ambrose (1908). The Principles of Electric Wave Telegraphy. UK: Longmans, Green and Co. pp. 380–382.
  3. doi:10.1098/rspl.1902.0034
    .
  4. ^
    doi:10.1098/rsta.1897.0001
    .
  5. ^
    ISBN 978-1886606210
    .
  6. ^ Stephenson, Parks (November 2001). "The Marconi Wireless Installation in R.M.S. Titanic". Old Timer's Bulletin. 42 (4). The Antique Wireless Association. Retrieved May 22, 2016. copied on Stephenson's marconigraph.com personal website
  7. ^
    ISBN 0906048249
    .
  8. ^ a b c Phillips (1980) Early radio wave detectors, p. 103-105
  9. ^ a b Phillips (1980) Early radio wave detectors, p. 98, 102, 106
  10. ^ Fleming, John Ambrose (1916). An elementary manual of radiotelegraphy and radiotelephony for students and operators, 3rd Ed. UK: Longmans, Green and Co. pp. 203, 208.

External links

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