Magnetic detector
Encyclopedia
The magnetic detector was one of the first practical devices able to make radio
signals audible through a pair of headphones
. It was not as sensitive as some detectors
but despite that, it was favored for early maritime use because of its reliability.
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 [1897]). 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:
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 frequency
excitation coil. Over this winding is a small bobbin wound with wire of the same gauge to a resistance
of about 140 ohm
s. This coil functions as the audio
pickup coil. Around these coils two weak permanent horseshoe magnet
s are arranged to magnetize
the iron band as it passes through the glass tube.
The magnet poles
are arranged to create two opposite magnetic field
s 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
. That radio signal in the excitation coil aids the reversal of the magnetization of the iron band as it passes under the audio pickup coil. This works by agitating the magnetic domains
to reduce reluctance in the iron band. Changes in the amplitude
of the radio signal change the rate of magnetic reversal in the moving iron band. This causes variation in the magnetic flux
through the audio pickup coil causing the current in the audio pickup coil to vary. The audio pickup coil is connected to a telephone receiver which converts the varying current to sound
.
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 significantly contribute to the total the magnetic field. Also, the radio signal applied to the excitation coil roughly matches
the impedance
of the excitation coil which is very low and requires a 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. Its sensitivity does not approach that of a good mineral detector, yet it is a reliable and surprisingly sensitive detector
for radio.
in 1902 through 1918, although it was soon supplanted by crystal receivers and multi-element vacuum tubes.
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
signals audible through a pair of headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
. It was not as sensitive as some detectors
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
but despite that, it was favored for early maritime use because of its reliability.
History
During his transatlantic radio communication experiments in December 1902 MarconiMarconi Company
The Marconi Company Ltd. was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal Company...
found the coherer
Coherer
The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
to be too unreliable/insensitive for detecting
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
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 [1897]). 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.
How it works
The Marconi version consisted of an endless iron band built up of 70 strands of number 40 gage silk-covered ironIron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
. In operation, the band passes over two grooved pulleys rotated by a wind-up clockwork
Clockwork
A clockwork is the inner workings of either a mechanical clock or a device that operates in a similar fashion. Specifically, the term refers to a mechanical device utilizing a complex series of gears....
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
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
wire. This coil functions as the radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
excitation coil. Over this winding is a small bobbin wound with wire of the same gauge to a resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of about 140 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s. This coil functions as the audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
pickup coil. Around these coils two weak permanent horseshoe magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
s are arranged to magnetize
Magnetization
In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...
the iron band as it passes through the glass tube.
The magnet poles
Magnetic pole
Magnetic pole may refer to:* One of the two ends of a magnet* The magnetic poles of astronomical bodies, a special case of magnets, two special cases of which are the Geomagnetic poles:...
are arranged to create two opposite magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s 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
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
is tuned and passed through the excitation coil, the other end of which is connected to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. That radio signal in the excitation coil aids the reversal of the magnetization of the iron band as it passes under the audio pickup coil. This works by agitating the magnetic domains
Magnetic domains
A magnetic domain describes a region within a magnetic material which has uniform magnetization. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction...
to reduce reluctance in the iron band. Changes in the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of the radio signal change the rate of magnetic reversal in the moving iron band. This causes variation in the magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
through the audio pickup coil causing the current in the audio pickup coil to vary. The audio pickup coil is connected to a telephone receiver which converts the varying current to sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
.
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 significantly contribute to the total the magnetic field. Also, the radio signal applied to the excitation coil roughly matches
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
the impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of the excitation coil which is very low and requires a 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. Its sensitivity does not approach that of a good mineral detector, yet it is a reliable and surprisingly sensitive detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
for radio.
Marconi Company usage
The "Maggie" was the "official" detector used by the Marconi CompanyMarconi Company
The Marconi Company Ltd. was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal Company...
in 1902 through 1918, although it was soon supplanted by crystal receivers and multi-element vacuum tubes.
Maintenance
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 the Marconi's Wireless magnetic detector.External links
- The Marconi magnetic detector From the book "A Handbook of Wireless Telegraphy" (1913) by J. Erskine-Murray. D.Sc.