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14.   About Electronic Magnetometers
and about Smoking


8. Oersted & Ampére

9. The Lodestone

10. Gauss

11. The Magnetic Sun

12. Fluid Dynamos

13. Dynamo in the
    Earth's Core

14. Magnetometers and
    Tobacco Smoking

15. Magnetic Reversals
    & Moving Continents

16. The Magnetosphere

17. Magnetic Planets

    Fluxgate Magnetometers

      For more than 150 years the basic instrument for measuring magnetic fields resembled Coulomb's--a magnetic needle suspended at its middle from a fine fiber, or some modification of that arrangement. It was a delicate instrument, of limited accuracy, not suitable for rough handling.

      Around the time of World War II electronic instruments came into use. One type, still widely used, is the so-called fluxgate magnetometer, based on the saturation of magnetic materials.

      A typical electromagnet, such as is used in a relay or machinery, has an iron core around which the current-carrying coil is wound. The coil's magnetic field is greatly strengthened by the iron, because the iron atoms (or arrays of such atoms arranged in crystals) are magnetic.

      In ordinary iron, the magnetic axes of its atoms point in random directions, and the sum of their magnetic fields is close to zero. When current flows in the coil, however, its magnetic field lines up the magnetic axes of atoms in the core, and they add their magnetism to the one created by the electric current alone, making it much stronger.

  But there exists an obvious limit to the process: when all atoms are lined up, a condition known as the saturation magnetization of the iron, the iron core can provide no further help. If one further increases the current in the coil, the magnetic field only increases by the amount due to the electric current itself, with no contribution from the core.

  Materials exist--certain ferrites--where saturation occurs abruptly and completely, at a stably defined level. If a large enough alternating current is driven through a coil wrapped around a core of such material, the core's magnetic polarity flip-flops back and forth, and saturation occurs in each half of the cycle, in symmetric fashion.

  If however such an electromagnet is located in an existing magnetic field, directed (entirely or in part) along the axis of the ferrite core, that symmetry is upset. In the half of the cycle in which the field of the coil is added to the existing magnetization, saturation arrives a bit earlier, because it depends on the total magnetic intensity, external plus that of the coil. In the other half of the cycle, where the magnetization due to the coil opposes that of the existing field, it happens a bit later, because the sum of the two is somewhat weaker than the field of the coil alone. That asymmetry can be sensed electronically, and this is the basis of the operation of the fluxgate magnetometer.

  It does not sound like a sensitive effect--but it can be made quite sensitive by various tricks (e.g. replacing the rod-like magnetic cores with rings). A typical intensity of the magnetic field near the Earth's surface is 50,000 nanotesla (nT), while the fluxgate aboard Voyager 2 has observed with fair accuracy the interplanetary magnetic field near Uranus or Neptune, typically 100,000 times weaker. The Voyager 2 instrument resides at the end of a long boom, keeping it away from the magnetic interference of the currents aboard the spacecraft. Even though such currents are quite weak, they create enough of a magnetic field to disturb the readings of the sensitive magnetometer.

  Such instruments must be calibrated against known fields from a coil or in some other way.   Other types of electronic instruments also exist, e.g. those based on optical properties of certain metal vapors, but they are beyond the scope of this quick overview. Another kind is the proton precession magnetometer, briefly described in a lesson plan of the web course "From Stargazers to Starships" and involving the process of precession. It is the basis of "magnetic resonance imaging," a medical procedure for viewing "soft" internal organs which x-rays cannot observe, without any of the radiation damage which x-rays cause .

A Magnetometer study on the effect of Smoking

  Sensitive electronic magnetometers have many uses. They are of course indispensable aboard satellites, and on airplanes mapping the local structure of the Earth's field, e.g. when searching for oil. Airport gates use them for the detection of firearms, while stores and libraries tag their materials magnetically and use such gates to prevent anauthorized removal. The navy uses them to detect submarines under water, and they help surveyors locate boundary stakes buried in the ground or hidden by vegetation.

  Perhaps the most striking use of such an instrument was in the medical experiments of Dr. David Cohen at the Massachussetts Institute of Technology (MIT). Cohen's lab was lined with screening coils whose current canceled most of the outside geomagnetic field. Inside the lab he built a small room which shielded out any remaining magnetic influence. It had five sets of walls nested one inside the other, like Russian matrioshka dolls, separated by alternate layers of iron (to keep out constant magnetic fields) and aluminum (to shield against electromagnetic fluctuations).

  No detectable magnetic field reached the interior of the room, and some extremely sensitive magnetic observations could be conducted there. Cohen experimented there with magnetic signals from the heart and the brain, but his most intriguing result, published in 1979, concerned the human lungs. Air passages in the human body are lined with hair-like cilia, constantly waving back and forth and thus slowly sweeping out any dirt or debris deposited in them (Cohen called them "the moving carpet"). To find how well the lungs cleaned themselves in this fashion, Cohen had a dozen volunteers inhale small amounts of iron oxide dust, which is harmless but can be magnetized.

  Over the year that followed the quantity of dust remaining in their lungs was measured periodically, as follows. First each subject stood between a pair of coils, through which a large current was briefly passed. This magnetized the dust grains inside the lungs and lined them up in the same direction; since the grains gradually shifted out of alignment, they needed to be remagnetized at each visit. The subjects then climbed into the shielded room, where the strength of the magnetization of their chest area was measured.

  During the year of observations the amount of dust declined steadily in all subjects, first steeply and then more gradually, ending at about 10% of the original level. This showed that the lungs cleaned away debris quite efficiently. The surprise came from 3 additional subjects, added as an afterthought, all of them heavy smokers. Their lungs cleaned themselves much more slowly, and after one year, about 50% of the dust still remained.

  Cohen concluded that heavy smoking not only deposited tars in the lungs but also impaired their capacity to clean themselves. He speculated this might explain why a combination of heavy smoking and exposure to asbestos was associated with lung cancer far more frequently than might be expected by simply adding together the effects of smoking and asbestos separately. Not only did the asbestos promote cancer, but the tobacco tars and smoke hampered the natural process by which the lungs were sweeping it away.

Further Reading

  Cohen, David et al., Smoking Impairs Long-Term Dust Clearance from the Lung, Science, 204, 514-7, 4 May 1979

For readers with a technical background: A History of Vector Magnetometry in Space by Robert C. Snare, Institute of Geophysics and Planetary Physics, UCLA.

Ness, Norman F., Magnetometers for space research, Space Sci. Rev.,11, 459-554, 1970

Next Stop:   15. Magnetic Reversals and Moving Continents

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   earthmag("at" symbol)

Last updated 25 November 2001
Re-formatted 19 March 2006