Build a Proton Precession Magnetometer

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    An educational "backyard" project, constructed using easily obtained electronic parts. A

    frequency counter is used to measure the post-polarizing pulse proton precession frequency. The

    measured frequency is related, by a physical constant, to the magnitude of the local geomagnetic



    For some background information and a description of a practical application for a proton

    magnetometer, see "The Amateur Scientist "column in the February 1968 issue of Scientific

    American. Construction of a dual coil magnetometer is described. Information in that article

    formed a basis for the details shown here.

    I constructed a fluxgate magnetometer several years ago. It was based upon Richard Noble's

    article in the September 1991 issue of Electronics World + Wireless World. With a chart

    recorder, it is possible to see the dirunal changes in the east-west component of the earth's

    magnetic field, after nulling out the overpowering total and north-south components.

    After finding the February 1968 Scientific American article, I thought that it would be an

    interesting project to try adding a frequency counter to the proton magnetometer.It would be an

    interesting "backyard science" project to use it to provide a measure of the earth's total magnetic

    field. The addition of a digital to analog converter can provide a output suitable for a chart


    However, a suburban backyard environment is a rather noisy one. Harmonics of the power line

    frequency extend well up into the audio frequency range. These compete with the decaying

    precession frequency tone. Connecting the sensor coils in differential series, sensor orientation

    and instantaneous sampling of the audio signal help in contending with the noise.

    From the physical sciences a quantity called the"Larmor frequency" defines the angular

    momentum of protons precessing in the presence of a magnetic field.

    There are currently quantum-mechanical views that explain particle precession, but a classical

    explanation seems a bit easier to comprehend. A proton, a charged particle, may be thought of as

    having definite "spin" about an "axis" and acts as a small magnet. An externally applied

  • Build A Proton Precession Magnetometer Page 2 of 14

    magnetic field does not alter the spin rate, but causes the particle to wobble at a slower rate about

    an axis of precession. This axis tends to align with an external magnetic field. However in weak

    magnetic fields, any alignment tends toward randomness due to thermal effects and other

    molecular interactions.

    The proton reacts to the perturbing effects of an externally applied magnetic force by precessing

    at a rate in accordance with a precise constant called the gyromagnetic ratio. For protons this

    quantity is equal to approximately 267.53 x 1E6 radians per second per Tesla or 42.58 mHz per


    In the northern latitudes of the U.S. the total magnetic field strength is in the order of 50,000 to

    55,000 nanoTesla and varies from location to location. Short period variations due to magnetic

    storms may reach several hundred nanoTesla. Diurnal variations caused by solar induced

    ionospheric currents are in the order of tens of nanoTesla. Presently, the long term trend of the

    total field is in the order of minus 90 nanoTesla per year ( steadily decreasing).

    The proton precession frequency detected by a suitable sensor in the geomagnetic field of the

    earth will be at a frequency in the audio range:


    42.58 mHz / Tesla x 52500 x 1E-9 Tesla= 2235 Hz

    In my northeast location the frequency readings currently average about 2271 Hz, corresponding

    to a total field of about 53,300 nanoTesla. This agrees quite well with the USGS readings shown

    for the Fredericksburg, VA monitoring station , 160 miles to the west. This figure also agrees

    with the value obtained using the fluxgate magnetometer that was calibrated using a Helmholtz

    coil. The fluxgate sensor was tipped upward from a horizontal position to nearly vertical to

    obtain the maximum reading of the earth field.

    I have noticed a decrease in the frequency readings of about six or seven Hertz over the past

    twelve months or so since the sensors have been in place in the backyard. Originally the

    frequency readings were around 2277 or 2278 Hz. This also seems to agree with the magnitude

    of the predicted long term variation shown by the USGS site.

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    This is a block diagram of a "listen only" version. The frequency counting circuitry is not used.

    Only the senor coil(s) ,audio amplifier and dc power source are included. A timer IC is used to

    provide switching contol to a relay that alternately connects the sensing coil between a polarizing

    current source and the input to the audio amplifier.(Click figure for larger diagram.)

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    This is a block diagram of a magnetometer design that adds the capability to measure the

    frequency of the voltage induced in the sensor coil by the precessing protons after the application

    of a polarizing current several seconds in duration. A four decade BCD counter dis- plays

    frequency to a selectable resolution of 1 or 0.1 Hz. A frequency multiplier method employs a

    phase locked loop to provide these resolutions using counter gate intervals much less than one


  • Build A Proton Precession Magnetometer Page 5 of 14


    I found the local super market to be a good source for coils forms on which to wind the

    magnetometer coils and contain the proton medium. Check the area where the spices are located.

    Particularly look for the store brand spices. I found that these use thin walled plastic containers

    that have encircling ridges at the bottom and just below the lid. These make a form on which a

    multilayer coil can be easily wound. (CLICK FIGURE FOR DETAILS )

    The above referenced page shows the particular size used. There are a number of sizes available.

    Also found some taller ones that would provide a coil length of about 3.75 inches. A somewhat

    larger container would conveniently allow the use of a larger wire size. There are advantages ---

    lower coil resistance, providing higher coil Q and possibly higher polarizing current (if the

    power supply can provide it ). A higher polarizing current increases the initial amplitude of the

    decay signal.

    The higher coil Q will sustain the ringing effect of induced by the decay signal for a longer

    period of time.Note that the coil inductance increases as function of the square of the number of

    turns while coil resistance increases as linear function of the number of turns. This suggests that

    the best results (high Q and tuned circuit selectivity) will be obtained using the largest number of

    turns and largest wire size that is practical.Also, and possibly most important, the coils will be

    tuned by the addition of a shunt capacitor---perhaps the most important component of all.

    The coil inductance should high enough to permit the use of a reasonably valued non-polarized

    capacitor. A higher Q will also aid in providing a narrower tuned circuit bandwidth--important in

    improving the signal to noise ratio and reducing the pickup of high order power line harmonics.

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    Notes on Sensor Construction

    1.It may be possible to place the 700 turns in four layers. However, as subsequent

    layers are added it becomes more difficult to maintain close spacing. Most likely it

    will take five layers. Actual turns count is not critical. If you have 700 turns before

    reaching the end of the bottle, continue winding to complete the final layer.

    2. Coil constructed as shown will provide an inductance of about 10 millihenries. An

    approximate formula (neglects a small multilayer correction factor of about negative 5

    percent) for calculating the inductance is:


    where: r=one half the bottle diameter in inches n= number of turns l= coil length


    3. A coil tuning capacitor for two sensor bottles as shown, connected in series, will be

    about 0.25 microfarads.

    4. After winding, fill the bottle with a "proton rich" fluid. Distilled water, kerosene,

    methanol have been used. Common isopropyl alcohol will work.

    5. Spice bottles are not designed to hold liquids. The lids may have a paper inner liner

    that should be discarded. If needed to stop leaking, try making a gasket from bicycle

    inner tube or similar material.

    In my backyard environment, for the best signal to noise ratio, I found that two

    identical coils were useful. These were connected in series and oriented for

    minimizing the level of power line harmonics. An orientation with the coil axes in line

    and electrically series opposing provided a degree of cancellatio


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