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by Shawn Carlson
Mar, 2000

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In January of last year I described a delightful device for detecting microfluctuations in the earth's magnetic field. The instrument was a sensitive torsion balance consisting of two small rare-earth magnets affixed to a taut nylon fiber with a tiny mirror attached to the fiber to reflect a laser beam onto a distant wall. When the instrument was properly nulled with additional magnets to cancel the earth's average magnetic field, an infinitesimal change in the earth's field rotated the rare-earth magnets and deflected the laser beam.

Originally developed by Roger Baker of Austin, Tex., this homemade magnetometer created quite a stir in the amateur community. But the device required constant visual monitoring to collect data, so it wasn't really suitable for serious science. Baker, however, suggested how someone could convert his unit into a research-grade instrument. This month I'm delighted to report that Joseph A. Diverdi, a chemist in Fort Collins, Colo., has met that challenge brilliantly.

Following Baker's suggestions, Diverdi placed the magnetometer at the center of a pair of Helmholtz coils, a special electromagnet that produces an extremely uniform magnetic field. Diverdi also designed a detector that could sense tiny displacements in the laser beam's position, and he developed a feedback circuit that runs just enough current in the coil to create a countermagnetic field that precisely cancels any external shift. The current necessary to keep the beam fixed thus tracks the changing field, and a personal computer can record these measurements directly through an analog-to-digital converter.

You'll find a complete description of the Baker-Diverdi magnetometer on Diverdi's Web site. I've reserved this column to give an overview of the device and to offer some fine-point kibitzing.

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For the Helmholtz coils, Diverdi started with two identical stiff cardboard rings about 6.5 centimeters (2.6 inches) in diameter, but you can use larger rings cut from a cylindrical oatmeal container. Mount the rings on a wooden base parallel to each other at a separation equal to their diameter [see illustration].

Next, purchase a spool of 30-gauge enamel-coated magnetic wire from an electronics supply store and neatly wrap each ring with 40 turns of the wire. Use one continuous length for both coils so that the same current passes through them and wrap them both in the same direction, either clockwise or counterclockwise. Secure the wire loops with a liberal dose of hot glue.

Diverdi soldered two lead wires to the coils and insulated the joints with shrink-wrap tubing. He hot-glued the tubing to the base of the assembly, leaving some slack so that the wires wouldn't break while the coils were being handled. Because the magnetic field is most uniform at the center of the Helmholtz assembly, be certain to position the rare-earth magnets of the magnetometer there.

To detect minute changes in the laser beam's position, Diverdi has devised a clever solution. He shines the beam on a small slide of frosted glass with two cadmium sulfide photoelectric cells (Radio Shack part no. 276-1657) positioned a few centimeters behind. The glass spreads the beam, which illuminates the photocells. If the beam is centered directly between the cells, equal amounts of light will shine on each of them. If, however, the beam is displaced even slightly, the output of the photocells will change measurably.

By covering the glass with a filter made from several layers of red cellophane, Diverdi makes the apparatus less sensitive to any stray light that could corrupt the measurements. The cellophane allows the red laser light through while blocking most other wavelengths.

Diverdi mounted the laser, Helmholtz assembly (including the magnetometer), nulling magnets and position-sensing detector on a sturdy wooden base. To isolate this equipment from household magnetic influences, he built a separate wooden enclosure to situate the instrument outdoors. He first constructed a frame slightly larger than the wooden base and about 0.6 meter high, and he nailed plywood sheets around the four sides. Diverdi also fashioned a removable plywood lid. He insulated the walls and lid with four-centimeter-thick sheets of construction insulation and attached sharpened wooden stakes at each corner of the walls to anchor the enclosure in soil. Last, he weatherproofed all exterior surfaces with a clear acrylic sealant.

You should also seal all joints with expanding foam insulation from a spray can, such as Touch 'n Foam. Furthermore, to prevent ambient light from disturbing the photoelectric cells, make the enclosure as impervious to light as possible and paint its interior a flat black.

Most delicate instruments are sensitive to temperature changes and so must be kept in controlled environments. Because it is easier to heat a volume than to chill it, scientists usually maintain a constant temperature by installing a heater to keep an enclosure warmer than its surroundings. Diverdi crafted a nifty homemade heater by using Nichrome wire and a computer fan, but a handheld hair dryer would probably work just as well.

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Diverdi also built his own thermostat circuit. I would do the same if only a few batteries were needed to power the heater (see the January column for such a circuit). But when wall current is required, as with a hair dryer, I prefer to buy my thermostats ready-made. Omega Engineering in Stamford, Conn., sells several high-precision temperature controllers, some with digital displays and programmable set points, for less than $200. I recommend the CN8591-T1 model ($165) in conjunction with a J-type thermocouple. Mount the controller and thermocouple sensor inside the wooden enclosure away from the heater's hot-air stream. Your instrument will consume less power if you periodically adjust the set point throughout the year to keep the interior about eight degrees Celsius (14 degrees Fahrenheit) warmer than the maximum expected temperature during any given season.

Diverdi partially buried his magnetometer by excavating a small plot (about six centimeters deep) well away from his house. He covered the depression with an oversize vinyl sheet and pounded the stakes of the wooden enclosure into place through slits he cut in the plastic. He then created a three-point leveling surface by driving three additional stakes into the ground at the points of an equilateral triangle inscribed within the enclosure's interior. Next, he insulated the floor space with plastic foam shipping "peanuts" and rested the wooden base, containing the laser, Helmholtz assembly, nulling magnets and position-sensing detector, on the stakes. Because the base will be stable but not watertight, you must choose a site that will have adequate drainage. Diverdi also secured a waterproof plastic cover over the entire wooden box for additional protection from the elements.

If you want to monitor the instrument's internal temperature with your home computer, you can piggyback the thermocouple signal from the controller's input. first, though, you must buffer each lead with a field-effect transistor (FET) operational amplifier such as the LF411CN, which has a low bias current. You can purchase this part on-line for about $1 from Pioneer-Standard Electronics in Cleveland.

But thermocouples are plagued with vexing subtleties, so the novice should use an analog-to-digital converter that comes with built-in hardware to interpret the thermocouple signals. National Instruments in Austin and Vernier Software in Portland, Ore., sell such systems.

Using his device, Diverdi has obtained some impressive results. If his example inspires you to follow suit, please share your experiences through the Society for Amateur Scientists's Web page.

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Phone: 1-401-823-7800

Internet: http://www.sas.org/

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