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.

Image: DANIELS & DANIELS

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.

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