Although you can build the gravimeter frame out of any nonmagnetic material, Baker recommends using window glass because of its low cost, low thermal expansion and ease of assembly. He cuts the plates with a carbide-wheel glass cutter so that the pieces fit together without gaps and then braces the structure with triangular glass supports. He glues them all together with silicone cement. Mistakes are easy to fix because the parts can be readily separated with a razor blade.

Image: DANIELS & DANIELS BAKER'S GRAVIMETER shown in horizontal cross section, can register extremely small movements caused by gravitational pulls. (In practice, the device is mounted vertically.)

For the electromagnet, Baker winds a coil utilizing two of the unused ceramic magnets as guides. Wrap a pencil with masking tape until the magnets slide onto it snugly. Cover the magnets with more masking tape and position them about one magnet diameter apart on the pencil. Then tape one end of 30-gauge enameled magnet wire (Radio Shack part no. 278-1345B) to the edge of one of the disks and begin wrapping the wire between them. Keep a few ounces of tension on the wire and continuously coat it with 24-hour epoxy as you go. Don't stop until the coil's edge is about two millimeters below the edges of the magnets. After the epoxy sets, warm the assembly in an oven at a low setting to melt away the masking tape adhesive, then separate out the bare coil. The finished coil will be about 0.5 centimeter long and 2.7 centimeters in diameter. Its DC resistance should be about 10 ohms.

The optical position sensor requires some care. You can use an ultrabright red LED (Radio Shack part no. 276-066B) and a phototransistor (part no. 276-145A), but you'll need to eliminate the outer cases to bring the active elements as close together as possible. Grind the casings down to the chips and then polish the ends with a fine-grit polish. A combination of toothpaste and elbow grease applied against the back side of a piece of soft wood works well. Install the circuit elements into their frames with silicone cement [see illustration below]. Next, blacken three small pieces of aluminum foil with a felt-tipped marker. Carefully epoxy two of them to the LED so that they form a narrow horizontal slit; Baker says his slits are about half a millimeter wide. Use the third piece to block the bottom half of the phototransistor. This trick sharpens the device's sensitivity because it causes the signal to crash rapidly to zero as the flag cuts across the narrow beam of light emerging from the slit. Assemble the rest of the instrument as shown.

Image: DANIELS & DANIELS GRAVIMETER is shown atop its metal housing. In operation, the device is slid inside the cylinder, an iron pipe wrapped with mu-metal to block the earth's magnetic field. The resistors glued to the outside of the housing act as heaters to keep the instrument's temperature constant.

Like almost all delicate instruments, Baker's gravimeter will produce spurious results if its temperature changes. Baker solved that problem by controlling its temperature. He installed the detector inside a metal sleeve, which he kept at about 10 degrees Celsius above room temperature by employing twenty 50-ohm 1¿2-watt resistors as small heating elements. Baker sensed the temperature using a Radio Shack thermistor, the output of which he compared with an adjustable set-point voltage, determined by a potentiometer. A circuit turns on the current through the resistors whenever the thermistor signal is below the set point and turns them off when the signal climbs above it. This setup, simple though it is, can keep the temperature constant to within a few hundredths of a degree C. The metal sleeve also serves to shield the apparatus from the earth's magnetism. It consists of an iron water pipe, three or so inches in diameter and eight inches long, wrapped with layers of thin foils made of mu-metal or permalloy interposed with sheets of a nonmagnetic material such as cardboard. You can purchase mu-metal foil through the Society for Amateur Scientists for $30 per square foot. In any case, the shield needs to extend at least two pipe diameters beyond both the top and bottom of the detector to attenuate magnetic fields that enter through its open ends

1 Comments

1. 1. williampengelly 12:09 AM 12/14/08

   I read this article about "Detecting Extraterrestrial Gravity" years ago in paper format, but no longer have my copy. I am grateful to Scientific American for putting there archives online and freely search able by everyone.
   thank you

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