To monitor the forces on the detector, you'll need to connect it to a computer. Baker uses the WinDAQ analog-to-digital converter, which runs from the Windows operating system. The WinDAQ/ Lite sells for about $100. Macintosh aficionados should check out the Serial Box Interface from Vernier Software. There are, of course, other options. For this application, the computer should display the data like a chart recorder, showing the shifting voltage across the coil versus time.

Install the instrument vertically on a stable base and set it on a concrete foundation as far from car and foot traffic as possible. Turn on the power and let the heater warm up. Then remove the insulation from the top and adjust the screw that raises and lowers the upper magnet while watching the output and find the location at which the signal from the phototransistor just barely turns on. This is an extremely tricky operation. At this point, a slight movement would turn the signal completely on or off. The float will naturally bob up and down at a frequency of about one second, which should be apparent on an oscilloscope. Replace the insulation.

Image: DANIELS & DANIELS OPTICAL SENSOR which can detect nanometer shifts in position, consists of an LED (left) facing a phototransistor, with an opaque flag that cuts the light beam between them (right). The beam passes through a narrow slit in the aluminum foil.

For fine adjustments, rest a ceramic magnet on top and move it around until the signal from the phototransistor just dims. The instrument should now record for weeks with only occasional adjustments to the outer magnet necessary to maintain its high sensitivity. If you are really observing extraterrestrial gravity, you should see a slowly varying sine wave that is in phase with your local tides. The detection will be less ambiguous if you live far from the shore so that secondary effects, such as swelling of the beach, do not account for the signal. Ambitious graviteers can carry out a Fourier transform on the data and look for excess power at around a period of 24 hours and also at 28 days.

You'll find more information about this project on Roger Baker's Web site. Please send your questions directly to the Society for Amateur Scientists through the ongoing discussion site. As a service to the amateur community, the society is selling the electronic components only (not the circuit board) for this project for $50 until December 2000. Mu-metal shielding is extra. You may write to the society at 4735 Clairemont Square, PMB 179, San Diego, CA 92117, or call 619-239-8807.

This article was originally published with the title Detecting Extraterrestrial Gravity.

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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