THERE'S A LOT GOING on down among the microns. What we perceive as a rigid surface squashes easily under a finger's gentle pressure when viewed from a distance of a millionth of a meter. Increasing the temperature sends objects at that scale into even more violent upheavals.

Biological processes reshape many living things on this scale. For example, every beat of an insect's dorsal vessel--essentially, its heart--flexes its abdomen by a few microns.

Now, thanks to John R. B. Lighton, a biologist at the University of Nevada, these tiny movements can be readily detected. (Lighton is not only a world-renowned physiologist but also a kindred spirit to amateur scientists everywhere, always striving to find the most direct and least expensive solution to vexing experimental challenges.) He realized that by detecting the microscopic flexings of an insect's body, he could in effect put a tiny stethoscope on the creature. This technique opens all kinds of micromotions for study, including the slight distortion of materials caused by changes in ambient temperature and pressure.

Lighton's ingenious method allows experimenters to embark on a fantastic voyage into the microscopic universe. Now anyone can detect movements as small as half a micron-about the wavelength of visible light-for less than $40.

Lighton senses micromotions by using minuscule magnets that he attaches to the moving objects. He then relies on a special sensor that picks up the variations in the magnetic field caused by the shifting magnet.

The sensitivity of Lighton's detector depends on the fact that all magnets are dipolar; they have a north pole on one end and a south pole on the other. These poles would cancel each other perfectly if they weren't separated by the length of the magnet. This self-cancellation quality makes the strength of a magnetic field fall quite fast over space. Tripling the range to the magnet weakens the field by a factor of 27-the cube of the distance. The size of the magnet sets the scale by which this falloff can be quantified. The closer together the magnetic poles are (that is, the smaller the magnet), the more rapidly the magnetic field changes over distance. That in turn produces a larger signal for a micron-size shift.

It's easy to get micromagnets. You can buy so-called rare-earth magnets from Radio Shack (part number 64-1895), which sell for less than $2 a pair. They are tiny disks about 0.48 centimeter in diameter and 0.16 centimeter high (3/16 by 1/16 inch). At the surface, the magnetic field, which is oriented perpendicular to the flat part of the disk, is about 20,000 times that of the earth.

If these magnets are too big for your project, then crush one with a pair of pliers. Made from a brittle ceramic, they will shatter into little shards. You need to make sure, though, that you know the direction in which the magnetic field of these shards points. Using nonmagnetic tweezers, position a fragment on a piece of wax paper. Placing the second magnet underneath the paper forces the fragment to align with the bigger field. Then dab a dollop of paint or five-minute epoxy over the magnetic speck. Once it sets, the magnetic fragment will easily peel off the paper. Make at least 10 of these magnetic chunks, all slightly different in size, to see which one works best for your application.

A Hall effect transducer (HET) senses the changes in a magnetic field. A modern-day silicon miracle, it is small, extremely sensitive and easy to use. Lighton recommends model SS94A1F from Honeywell Micro Switch in Freeport, Ill.; call (800) 537-6945 for a distributor. A bargain at less than $20, this device changes its output by 25 millivolts for each one-gauss shift in a magnetic field.

Secure your HET no more than one centimeter away from your subject. For instance, if you are monitoring insects, you can epoxy the HET to a rigid piece of plastic and hold it above the subject with a device called Helping Hands, a soldering aid sold by Radio Shack. A HET records all magnetic fields, including the earth's. This indiscriminateness means that the detector will always produce a large constant voltage signal (created by the earth and the magnet). On top of this voltage constant will be the small changing signal created by the magnet's motion.

You can forget about trying to boost the signal with a single operational amplifier (op-amp). A single op-amp cannot accurately amplify a small signal on top of a voltage constant. What you need is an instrumentation amplifier. Like op-amps, instrumentation amplifiers are available as inexpensive, integrated circuits. Entry-level devices cost about $5; the Cadillacs of these chips sell for about $20. The AD524 from Analog Devices in Norwood, Mass., works well; to order, call (800) 262-5643, extension 3. You can also construct an instrumentation amplifier from three type 741 op-amps.

If you're monitoring temperature or another signal that varies slowly, use Lighton's slowly varying signal rendition of the circuit. For flexing insect abdomens and other activities that change significantly over 30 seconds or so, use the quickly varying signal circuit. The circuit employs a clever technique that should be in every amateur's (and professional's) tool kit.

The trick begins by splitting in two the voltage from the HET. One signal goes into the amplifier's positive input. The other goes into a low-pass filter that only passes signals that oscillate slower than about one cycle every 30 seconds. Because an insect's heart contracts in much less time, the filter strips out that signal and passes the large constant voltage (the DC offset). This filtered voltage is then fed into the instrumentation amplifier's negative input. An instrumentation amplifier boosts the difference between its two inputs, so the troublesome offset voltage is automatically subtracted, leaving only the coveted signal.

Signal wires can introduce extraneous signals. They act like antennae, picking up electromagnetic energy, such as emanations from 60-cycle power lines, and then dumping it directly into your amplifier. To minimize the effect, keep the leads between the HET and the amplifier short. Additionally, you should use shielded wire. Lighton relies on three-core shielded cables. An electronics store may stock them, or you can make your own. Twist together three different colored wires, one each for the positive, negative and signal leads of the HET. Wrap the wires inside a layer of aluminum foil and connect the foil to the circuit's ground with a short wire. For protection, add a layer of duct or electrical tape around the foil. The filter circuit provides another barrier to power-line noise. Finally, encase all your electronics inside a grounded metal project box.

You can read the output with a digital voltmeter or, better yet, use an analog-to-digital converter to record the data into a computer. Several software packages that link the signals to your computer are available [see "The New Backyard Seismology," Amateur Scientist, April]. Use shielded coaxial cable for the output connections, to prevent the HET from detecting the signal.

Lighton obtained some remarkable results after he superglued a whole rare-earth magnet to the abdomen of a Blaberus discoidalis nymph, a relative of the American cockroach. With the instrumentation amplifier's gain set to 100, the signal caused by the contractions of the dorsal vessel-the insect's heartbeats-is striking. After about 70 seconds of recording data, Lighton waved his hand in front of the nymph. The insect's heart stopped beating for several seconds. According to Lighton, that happened because the creature's nervous system may be too limited both to maintain circulation and to attend to stimuli.

Of course, any crawling by the insect will disrupt your results, so record data only when it is still. If the insect moves, it will generate a huge voltage signal that jumps well off the scale. In fact, Lighton reports that large signals occur whenever the insect opens its spiracles to breathe, about once every five to 30 minutes. By lowering the gain of your instrumentation amplifier, you can also monitor insect respiration.

Recording the vital functions of insects is just one of the experiments you can do. By attaching the magnet to the bottom of a heavy pendulum and affixing the HET to the floor one centimeter below the magnet, you can make an extremely sensitive seismograph. It can significantly extend the lower range of an amateur seismography station, as described in the April column.

By connecting the magnet to a sheet of Mylar stretched tightly over the mouth of a jar, you might record the atmospheric pressure that pushes against the membrane. Other suggestions appear on the World Wide Web site of the Society for Amateur Scientists. I invite you to invent, experiment and discover-and let me know what you find.

For more information about this project, send $5 to the Society for Amateur Scientists, 4951 D Clairemont Square, Suite 179, San Diego, CA 92117, or download it from the SAS Web site.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.