Getting a Charge out of Rainby Shawn CarlsonOne of my best friends in college was a burly, carrot-topped Irishman named Michael North, affectionately known to us as Red. He was a forestry major, a brilliant chess player and one of the physically strongest men I have ever known. Red spent his summers working for the U.S. Forestry Service as a logger and occasional fire lookout in tall towers on the high peaks of the Sierra Nevada in California.
One afternoon he took a nap that nearly cost him his life. By the time the thunder woke him, it was too late to escape. The black base of the thunderhead's anvil was already poised directly overhead, only a few hundred feet above the tip of the pointed, all-metal tower. With only seconds to act, he grabbed four glass beer steins and set them on the metal floor. He hurriedly positioned a small wooden stool onto the glasses and crouched his 250-pound body atop the stool. Within moments the first bolt hit, sending the tower reeling and temporarily knocking him off his insulated perch. Over the next three hours, by Red's count, lightning struck the tower 30 more times.
The power of lightning, as I'm sure Red agrees, is one of the most awe-inspiring phenomena in nature. (If you're not already convinced, see the article on lightning.) Yet despite both the fascination and terror it evokes in most people, scientists still do not understand exactly how lightning comes about. It is clear that cosmic rays liberate large quantities of both positive and negative charge in the atmosphere by ripping away electrons from atoms in the air. These charges collect on the water droplets that make up the thundercloud. But even the experts are not sure just how these drops routinely acquire tens of millions of excess positive or negative charges to become highly charged themselves. And it is still quite a mystery how these charged droplets manage to separate from one another, forming distinct positive and negative regions within the cloud that are intense enough to generate thunderbolts. Lightning is simply too dangerous an animal to be let loose inside the amateur laboratory. But it is only one electrical component of a storm. Raindrops carry their charge with them as they fall, removing it from the cloud and depositing it into the earth. Although lightning is more dramatic, rainfall may actually transport more charge to the ground. The device described here will let you measure the charge on individual raindrops as they fall. This instrument works on the principle of electrical induction. A charged drop falling through a metal cylinder will momentarily change the electrical potential of the cylinder. A positively charged drop will raise the potential, whereas a negatively charged drop will lower it. The size of the change reveals the magnitude of the charge on the raindrop. Furthermore, the drop escapes the apparatus unscathed, and so additional measurements of it can be made. (You might, for example, want to try to measure the mass of each drop. Can you figure out how?) Our instrument is made of a plastic drainpipe 10 centimeters (four inches) in diameter and 94 centimeters (37 inches) long, some plastic funnels of the same diameter, two nested metal cylinders and a handful of electronic components. For the cylinders, I used two soup cans, one large enough to fit comfortably over the other. Electrically connect wires for the signals to the outside of the smaller can and the inside of the larger can; then insulate the cans by dipping each one in latex-based enamel paint. After the paint dries, secure the painted metal cans inside the center of the plastic drainpipe, as shown in the illustration on this page. I wrapped the outer soup can about two centimeters (one inch) from one end with three layers of double-sided foam tape (3M Corporation, part no. 051131-06439). This process builds up a flexible ring of foam. Do not remove the protective backing from the last layer of tape. Rather leave it attached so you can slide the assembly into position and have friction hold it in place. The two inverted funnels above the soup cans act as baffles to keep everything dry. And having a set of funnels to act as "rain caps" is convenient. Select a cap that will let drops enter the instrument at a manageable rate. Use a small aperture cap in a downpour and a larger aperture cap during drizzle. The bottom funnel lets you collect the drops for any chemical analysis you may wish to do. Cut the two inverted funnels so that their openings are about two centimeters narrower than that of the inner can. To fix the funnels into place above and below the cans, you'll need two rings for the inside of the drainpipe. Carefully slice two short segments from the end of some extra drainpipe and remove a centimeter or so from each ring; when compressed, the rings should fit snugly inside the drainpipe. Use Plastix (Loctite Company, item no. 82565) to rejoin the cut ends and to glue the compressed rings to the funnels. You will also need to cut slots into the pipe as shown. Use these openings to swab inside the pipe thoroughly with epoxy. Then push the funnel into position. When the epoxy sets, carefully dope the joints with silicon aquarium cement to create a waterproof seal. You want any drops that strike this funnel to exit through the slots, not drip farther into the instrument. The bottom funnel should be epoxied in place similarly. The instrument stands on a plastic flange that is fixed to a weatherproof base. Just push the assembly over the flange; do not attach it permanently.
Position this circuit between the two cans. The four-wire telephone cable runs inside your home where you can remain warm and dry. If at all possible, read this signal directly into your computer using an analog-to-digital converter. This capability isn't as exotic as it sounds. For $310, the Multipurpose Lab Interface, made by Vernier Software (Portland, Ore., telephone: 503-297-5317), will serve nicely. Alternatively, you could feed the signal into Gilbert's other nifty circuit. This dual peak detector saves exactly half the maximum voltage, which can be read with a handheld digital voltmeter. A positively charged drop results in a positive final voltage, whereas a negatively charged drop yields a negative final voltage. Using the peak detector requires good reflexes. Momentarily depress the reset button and then push down on the "sample" button. The output voltage of the peak detector will jump when a charged drop passes though the instrument. Quickly release the sample button to prevent another drop from upsetting the measurement. Record this voltage with your voltmeter. When you're ready, press the reset button, hold down the sample button, and await the next drop. Charged drops typically give readings of about 0.3 volt. Because larger charges produce larger voltage spikes, measuring the maximum voltage provides an estimate of the charge on the drop:
The electronic components required for these circuits can be purchased from Future Electronics in Bolton, Mass.; call (800) 655-0006. For more information about this project, check out the Society for Amateur Scientists's World Wide Web site or call (619) 239-8807 or (800) 873-8767. You may write the society at 4735 Clairemont Square, Suite 179, San Diego, CA 92117. |