Launch Notes: Two Fields Before Liftoff
1The Sandhill Rocketry Club meets on a mowed field outside town for its spring launch day. This year the club's biggest rocket carries a small altimeter, a device that records how high the rocket flies. The altimeter sits sealed inside the body tube, with no hole for a switch. Instead, it has a magnetic switch: when someone holds a small magnet against the outside of the tube, the switch inside closes and the altimeter's light blinks on. With no magnet nearby, the light stays off.
2Eighth grader Leocadia Petrov wanted to know how far the switch's magnet could reach and whether the tube's material mattered. She set the switch on a block and slid the magnet toward it along a ruler, three times for each setup, writing down the distance at which the light first came on. She tested the switch with only air in between and with flat wall pieces between the magnet and the switch: cardboard, fiberglass, aluminum and steel. Every wall piece was 1 mm thick.
3On launch morning the air was cold and very dry. When Tobiah Rinaldi unfolded the rocket's thin plastic parachute, it crackled, clung to his sleeve, and bits of dry grass jumped up onto it. Club advisor Mr. Cornelius Abara helped him build a foil-leaf jar: a paper clip pushed through the jar's cardboard lid, with two thin strips of aluminum foil hanging side by side from its hook inside. When a charged object comes near the top of the clip, the two foil leaves swing apart, even if nothing touches the clip.
4Before each reading, Tobiah rubbed the parachute 10 times with a wool scarf. Then he held it still at a measured height above the clip and read the gap between the tips of the foil leaves (Table 2). He also held a parachute that had not been rubbed 2 cm above the clip.
5Last, Tobiah set the whole jar inside a cardboard box and held the rubbed parachute 5 cm above the box: the leaves spread to 18 mm. He lined the inside of the box with aluminum foil on every side, leaving only a pencil-sized peephole to look through, and tried again. This time the leaves did not move. When he lifted the jar out of the foil box, the same parachute at 5 cm spread the leaves to 18 mm again. Mr. Abara then set a compass inside the foil-lined box, which sat still on the table, and watched it through the peephole. As he brought the switch magnet near the outside of the box, the needle turned toward it.