Club Notes: Magnets in a Smart Helmet
1The bike-safety club at Juniper Hill Middle School is designing a smart helmet for younger riders. Two of its features use magnets. The chin buckle has a disc magnet inside one half and a steel plate inside the other, so the halves pull together and click shut. The rear light flashes faster as the bike speeds up. To measure speed, a small magnet clipped to a spoke of the front wheel passes a sensor on the bike's fork once on every turn of the wheel. The magnet and the sensor never touch, and the bike's computer passes the count on to the light. Club advisor Ms. Aurelie Fonseca explained that a magnet is surrounded by a magnetic field, and that a piece of steel or another magnet in that field feels a force.
2In Test A, Mirela Danso set the magnet half of the buckle on a digital kitchen scale with a plastic platform and pressed zero. Cyrus Petrakis clamped the steel half in a wooden stand directly above it and lowered it to each gap in Table A, never letting the halves touch. When something pulls up on the half on the scale, the reading drops below zero; a reading of −70 g means an upward pull equal to the weight of 70 g. Then they swapped the halves: the steel half sat on the zeroed scale, and the magnet half was clamped above it at the same gaps. They took three readings for every setup and averaged them. In a check run, a plain plastic half with no magnet or steel held 2 mm above the magnet half left the scale at 0 g.
3In Test B, the club turned the bike upside down on a stand. Where the speed sensor normally sits, they taped a phone running a magnetic sensor app, which reads in microteslas (µT). They subtracted the reading the phone gave with no magnet nearby, which comes from Earth's field. Cyrus set the spoke magnet to pass 6 mm from the phone at its closest point and turned the wheel slowly by hand. Table B shows readings with the magnet at points along its path, before (−) and after (+) the closest point. Next, they changed how far the magnet passed from the phone and recorded only the highest reading during each pass (Table C). They repeated every gap in Table C with a piece of plastic mudguard and then with a folded rubber glove filling the gap, and the highest readings matched the readings with only air in the gap. The club's sensor counts a pass only when the reading is 150 µT or more.
4Last, Mirela wiped the helmet's clear plastic visor with a dry cloth to clean it. When she held it 2 cm above some small bits of tissue paper on the table, the bits jumped up onto the visor. A visor that had not been wiped, held at the same height, did not lift any bits.