1198. How does the tachometer in a new car work? It looks like a magnet wrapped with wire that's located very near a saw-toothed wheel that spins as the engine turns. — TR, Provo, UT
The device you describe is essentially an electric generator. The toothed wheel is made of pure iron so that its teeth can become temporarily magnetized while they are close to the permanent magnet. When a tooth becomes magnetized as it approaches the permanent magnet, or demagnetized as it moves away from the permanent magnet, it changes the shape and strength of the magnetic field around the permanent magnet. Since changing magnetic fields produce electric fields, the tooth's movement causes an electric field to appear around the magnet. This electric field pushes on mobile electric charges in the wire coil wrapped around the magnet and generates electricity. The current in the coil flows one way as a tooth approaches the magnet and reverses when that tooth moves away from the magnet. Also, the faster the tooth moves, the stronger the change in the magnetic field and the higher the voltage generated in the coil. The tachometer can tell how fast the engine is turning by how frequently the current in the coil reverses directions or by how much voltage the coil generates.