The Speedometer was first patented by German engineer Otto Schulze in 1902.
The Speedometer is one of those dials on the dashboard of a car. It measures miles per hour—distance traveled in a certain amount of time. If you were driving to someplace 50 miles away, you could get there in an hour if you drove 50 miles per hour (mph) the whole way. But there will be town roads as well as the highway, and stop signs and school zones, maybe a potty break. So you won’t be traveling at the same speed the whole trip. The speedometer tells you how fast you’re going right now, whenever you look at it.
Here’s how it works—though I still can’t believe how brilliant this idea is. Remember the driveshaft? The pistons in their cylinders push the driveshaft round and round, so it turns the wheels of the car. I’m going to attach another shaft to the drive shaft. The second shaft works the speedometer. When the driveshaft turns around, this speedometer shaft turns around, too.
At the other end of the speedometer shaft, I attach a magnet inside a metal cup. Both spin on the shaft—but the magnet is attached firmly to the speedometer shaft; the cup isn’t. The cup spins freely.
Imagine now, that the speedometer shaft is spinning as fast as the driveshaft. When the drive shaft speeds up, the speedometer shaft speeds up, too. The magnet is spinning around because the speedometer shaft is turning. The metal cup is attracted to the magnet. it spins freely and tries to keep up with the magnet.
The cup can’t keep up with the magnet, though, because I attached a hairspring to it to hold it back. The cup gets tugged by the magnet but it can only move so far. Get this: on the back of the cup, there’s a pointer with a dial behind it. The dial shows numbers, and miles per hour. The driveshaft spins around, the speedometer shaft spins around, the magnet spins around, the metal cup tries to spin around—but only moves so far because of the hairspring. It only moves far enough to show on the dial how fast you’re going.