Tag Archives: magnet

The Speedometer

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 drive shaft? 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, the speedometer shaft is spinning as fast as the drive shaft. 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, 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.


The history of the speedometer and odometer


Back to the beginning of The Western Civ User’s Guide to Time & Space

The Chinese invent the compass

A thin piece of magnetized iron in the shape of a fish (or a shallow boat) floats in water and points north.

The magnetic compass was invented in China sometime between the 2nd century bc and ad 1st century. They used it to make sure streets and houses aligned with the Earth in a harmonious way—what is called feng shui. The Chinese later figured out they could use a compass for finding their way on the ocean (ad 1040-44).

This carefully-balanced magnetized iron spoon points north with its handle.

Magnetic fields

Well, everybody who reads this blog called me this morning in a panic because of that last post. The post where I explained how we’re all doomed. I went to bed before going on to explain that we’re not doomed. Sorry, gang.

Probably I should have mentioned: the molten metal inside the Earth is magnetic. It behaves like a giant magnet. Magnets have 2 poles and so does Earth, thanks to its sizzlin’ hot molten metal core. If you’ve ever put a magnet on a sheet of paper and sprinkled metal filings onto it, you’ve seen the filings arrange themselves into circular patterns around both poles. These are magnetic fields.

There are gigantic magnetic fields around Earth’s north and south poles that extend way out into space. You can’t see them. You know what they do? They act like a shield against whatever dust the Sun keeps shooting at us. That’s why we’re not gasping for oxygen and wiping sun-dust off our computer screens.

Pretty neat, eh?

What is Earth’s Magnetic Field?


I add this link in the interests of science: