by Chris Woodford. Last updated: December 5, 2013.
Excuse me, sir, have you any idea how fast you were going? That's the question every motorist dreads being asked by a police officer at the side of the road. If you were staring straight ahead, not looking at the dashboard, you might have only a vague idea what to say. If you were looking at the speedometer, on the other hand, you'll know the answer exactly, possibly to within one or two kilometers or miles per hour. Have you ever stopped to think how a speedometer actually works? It's a really ingenious use of electromagnetism!
Photo: Speedometers might look like moving-coil meters (voltmeters, ammeters, and so on), but they work in a totally different way.
How to measure speed
If you've read our article about motion, you'll know that speed is very simply defined: it's the distance you travel divided by the time you take. So if you go 200 kilometers and it takes you four hours to do it, your average speed is 50 kilometers per hour. Measuring your average speed after you've traveled is not actually that much help, especially if a police officer is asking you questions. How fast were you going sir? Erm, pull me over again in a couple of hours, when I get to my destination... and I'll divide the distance I've gone by the time it took... and then I should be able to give you some kind of an answer. Okay?
What we're talking about here is average speed; what you need to know as a motorist is your instantaneous speed: the speed you're going at any given moment. Figuring that out is a lot harder than you think. If you've seen traffic cops (or speed cameras) by the side of the road, you'll probably be aware that they use radar beams to check speeds. The radar gun (handheld or mounted inside the speed camera) shoots an invisible electromagnetic beam at your car at the speed of light. Your car reflects the beam back again, modifying it very slightly. The gun figures out how the beam has been affected and, from that, calculates your speed. Now in theory we could all have radar guns mounted in our cars, shooting beams out at lamp-posts and buildings and waiting for the reflections to come back—but that's an awful lot of bother! Isn't there a simpler way of finding out how quickly we're going?
Photo: Measuring speed with a radar gun. Photo by Heather M. Norris courtesy of US Air Force and Defense Imagery.
What we really need is a way of figuring out how fast the car's wheels are turning. If we know how big the wheels are, we can then figure out the speed fairly easily. But how do you measure a wheel's rate of rotation? Even that problem isn't simple. Imagine how much harder it must have seemed in the early days of motoring, back in 1902, when German engineer Otto Schulze invented the first practical solution: the eddy-current speedometer.
How mechanical (eddy-current) speedometers work
Here's what we want out of our speedometer. We have the car's wheels rotating at a certain speed and we want to know, with a simple pointer and dial, what that speed is. So we need to connect the spinning wheels to the pointer in some clever fashion. Even that is pretty tricky: the wheels are racing around but the pointer, some distance away, merely flicks back and forth. How do we convert continuous, spinning motion into intermittent, flickery, pointer motion? The answer is to use electromagnetism!
The shaft that turns the car's wheels is connected to the speedometer by a long, flexible cable made of twisted wires. The cable is a bit like a mini driveshaft: if one end of the cable rotates, so does the other—even though the cable is long and bendy. At the top end, the cable feeds into the back of the speedometer. When it rotates, it turns a magnet inside the speedometer case at the same speed. The magnet rotates inside a hollow metal cup, known as the speed cup, which is also free to rotate, though restrained by a fine coil of wire known as a hairspring. However, the magnet and the speed cup are not connected together: they're separated by air. The speed cup is attached to the pointer that moves up and down the speedometer dial.
How does it all work? As the speedometer cable rotates, it turns the magnet at the same speed. The spinning magnet creates a fluctuating magnetic field inside the speed cup and, by the laws of electromagnetism, that means electric currents flow inside the cup as well. In effect, the speed cup turns into a kind of electricity generator. But, unlike in a proper generator (the kind that makes electricity for your home in a power plant), the currents in the speed cup have nowhere to go: there's nothing to carry their power away. So the currents just swim about uselessly in swirling eddies—we call them eddy currents for that very reason. Since they're electric currents, and they're moving in an electrical conductor inside a magnetic field, another law of electromagnetism says they will create motion. How? The currents actually make the speed-cup rotate in such a way that it tries to catch up with the spinning magnet. But the hairspring stops the cup from rotating very far so it just turns a little bit instead, pulling the pointer up the dial as it does so. The faster the car goes, the faster the cable turns, the quicker the magnet spins, the bigger the eddy currents it generates, the greater the force on the speed cup, and the more it's able to pull the pointer up the dial. If you can't picture all that clearly, take a look at the little animation below.