
How to measure up speed
If you"ve check out our article about motion, you"ll know thatspeed is an extremely simply defined: it"s the distance you travel dividedby the moment you take. For this reason if you walk 200 kilometers and also it takes youfour hrs to do it, your typical speed is 50 kilometers every hour.Measuring your typical speed after ~ you"ve travel is not actuallythat lot help, particularly if a police officer is asking youquestions. How fast were friend going sir? Erm, traction me end again in acouple that hours, as soon as I get to mine destination... And I"ll division thedistance I"ve gone by the time it took... And also then I need to be ableto offer you some kind of one answer. Okay?

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Mechanical (eddy-current) speedometers
Here"s what we desire out of our speedometer. We have actually the car"swheels rotating in ~ a certain speed and we want to know, v a simplepointer and dial, what that rate is. So we require to affix thespinning wheel to the reminder in some clever fashion. Even that ispretty tricky: the wheels room racing around but the pointer, somedistance away, just flicks back and forth. Just how do us convertcontinuous, rotate motion right into intermittent, flickery, pointermotion? The prize is to use electromagnetism!The column that turns the car"s wheels is linked to thespeedometer by a long, flexible cable do of twisted wires. Thecable is a bit like a mini driveshaft: if one finish of the cablerotates, so does the other—even despite the cable is long and bendy.At the peak end, the cable feeds into the earlier of the speedometer.When that rotates, it turns a magnet inside the speedometer situation at thesame speed. The magnet rotates within a hollow steel cup, known asthe speed cup, which is also complimentary to rotate, despite restrained by afine coil of wire known as a hairspring. However, the magnet andthe speed cup room not associated together: they"re be separate by air.The rate cup is attached come the reminder that move up and down the speedometer dial.
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How go it all work? as the speedometer cable rotates, the turnsthe magnet in ~ the very same speed. The turn magnet creates afluctuating magnetic field inside the speed cup and, by the legislations ofelectromagnetism, that means electric currents flow inside the cup aswell. In effect, the speed cup turns into a kind of electricitygenerator. But, uneven in a ideal generator(the sort that makes power for your home in apower plant), the currents in thespeed cup have nowhere to go: there"s nothing to carry their poweraway. So the currents just swim about uselessly in swirlingeddies—we speak to them eddy currents because that that very reason. Sincethey"re electric currents, and they"re moving in one electricalconductor within a magnetic field, another law that electromagnetism claims they will produce motion.How? The currents actually make the speed-cup turn in such a means that that tries tocatch up with the turn magnet. However the hairspring stops the cup fromrotating an extremely far therefore it simply turns a tiny bit instead, pulling thepointer up the dial together it walk so. The much faster the automobile goes, thefaster the cable turns, the faster the magnet spins, the bigger theeddy currents the generates, the better the pressure on the rate cup,and the an ext it"s maybe to traction the guideline up the dial. If you can"t snapshot all that clearly,take a look in ~ the tiny animation below.