Whether you’re driving down the highway or cruising down Main Street, most of us don’t have to think twice about when to look at our car’s speedometer. It’s a constant companion, telling us how fast we’re going and, for some of us, it’s the only way to keep track of our speed. But how exactly does it work?
A speedometer’s basic function is to measure the rate of change in distance, or displacement. But there’s more to it than that, since velocity is a vector quantity with a direction and displacement does not. To determine the direction of velocity, we can divide the distance traveled by the time taken to travel that same distance. But that only tells us the average speed of the trip, not your instantaneous speed.
To find your instantaneous speed, the shaft that turns the wheels of your car also rotates a magnet inside the case of your speedometer. The magnet’s rotation causes it to generate electric currents within the metal cup that contains it, which then creates a fluctuating magnetic field. That fluctuating field causes currents to flow in and out of the speed cup, producing a sort of electricity generator that emits tiny electric pulses. As the magnet turns, those pulses are amplified by a circuit, which translates them into your instantaneous speed and then displays it on the digital display.
Looking at your speedometer on a regular basis is one of the best ways to stay safe on the road, as it helps you keep your eyes on the road ahead of you and in your side and rearview mirrors. But don’t become so attached to the number on your speedometer that you ignore important driving cues, like checking your tire pressure and maintaining a proper braking distance.