This Magnetic Moment: An Introduction to the MetroCard

If you live in New York City, you are familiar with the MetroCard: it is your key to the city’s public transportation system, your eighty-dollar-a-month unlimited pass or dwindling pay-per-ride card or fleeting single fare. It is the thing you take for granted until you find yourself without it in a station with no agent and a broken vending machine, standing sadly by the turnstile as a stream of people swipe their cards and pass through the gate and board the train without giving you so much as a sympathetic smile. It is the thing you take for granted knowing how to use until you are caught behind a group of tourists who approach the turnstile uncertainly in front of you, as one would approach a large and unfriendly and perhaps rabid dog. You watch as, after some discussion and a few false starts, they drag their collective card slowly through the machine, failing to notice the screen reading “please swipe your card again at this turnstile” until after they’ve given the unyielding iron bar a few sincere pushes. Meanwhile, you hear your train come, and you hear it go, but the good part inside of you swallows your fury and provides the tourists with a friendly tutorial on the speed with which a MetroCard should be swiped.

But as common as the MetroCard is to us, and as familiar the anecdotes, few of us have any idea how the thing actually works. Because I didn't know either when a friend asked me a few weeks ago, I began researching it. And because I had so much fun learning about it, I'm going to devote a few blog entries to the workings of the MetroCard, our key to the city...

The technology on the card itself—the magnetic stripe—is actually fairly uncomplicated. Magnetic stripes are made out of thousands of tiny particles of iron oxide, each about 20 millionths of an inch long. Because iron oxide is a magnetic material, all of these little particles are oriented in one of two directions-- you can think of them like arrows pointing north or south. When the card is blank, all of these little magnetic particles are oriented in the same direction, and by switching some of the particles around, the stripe becomes “encoded.” Just like binary code (0’s and 1’s) can be interpreted as characters and words and whole computer applications, the magnetic orientation of these particles can be read as information regarding your card’s serial number, balance, expiration, and so on.

The magnetic stripe on a MetroCard contains information in exactly the same format as virtually all other cards with magnetic stripes. Though you can’t see it, the stripe carries three different “tracks,” each with a different type of data. Track one on a driver’s license holds information about your name and address, along with the location the card was issued. On a MetroCard, tracks one and two hold information about the card type, expiration date, times used and remaining card value, while track three is encoded with a unique serial number. One inch of one of these tracks holds 210 bits of data, which translates to about 70 characters. Each time you swipe, your card is both read and rewritten to reflect your new balance. And how does that work? I'll tell you later this week.

(I created the handy drawing above using Microsoft Paint, an application that I don't believe has been updated with a new version since I was in elementary school. Speaking of simple technology...)