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Hard Disk Drives record data using a technology long known to baffle juggalos. The read/write head uses magnets to store information on those disks. How? Why? What does that even mean? Let’s jump in.
What makes a magnet a magnet? Moving electricity. When you get down to the atomic level atoms are magnetic because their electrons are spinning. Glomp a bunch of those atoms together (like sticking magnets one to another) and you have a grain. Get enough grains lined up in the same direction and you have a permanent magnet.
It’s not just spinning electrons; any movement of charge creates a magnetic field. Let’s say you have a wire (lucky you!) and you’re running electricity through it. Don’t ask me what for; you’re the one with the wire. But that current in the wire will produce a magnetic field in circles around the wire. Or you can take a coil of wire, send electricity through it. That gives you a strong magnetic field inside the coil, all pointing in the same direction. The word for that, incidentally, is a ‘solenoid.’
In that picture, the candy-canes represent magnetic field lines and the wire represents, well, a wire. To represent an actual solenoid the candy canes ought to continue on all the way around and join back up on the other side. Magnets don’t get a start and a finish, they only work in loops. The model does work very well in one respect though, look at those candy canes. The space inside the wire is mostly candy cane. The space outside the wire is mostly not. You have the same number of lines inside the cylinder as outside, but the outside they have so much more volume to spread across. What that means is you have a strong magnetic field in the middle of the loops of wire and a weaker one looping around outside.
The whole electricity-and-magnetism thing works in reverse too; if you wave a wire through a magnetic field you’ll get a current in the wire.
Actually, if you’ll excuse a brief side trip, a great deal of modern technology is based on getting currents by waving wires at magnets or pushing magnets around with electricity. A generator makes electricity by using an engine (burning gasoline, or what have you) to turn a wheel of magnets past wires. Most power plants do the same thing, only varying what they’re using to turn the crank. An electric motor (like, say, in your car windows) does its thing by using currents to pull magnets in a circle. I’d go into more details but all that’s ancillary to my main discussion. The point is you can use a magnet to produce current in a wire, and you can use a current in a wire to magnetize something.
Okay, let’s move this back in the direction of hard drives. What happens if you apply a magnetic field to a bit of iron? Well, the iron moves, right? I mean that’s what makes a magnet a magnet. Let’s say though, that you wrap a bit of iron in a coil of wire and you run electricity through it. Here, let me show you what I’m talking about:
Okay, ignore the sausage for a moment. What happens if you run a current through the noodles? You’d get a magnetic field through the center (remember the solenoid up there?) Only now we’ve got a hunk of kielbasa (iron) taking up that space. The magnetic field lines will follow the metal; instead of coiling around the solenoid the field lines will run through the iron until that’s no longer an option. The magnetized metal acts like a horseshoe magnet, and you get your magnetic field lines in between the prongs.
Now here’s the kicker. What happens if you move that kielbasa magnet next to a surface that you could magnetize? Recall how we were talking about the grains up above? If you push enough current through that wire you get a large enough magnetic field that you can use it to align the grains in your surface. Presto change-o you’ve got a magnetic field embedded in that surface. Move your sausage away, turn off the power and you’ll still have magnetism there. You want to flip a bit? Run the current through the wire the opposite way. The polarity flips on your magnet (yeah, polarity actually means something outside of Star Trek technobabble) and your magnetic chunk in that surface points in the other direction.
And that’s how you can store information on a metal platter. Great! I’ve got all kinds of things I want to write down. But how do I read them once they’re on there? That’s a story for another day. Join us fortnight next when we explore the practical implications of reading information off of disk drives in “Kelvin can’t wait” or “Spinning sausages at 7200 rpm”
This is part 23 of my ongoing series on building a computer, the Christmas Miracle way. You may find previous parts under the tag How to Build a Computer. This week’s post has (again!) been brought to you by Ricochet’s own Group Writing! December’s theme is Veneration and there are plenty of spots waiting for you to fill one. Do your Group Writing today!