There’s a beautiful simplicity about what the toaster is up to. When you put the bread in place, it rests on a spring-loaded tray. The springs underneath are pushing the bread up to its “popped” position, high above the heating elements. But you’re strong enough to push the bread down in spite of those springs. And once the tray reaches the bottom of the toaster, a protruding bit of metal fills the gap in not one, but two circuits. One of those circuits deals with the heating, so electricity starts to flow around the toaster to heat the bread.
But the other circuit is a lot more interesting. The electrons in that circuit shuffle along and around a section of wire that is wrapped around a small lump of iron. It’s a bit like a helter-skelter ride for electrons—they spiral around and around the iron and then out and along the rest of the circuit back to the plug socket. That’s all. But because magnetism and electricity are so deeply intertwined, when an electric current runs through a wire, it creates a magnetic field around that wire. Sending electrons around a coil of wire means that each time the electrons loop around, they’re adding to the same magnetic field. The iron core in the middle of the coil reinforces the magnetic field and makes it even stronger. This is an electromagnet. When an electric current is running through the wire, it’s a magnet. When the current stops, the magnetic field goes away. So when you push down the lever on the toaster, you’re switching on a magnetic field at the base of the toaster that wasn’t there before. Since the bottom of the bread tray is made of iron, it sticks to the magnet. In other words, while I’m poking about in the fridge, a temporary magnetic field is holding the bread tray in place. The toaster has a timer on the side, and the clock starts when the circuits are connected. When the time is up, the timer cuts the power to the whole toaster. Since there’s no power to the electromagnet, it stops being a magnet. Nothing is holding the bread down anymore, so the springs pop it up.
I sometimes forget that I’ve unplugged the toaster, but I find out pretty quickly. If I try to push the lever down, it pops straight back up, even if I push it down all the way. That’s because there’s no power to the electromagnet, so it can’t hold the bread tray down. It’s such a simple system, and stunningly elegant. Every time you make toast, you’re taking advantage of this very fundamental connection between electricity and magnetism.
Electromagnets are very common because it’s really useful to be able to switch magnets on and off. They’re in loudspeakers and electronic door locks and computer disk drives. They must be continually powered, otherwise the magnetic field vanishes. The kind of magnets you stick on your fridge are called permanent magnets—you can’t turn them on or off, or change the magnetism, but they don’t need any power. Electromagnets do exactly the same job as a fridge magnet when they’re turned on, but they can conveniently be turned off just by stopping the current.
We’re surrounded by small, local magnetic fields, some permanent and some temporary. They are almost all made by humans, either to do a useful job, or as a by-product of something that’s doing a useful job. The magnetic fields don’t reach very far, and so they’re only detectable very close to the magnet. But these are just tiny local glitches in a much bigger magnetic field that stretches around our planet, and this one is entirely natural. We can’t feel it, but we use it all the time.
IT’S EASY TO take a compass for granted, especially if you do a lot of walking, when it’s very handy to be accompanied by a needle that always points north. But imagine getting ten compasses, or twenty, or two hundred. You spread them out across the floor, they all point north, and suddenly you see that this isn’t just something that happens when you get a compass out. It’s there all the time, and it’s consistent. You can take your compass collection anywhere on the globe, unpack it, set it out, and all the compasses will swing around and agree on where north is. The Earth’s magnetic field is always there, flowing through cities, deserts, forests, and mountain ranges. We live inside it, and although we never feel it, a compass will always remind us that it’s there.
A compass is a brilliantly simple measuring device. The needle is a magnet, and so one end of it behaves very differently from the other end. Unhelpfully, these two ends of the magnet are called the north and south poles, but it’s just a way of saying that one behaves like the magnetic north pole of the Earth, and the other behaves like the magnetic south pole. If you take two magnets and move them about near to each other, you’ll see very quickly that it’s very hard to push the two north poles together, but that a north pole and a south pole will attract each other very strongly. This is why it’s easy to detect the direction of a magnetic field; if you put a small mobile magnet inside a magnetic field, it will spin around until its north and south ends are aligned with the field. And that’s all a compass is: a mobile magnet that gives away the direction of any magnetic field you bring it into. We can’t see the vast magnetic field of the Earth, but we can see the compass needle respond to it. It’s not just the Earth’s field that compasses sense, either. Take a compass around your home, and you’ll detect for yourself the magnetic fields that surround plug sockets, steel pans, electronics, fridge magnets, and even any iron that’s been close to a magnet recently.
Compasses, obviously, are mostly used for navigation. Finding your way about on the surface of a sphere is always going to be tricky, but the Earth’s magnetic field has provided a fabulously reliable tool for explorers for centuries. The Earth has a magnetic north pole and a magnetic south pole, and anyone with a compass can orient themselves toward one or the other. As a navigational tool magnetism is straightforward, it’s cheap, and it never runs out. However, it does have a few caveats attached to it. Caveat number one sounds unexpectedly serious: The magnetic poles aren’t fixed in one place. They wander, and they can travel a very long way.