字幕表 動画を再生する 英語字幕をプリント Let's switch it on. Let's see what it does. Through this coil of thick wire, we're about to pass a huge alternating electric current. On top is a 1 kg aluminium plate. So we hear that noise. What's that noise? It's the vibration of the plate, because it's vibrating at two times the frequency of this one. Woah! Woooaaaahhhhhhh!! hahaha, how does it do that? It's magical To find out, I've come to the place where it all started - The Royal Institution in London This is the key to Faraday's magnetic lab. It's amazing that the lock still works. From the 1870's on this became a store room, which is why it survived and it survived intact, all the joinery giant electromagnet are exactly the same as Faraday left it. So this is exactly as Faraday would have had it. That's right, yup. In Faraday's time it was known that electric current creates a magnetic field, but it remained an open question whether the reverse is possible -- if a magnetic field could generate electric current. Faraday answered this question with his most famous apparatus. Faraday's electromagnetic induction ring. Which is this. In August, 1831 Faraday wrapped two coils of insulated wire around this iron ring. But in 1831 you could not go down to your local electrical hardware shop and ask for x hundred meters of insulated wire, you had to insulate the wire as you went. So as you pushed and pulled the wire in and out of the ring you had to insulate it. It takes 10 working days, which was a huge investment of time. But the investment paid off. When Faraday connected a battery to one of the coils, he saw a brief pulse of current in the other coil. And when he disconnected the battery, he saw a pulse of current in the other direction. He realized that current was induced in the second coil only when the magnetic field through it was changing. And if they hadn't been wrapped on the same ring, Faraday may have noticed that the two coils repel each other when the current is induced and that's due to the interaction of their magnetic fields. Which brings us back to this. Through the bottom coil we are passing a huge electric current: 800A which alternates in direction 900 times per second. This ensures there will always be a changing magnetic field above the coil. Instead of a second coil we're using the Aluminium plate, but the principle is the same, the changing magnetic field induces currents in the plate that create an opposing magnetic field -- so it levitates. How awesome is that?! This current is not only good for levitating the plate. It can also make lightbulbs glow. A gift. Uh, thank you. Oh. That is cool. Not too close because it will burn the lamps. Can I put it there? Yeah. And just as current in a toaster element heats it up, the induced current in the plate dissipates its energy as heat. And some water too Thank you. Yeah to see the temperature. Check out how hot this plate is. Oh, that is nuts! Is this your favorite demo? It's a flying BBQ or something Tell me this is not the best dinner table centerpiece. It levitates, gives you light, and you can cook on it. and all the while you're demonstrating Faraday's Law of electromagnetic induction.