Hard Tweets Explained: Induction

Growing up in the 1970s, fondue was a thing. Not a frequent thing. But a thing. For example, I’m quite sure that we had it on New Year’s Eve at least once. (Although my memory says we had it every New Year’s Eve, I’ve learned from my kids that anything we do once is remembered as something we do every, so I don’t trust my memory on this one.) I’ve always loved the smell of wine boiling, and I’m a huge fan of cheese, so putting those two things together really triggers the sense memory for me.

All your cheese are belong to us

All your cheese are belong to us

So when my wife suggested that we should take the kids to The Melting Pot, which is a fondue restaurant chain, I thought, “that sounds like a great idea!” I live in the middle of nowhere and this was more than an hour drive, but the way things worked out, I was going to be driving the kids and meeting her there. And I kind of like driving with the kids, because we can be silly and we can crank the music really loud. So the travel wasn’t so bad. But I don’t think we’ll be going back.

There is a term in electrical engineering called an impedance mismatch. Suppose you have modern 8 ohm speakers, but your stereo amplifier was designed to drive old-fashioned 4 ohm speakers. Ohms (Ω) are a measure of resistance. And another name for resistance is impedance. So your 8Ω speakers and your stereo designed for 4Ω speakers have an impedance mismatch. Connecting them results in bad things happening, like your speakers getting overdriven to the point of destruction, or your amp turning into a bonfire. There is nothing wrong with the speakers, and there is nothing wrong with the amp—they just don’t go well together.

There is an impedance mismatch between youngish kids and fondue restaurants. Fondue restaurants are expensive. Kids don’t enjoy them enough to justify that expense. Fondue restaurants are slow. Really slow. No—slower than that. Kids are impatient. Fondue restaurants serve huge portions. My kids barely eat. Fondue restaurants have a panoply of wonderful rich smells. My kids are downright obnoxious when it comes to wonderful rich smells.

So as we are waiting in our booth, and waiting, and waiting, I decided to teach the kids about induction. Unlike the fondue pots of my youth that were heated by flaming cans of Sterno, these pots on our table were heated by an induction cooktop. I understand these are quite common in other parts of the world, but my kids had never seen one, and they are pretty magical. I could see them wince when I smacked my hand down onto the surface right next to the pot of boiling water. And then they each had to try it. Tentatively at first. Then with gusto.

WARNING: Descriptions of electrical engineering topics may induce drowsiness

WARNING: Descriptions of electrical engineering topics may induce drowsiness

The way an induction cooktop works is really neat. Below the surface there is a coil of wire. In that coil there is an alternating current. When electricity runs through a coil, it makes a magnetic field. So you can think of an electric coil as being just like a straight magnet. If you reverse the flow of electricity, the direction of the magnetic field reverses. So if + is going in on the left and – on the right, you might have north going up; and then if you put – on the left and + on the right, you would have south going up. Alternating current means the + and – are flipping constantly. Typically 60 times a second here in the USA. So that magnetic field is going north-south-north-south really fast.

Now put a ferrous metal (the kind of metal a magnet would stick to) near that alternating magnetic field. The atoms in ferrous metals like to line up with magnetic fields. So when you put them next to an alternating magnetic field, all the atoms in that metal are going to start flipping back and forth really fast. Another word for “atoms moving fast” is “heat.” So whatever is made of that metal is going to start getting really hot. And if that whatever happens to be a pot full of cheese and wine, you get fondue!

The magnetic field is inducing motion in those atoms. So the process is called induction. As so often happens with technology, it was discovered pretty much simultaneously by two different people, back in 1831: Joseph Henry in the USA, and Michael Faraday in Britain. Your rechargeable electric toothbrush uses induction. There is a coil in the stand, and there is a coil in the toothbrush. A current in the stand coil induces a current in the toothbrush coil, which then charges the battery.

Remember when MP3 players were new, and you had to stick this contraption into the cassette player in your car so you could get music from the MP3 player’s headphone jack into your car stereo? That was induction, too.

And those giant metal things on the poles outside your house that step current down from power line level to house levels? Induction.

This picture links to the site I stole it from

The circuit that matches fondue restaurants with young kids is a bit more complex

And now I’m going to blow your mind: an inductor is also a great way to solve an impedance mismatch between two circuits! See what I did there? Fine. Be that way.

With that, I think we’ve got enough background to explain the tweet. A good friend told me that she was going to be tied up with an Honor Society induction ceremony. And having just had this fondue experience, I immediately thought of a bunch of kids in caps and gowns standing around a big coil of wire. I’m weird that way. And that got us to the tweet.

When the illuminati (or whoever it is who decides what words we use for units of measurement) were deciding what to call the units of measure for inductance, they had Henry and Faraday to choose from, since they both discovered the effect at about the same time. They went with henry (in lowercase, because the illuminati are a bunch of elitist jerks or something). Faraday’s consolation prize was to be the units for capacitance (the farad).

Go read the tweet again. Henry. Check. Recoiled (like the coil in an inductor). Check. Induction. Check. Faraday. Check.

Homework: Go to a fondue restaurant chain and put your phone onto the cooktop surface and find out how much of your phone is ferrous metal.


2 thoughts on “Hard Tweets Explained: Induction

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