What engineers talk about over beer

I'm sitting in my living room blogging, and my husband is chatting with a friend of ours from the SCA who happens to be an engineer.

The hubby is describing our new electrical system and mentioned the handy voltage detector pen that we have. Basically you move the thing near a wire, and if it glows red and beeps, the wire is live. This is super-handy when you have no idea whether the wire you just uncovered in the wall you're ripping out is in fact connected to anything - it's nice to know if it's live before you grab it.

So as we're talking about this handy volt detector gadget, our engineering friend looks puzzled and says, "Cool, but how does it work?"

"You're not closing a circuit, so what's making it work?"

Of course he's right - you just move the detector near the wire and it beeps and glows. There's no circuit, especially if you're wearing thick-soled rubber shoes. So... I do what any intelligent person does in this day and age - I look it up on Google. And here's the description I found, available at http://en.allexperts.com/q/Physics-1358/2008/12/Voltage-detector.htm.

'The "circuit" that is being created to light up the LED is not between the 120 AC voltage and ground. It is a circuit going through a looped wire and then through the LED. What does a looped wire do? When you change the magnetic flux through a looped wire, you create a current in the wire. And where does the magnetic field come from? From the wire, carrying a current, which changes direction sixty times a second. That is a measurable change in the current, which causes a measurable change in the magnetic field coming from the AC wire, which causes a measurable change in the magnetic flux through the looped wire, which induces a current through the loop, which causes the LED to glow.

This induced current would be nowhere near enough to cause an incandescent or fluorescent bulb to glow. However, you don't need a lot of current to light up an LED. Thus, you can safely detect AC in a wire, without having to short the wire to ground.'

Well, that's it for your physics lesson for tonight. And now you know what engineers talk about over beer. :)

Oh and if you don't have one of those voltage detector pens? You need one.