More physics with aluminium foil

I gave a talk to the Junior Naturalists in Hamilton last Friday. It had some similarity to the talks I gave in June to the Osborne Days (year 12 and 13 school students), but I needed to change a few things because 1. The audience was younger, and 2. I wasn’t prepared to cart voluminous apparatus across from the University to Hamilton Gardens and back on a dark night.

The mobile phone in foil experiment works pretty well, and is simpler to do than the mobile phone in water experiment, and this time I extended it to a radio in aluminium foil. (It shouldn’t receive anything – the foil reflects all the waves and the radio goes quiet.)Now, I tried to test this a couple of evenings previously at home (to avoid embarrassing situations where your experiment doesn’t work.) But we’d run out of foil at home, and I had to improvise by sticking my pocket radio in a saucepan and putting the lid on.  This is when I got a bit of a startling result – on FM, the radio went silent as I expected. But on AM, it still continued to pick up stations while completely surrounded by metal.

Being the physicist I am, I wondered why? Was it to do with the wavelengths. FM and AM have very different wavelengths (AM is larger by about a factor of 20) and I know that the skin depth of electromagnetic waves (broadly speaking, their penetrating power) is greater with greater wavelengths.  Was my AM managing very slightly to penetrate the saucepan?  I did some rough quantitative estimates, but wasn’t convinced that the difference was great enough.

So then, finally, just before the talk on Friday night. I got hold of some foil. I wrapped my radio in it, and, hey presto, the AM signal was cut out just like the FM. Well, that puts pay to any theory that it was due to the wavelengths and skin depth. But it still leaves me with a burning question – why does my radio still receive AM when in a metal saucepan with the lid on?

Maybe there’s a tiny gap somewhere around the lid, but I’m not sure why that should let the AM signal in but not the FM.  Currently this effect has me stumped.  Has anyone else observed the same? Worth doing a science fair project on it?

Which brings me to my last point – I am totally gutted that I’m not available to be a judge at the Waikato science fair last year. I’ll still come along on 19 August to look at the posters and talk to the presenters, but I can’t make the judging part of it. I know I’ve bleated on occasionally about too many energy-saving light bulbs etc, but judging is such a great thing to do and I’ve loved it in the last few years. The best thing is getting to talk with future scientists about what they’ve done – some of the enthusiasm is quite something. I hope to be back next year.

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