Detecting weak electric fields with your bare hands

Detecting when an electric blankets is switched on

I have often wondered about the effect that electricity may have on our bodies. Years ago I discovered that I could tell when Mrs Piffle’s electric blanket was switched on while she was laying on it by running my finger very gently down her arm. When the blanket was switched off it felt smooth. When it was on I could feel a slight vibration which is hard to describe. It certainly felt less smooth. We did a series of tests with her turning it through the different settings – sometimes on sometimes off, and I could always tell when it was on or off. Just for the record, the electric blanket was on a foam mattress on a wooden base.

Do I have super powers? Am I particularly gentle person? Am I just awesome? Probably not. If I can detect this when some others can’t it is more likely that I benefit from baby bottom soft skin on my hands from years of office work. A few other people who have tried it, including Mrs Piffle have been able to detect the effect too, so it seems to be common, but due to the weakness of the effect maybe we just don’t usually notice it.

Electricity from a TV antenna

I had also noticed a similar, but stronger effect when I rubbed my finger along the rabbit ears antenna that was connected to our previous television but only when the TV was turned on. I’ve read that leakage can occur within TVs resulting in some power being released into the antenna socket. So, that probably explains that one.

Power from a laptop

I had forgotten about it until recently after my wife got a Macbook Pro. She was sitting on the bed one day with the laptop connected to the charger and I touched her arm and got exactly the same vibration. We tried unplugging the power connecter from the side of the laptop and the effect stopped, plugging it back in and it returned. Another series of blind tests showed that I could tell every time if it was connected or not. We did another series later and at that time the effect felt much weaker, so there must be something else at play. These laptops are in an aluminium case which is probably relevant. It’s interesting to note that the effect only happens when it is plugged into mains power. We then tried with the lid closed in sleep mode and the effect was still there, and then with it shut down, but still plugged into the power adapter and again the effect was still there.

Mrs Piffle can even notice it herself through the tips of her fingers sometimes when she is resting them on the metal case in front of the keyboard.

The power adapter for the Macbook has a fairly low output voltage of just 16.5v – not what I would have thought could be detected with your finger.

Nothing from an iPad

We also have an iPad so I did a few tests with that – after all it also has an aluminium body, but I could not detect any difference.

Is it a form of mains hum?

The mains power her is 240v 50Hz. Perhaps the higher mains voltage is relevant, perhaps not. The frequency of the vibrations seems identical in all tests so perhaps it is 50Hz from our mains electricity, but I can’t think of away to measure it with the equipment I have. It’s odd and very interesting, at  least to me.

My technique

I found I got best results when you rub an area of skin that has no hair as the roughness of the hair makes it more difficult. Inside arms and the side of the neck seems to work well (except with the rabbit ears antenna – I rubbed my fingers up along its … ears?:)). Sometimes I use a fingertip, other times I find a knuckle works better.

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Posted on July 15, 2012, in Electronics, Science and tagged , , , . Bookmark the permalink. Comments Off on Detecting weak electric fields with your bare hands.

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