Thursday, January 6, 2011

How Electrically Active Are Dental Fillings?

One of the more bizarre news stories today is this one at BBC News:

Iran detains American woman over 'spy device' in teeth

On the surface, this is just another story about a failed state, so eager to make spying arrests for propaganda purposes that it is willing to pass off an ordinary dental filling as an electronic device — some sort of microphone, officials there are suggesting.

From my own experience as a guitarist, I can tell you that microphones that small exist. I can also tell you that microphones are useless without at least a wireless transmitter, including a power supply on the order of two AA batteries — not so easy to hide in a belt pack onstage, though we do our best. And that’s just for a wireless signal that you hope will make it to the side of the stage; you’d need more power to transmit over a longer distance. A microphone could also be connected to a recorder, but those are even larger. So the suggestion that all this was shrunk down to a few cubic millimeters and hidden in a tooth is not to be taken seriously. It is the crazy claim of a desperate government.

But there is more to the story than this. When dental fillings are made of metal, and particularly when they are constructed of several metal elements together, they are electrically active. They create electric currents from the fluids that are naturally present in the mouth. If the border patrol were scanning for electronic devices, a dental filling would come up with the same electromagnetic signature as a battery or microphone. Usually, this would be a very small electromagnetic pattern, but if a filling is large and is patched together from dissimilar metals, it could be more prominent. Then, if the border patrol is sufficiently paranoid, they could scan a dental filling and imagine a microphone.

If you took a high school physics class, you probably saw a physics teacher throw together some water, mineral salts, and metals and create a battery. If you put metal and water together, you almost can’t help creating an electric current. It is safe to say that all metal dental fillings create electric currents, and this is something you can detect using an ordinary electric multimeter.

We really don’t know what the consequences of having random electric currents in the mouth are, but there are at least two reasons why some scientists are concerned. First, electricity slowly dissolves any metal that might be present. In the mouth, of course, most of the metal is mercury. Second, the mouth is very close to the brain, which is known to be affected by electrical activity. Scientists have been trying to quantify these effects, but the results so far are really just guesses, and there could be other effects we haven’t thought of yet.

Probably, I hope, the electric currents that most of us have running around our teeth are just one of those curiosities of physics. But for one woman who was stopped at the border of Iran, the electricity was enough of an excuse to get her arrested and charged with spying.