Thursday, January 07, 2010

Hand Capacitance

Ever notice that when you touch the radio the volume changes and/or the station gets clearer? Isn't that completely infuriating? It's caused by hand capacitance. It is the power behind that phenomena and also he way we play the Theremin. The effect is actually very subtle and really interesting.Here's the basic idea. Your hand has a certain measure of resistance, as all matter has a certain amount of resistance. It's a relatively high amount of resistance, but also varies. You can even use yourself as a connector between two components and allow the current to pass through you. Your hand can be used as the dialectic of a capacitor. This would be unsafe at high voltages of course. This part of the idea is pretty intuitive.

Here's how that works: A capacitor is a passive electronic component. In it's simplest form consists of a pair of conductors separated by a dielectric. A dielectric is just an insulator. So on a touch pad you may be the dielectric between two copper conductors. Chemically the human body is about 75% water, that H20, Oxygen and Hydrogen. Water is not a good conductor of electricity. That's why we are a good dielectric.

1. Oxygen (65%)
2. Carbon (18%)

3. Hydrogen (10%)

4. Nitrogen (3%)

5. Calcium (1.5%)

6. Phosphorus (1.0%)

7. Potassium (0.35%)

8. Sulfur (0.25%)

9. Sodium (0.15%)

10.Magnesium (0.05%)

Less intuitive is how it can work at a distance. In this application an antenna is essentially a proximity detector. The capacitance you provide drops off as you move away. Even though we're measuring this effect in picofarads the change is audible. Variable capacitors and whip antennas are particularly vulnerable.

What's happening in this case is that you are in the magnetic field of the radio. The antenna is an electrode. Electrodes are conductors that make contact with non-metallic parts of circuits. This can include dielectric even a vacuum through induction. Your proximity to the electrode changes the amount of current moving through the circuit because your body has less resistance than the air space between you and the radio. That changing distance is a change in coupling capacitance. The other part of the circuit is the Earth itself.