How the heck does this laptop glidepoint thingie know it is my fingertip? I can’t move the cursor with my fingernail or my knuckle, but I can with my fingertip or…and don’t ask how I know this… my nose. What gives?
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O p a l C a t
www.opalcat.com
Basically the glidepoint surface contains a grid of fine wires. The system determines where your finger is by checking the inductance of the wires; it changes in the ones you are touching. (Simplified explaination.)
Your fingertip and nose are fleshy, which means they have a fairly high water content and have a larger effect on the inductance. Your fingernail and knuckle bone (the skin is very thin over your knuckle) have a very low water content and so have less effect.
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Actually, it is a capactive sensor. The dielectric constant of water is very high (about 80) compared to air (about 1) or solids (usually less than 5 or so). The water in your finger changes the capacitance sensed by the wire grid. The point in the grid where the change is greatest is where the sensor detects the touch. You can check out www.glidepoint.com for more info.
Arjuna34
Hey OpalCat! What timing - this very question was on my list of things to ask today!
tanstaafl: But why, then, does an item that I know to conduct electricity (copper wiring) not have any effect at all?
Arjuna34: I’m not really sure I understand your answer. Could you put it in non-electrical engineer lingo, please? From what I understand you’re saying, though, anything that conducts electricity reasonably better than the surrounding air will activate the touch pad. Copper wire (which surely has electrical conductivity at least equal to, if not greater than, my finger) doesn’t. Why?
~ Complacency is far more dangerous than outrage ~
Sorry for the EE lingo! 
The dielectric constant of a material isn’t how well it conducts electricity, but sort of how efficiently electric charge can be stored in the material. Conductors like copper can’t store much charge at all- since they conduct, the charge flows away. Air has a dielectric constant of about 1.00, while water is about 80. Most plastics are about 2. Marble is about 8- this might be high enough to trip the touch pad. None of these materials are good electrical conductors (i.e. electricity won’t flow through them), but they can store electric charge (think static electricity).
Anyway, you can think of the wire grid as putting a small, grid-like electric field over the touch pad. The amount of electric charge in this field depends on the dielectric constant of the material over the touch pad. If it’s just air, it’s not much charge. If you put your finger on the touch pad, the electric field increases only where your finger is, because the dielectric constant there is higher (due to the water in your finger). The energy needed to increase the electric field comes from the voltage on the wires. The touch pad senses this increased field (by measuring voltage and current on the wires, which changes when the electric field changes), and since it’s a grid, can figure out where it happened.
If there weren’t a plastic cover over the grid of wires, you’d touch the wires directly, and screw things up since your finger would conduct electricity directly from one wire to the next, instead of interacting via electric fields over the wires.
Arjuna34