John Summerfield wrote: : Dean S. Messing wrote: : : > I took think the pad is a wee bit sensitive to the touch when I'm : > banging away at the keyboard. Sometimes my thumb will brush it and : > (at least w/in emacs) the "point" will just to where the cursor sits : > and I'm now entering text in the wrong place. Thank goodness for the : > powerful "undo" features of emacs. Solution: thumbs up! : : Touchpads are misnamed; they don't require touch. There's some kind of : proximity detector involved. I believe it is a capacitive sensor and depends on the Dielectric Constant of the material between the pad and one's finger. I just did several little experiments to check. (I'm a scientist, after all :-) First, I took my finger and put it really close to the pad w/o touching it. No cursor motion. (Air has a small dielectric constant.) Then I took a thin piece of paper and tried to move the cursor by sliding finger over paper. No go unless I pressed very hard. Paper is pretty incompressible so I don't think it was that my finger was getting closer. Rather the surface area of the contact was increasing so the capacitive effect was greater the harder I pressed. Interestingly, through a much thicker envelope glue label, my finger had no trouble moving the cursor at normal pressure. My guess is that the glue in between the label and the backing has a high Dielec. Const. Finally, I used my finger nail. Being an insulator, it had no effect even pressing hard. As an aside to this aside, I once worked for a company that took security very seriously. They had RFID-activated doors installed all over their laboratory to keep the secrets in and the intruders out. Problem was that each door had (on the inside) a capacitive "rail" that one could push against to open the door normally from the inside. These were double doors with a small space between them. I wondered one day if a wire coat hanger, bent like a hook, with me touching one bare end from the insecure side and the other bare end through the space touching the railing would release the lock. Voila! So much for the $50K or so they spent on internal building security. Ain't physics fun? <snip> Dean
