Several co-workers and myself have been debating on what a laptop touchpad is sensitive to. We have found that objects other than your fingertip (i.e. pencil eraser, cell phone, back of fingernail) do not activate a touchpad ( a
touchpad is a mouse substitute frequently found on laptop computers). So what makes it work? Is it the ridges on your fingertip? The heat from your body? Surface area? A certain amount of friction?
Any ideas?
Thanks,
GIPNOR
I know this is backwards logic, but I can offer some thoughts here. I’ve left my laptop in the car on more than one occasion ( sadly… ). It turned on just fine, and the touchpad worked just fine- even though the surface of it felt HOT to my fingertip’s touch. If it was warmer than my body temperature, how did it sense my body heat??
I’ll agree that other objects besides a finger don’t work well, but I haven’t tried dragging a gherkin across it’s surface. Surely a job for the Straight Dope Labs 
Cartooniverse
It senses electrical capacitance from your finger. Have you ever seen a touch lamp (a lamp that turns on when you touch the metal base)? A laptop touchpad is basically a large array of tiny touch-lamp on/off switches. As you drag your finger across it, you turn on/off different switches in the array, which is interpreted as a “mouse” movement by the driver software. Naturally there are a huge amount of complications and details, but that’s basically what happens.
Here’s a link to a module: http://www.glidepoint.com/modules/tsm9957.html
Water has an unusually high dielectric constant (a constant of materials that affects how much capacitance something has), which makes it easier to sense with a touch pad. Fortunately, fingers are mostly water. A water balloon, or condom filled with water, should work on a touch pad. I’d guess a wet gherkin might work also 
Arjuna34
Oddly enough, however, it doesn’t like to work immediately after I’ve washed my hands…
It’s probably sensing sweat/oils on the surface of your skin, which get washed away. If you press harder (to put the water in your skin closer to the touchpad), it should work better. The capacitance between your finger and the touchpad increases proportionally to the decrease in distance, so the closer you get, the better it works.
Unless your hands are still wet, which means that your providing too much capacitance (with water directly on the touchpad surface).
Arjuna34