HOW THE TRACKPAD/TOUCHPAD WORKS?

The trackpad, also known by the less-descriptive term touchpad, was invented
by a fellow named George E. Gerpheide in 1988. Gerpheide and his small
company patented the technology and called it GlidePoint, but it didn?t see
commercial success until Apple Computer introduced its PowerBook 500 series
laptops with trackpads in 1994. Since the trackpad mechanism doesn?t have any
moving parts, it?s less prone to being fouled by dirt and dust.
The trackpad relies on a principle called coupling capacitance to know where
your finger is. This has been defined as ?the transfer of energy from one circuit
to another by means of the mutual capacitance between the circuits.?
Capacitance is a measurement of the ability to store electric charge ? it?s the
potentially-lethal charge stored in a very large capacitor that makes opening up a
television or CRT monitor dangerous. Underneath the trackpad?s surface are
two layers of tiny rows of parallel wires known as electrodes. One row runs
from side to side, and the other from top to bottom, with a thin layer of material
between them. The two layers form a grid ? any point on the grid can be
identified by the two wires that cross paths at that point. Alternating current <>
runs through the wires, and there is a measurable capacitance between the two
layers of electrodes. When you touch the pad, a tiny amount of current from one
of the wires travels through the surface of the pad, and through your fingers, to
another wire. This miniscule change in the current through the wires changes the
capacitance between the two wires that cross paths under your finger. This
change in capacitance can be measured, and is associated with that spot on the
grid, so that the computer knows that?s where your finger is. As you move your
finger across the tiny grid, the computer?s cursor tracks a similar path across the
screen.
Since trackpads don?t rely on pressure or heat, you can?t use one if you?re
wearing mittens or have particularly greasy fingers ? electricity has to be able to
flow through your fingertip. So while these devices are generally impervious to
the dirt and dust that can foul a trackball, water droplets on the pad will confuse
the circuitry, since water conducts electricity. Likewise, if you accidentally
touch the pad with two fingers at once, it detects a change in capacitance at two
non-adjacent points on its grid, and doesn?t know what to do with that
information, so the cursor may jump around erratically on the screen.
That?s the one major drawback to trackpads from a technological standpoint ?
they actually measure position, rather than movement. Trackballs and mice can?
t only know when they?re being moved, so they just tell the computer to move
the cursor in a given direction ? there?s no way to make a jump from one point
to another without passing through the points in between. If you touch a
trackpad with multiple fingers, or get it wet, it will tell the computer to display
the cursor at non-adjacent locations, which results in cursor behavior that?s
largely inconsistent with what we expect from our computers. <> The trackpad?
s inventor saw the device as a more natural way for humans to interact with
graphical human-computer interfaces, but this unnatural behavior (and the
advent of affordable optical mice that also benefit from a lack of moving parts)
has limited the appeal of the technology to laptops and devices like Apple?s
iPod. - Thanks Joe Elliot