| Similar to a mouse, a touchpad is a device for moving a pointer around a
computer display. Unlike a mouse, the touchpad does not need to be physically
moved, making it ideal for use in a notebook computer. Additionally, a touchpad
is a sealed unit so it is unaffected by crumbs or dust, a real asset for a
portable device that might be used in almost any environment.
George E. Gerpheide invented the touchpad in 1988. Apple Computer licensed his
design and first used it in its PowerBook notebook computers in 1994. Since
then, the touchpad has become the most common cursor control device for notebook
computers. Alternative notebook cursor control devices include the trackball and
the TrackPoint pointing stick IBM uses in its ThinkPad computers in conjunction
with a touchpad.
A touchpad contains several different layers of material. The top layer is a
protective coating designed to let your finger glide over it while protecting
the rest of the unit from wear and contamination. Below the top layer are two
layers of electrodes in a grid arrangement. The upper layer uses vertical
electrode strips while the lower layer uses horizontal strips. A very thin layer
of insulation separates each layer, and the layers are connected to an
integrated circuit. The electrodes are charged with alternating current, and the
integrated circuit constantly measures the capacitance of the horizontal
electrodes to each of the vertical electrodes. When the user touches the pad,
the capacitance between the two layers of electrodes changes since the
dielectric properties of a finger are very different from the air it displaces.
By measuring this change in capacitance, the integrated circuit knows exactly
where the pad was touched. As a finger moves over the touchpad, the changes in
location are noted by the integrated circuit and translated into cursor
movements. Most touchpads let you tap the pad instead of clicking a button. The
integrated circuit registers this, as well. Many touchpads also include buttons that work like
mouse buttons.
A few desktop keyboards include a touchpad; however, they have not proven
popular with users. Some proponents of touchpads feel they are less likely than
a mouse to cause repetitive motion injuries; not all experts agree with this
assessment.
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