A touchpad (also trackpad) is a pointing device consisting of specialized surface that can translate the motion and position of a user's fingers to a relative position on screen. They are a common feature of laptop computers and also used as a substitute for a computer mouse where desk space is scarce. Touchpads vary in size but are rarely made larger than 40 square centimeters (about 6 square inches). They can also be found on personal digital assistants (PDAs) and some portable media players.

Operation and function

Touchpads operate in one of several ways, including capacitance and conductance sensing. The most common technology used today entails sensing the capacitance of a finger, or the capacitance between sensors. Because of the property being sensed, capacitance-based touchpads will not sense the tip of a pencil or other similar implement. Gloved fingers will generally also be problematic (such as in a cleanroom environment). Likewise, moist, sweaty, or callused fingers will also affect capacitance sensing. However, some touchpads, such as have been manufactured by Elographics, sense electrical conductance. In the case of the Elographics touchpads, there are little nipples that separate a flexible top surface from a lower surface. When the top surface is pushed into contact with the bottom surface, using either a finger or any other device, the point of contact can be sensed due to the signal being conducted. In this case, one can use a finger or a wooden stylus, and performance will not be affected by sweat or calluses, for example.

Like mice, touchpads are relative motion devices. This means that the relative motion of a finger on the touchpad surface causes the relative motion of the cursor on the screen. The buttons below or above the pad serve as standard mouse buttons. Depending on the model of touchpad and drivers behind it, you may also click by tapping your finger on the touchpad, and drag with a tap followed by a continuous pointing motion (a ‘click-and-a-half') . Touchpad drivers can also allow the use of multiple fingers to facilitate the other mouse buttons (commonly two-finger tapping for the center button).

Some touchpads also have “hotspots”: locations on the touchpad that indicate user intentions other than pointing. For example, on certain touchpads, moving the finger along an edge of the touch pad will act as a scroll wheel, controlling the scrollbar and scrolling the window that has the focus vertically or horizontally depending on which edge is stroked. Apple uses two-finger dragging gesture for scrolling on their trackpads. However, these are driver dependent functions and can be disabled. Also, certain touchpad drivers allow for tap zones, regions whereby a tap will execute a function. For example, pausing the media player or launching an application.