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A trackball is a pointing device consisting of a ball housed in a socket containing sensors to detect rotation of the ball about two axes—like an upside-down mouse with an exposed protruding ball. The user rolls the ball with the thumb, fingers, or the palm of the hand to move a cursor. Large tracker balls are common on CAD workstations for easy precision. Before the advent of the touchpad, small trackballs were common on portable computers, where there may be no desk space on which to run a mouse. Some small thumbballs clip onto the side of the keyboard and have integral buttons with the same function as mouse buttons.
When mice and trackballs still had chopper wheels, trackballs had the advantage of being in contact with the user's hand, which is generally cleaner than the desk or mousepad and doesn't drag lint into the chopper wheels. The late 1990s advent of scroll wheels, and the replacement of mouseballs by direct optical tracking, put trackballs at a disadvantage and forced them to retreat into niches where their distinctive merits remained important. Some mice, in place of a scroll wheel, acquired a small trackball between the ears, useful in maps, badly laid out Web pages, and other circumstances calling for scrolling in two dimensions.
Large tracker balls are sometimes seen on computerised special-purpose workstations, such as the radar consoles in an air-traffic control room or sonar equipment on a ship or submarine. Modern installations of such equipment may use mice instead, since most people now already know how to use one. However, military mobile anti-aircraft radars and submarine sonars tend to continue using trackballs, since they can be made much more durable, more fit for fast emergency use and, in the case of large and well made ones, allow easier high precision work.
Trackballs have had some limited use in computer and video games, particularly early arcade games (see a List of trackball arcade games). One of the more famous games to use one is Centipede. "Football", by Atari, was the first arcade game to use a trackball, released in 1978 for the arcade - though Atari spells it "trak-ball". Console trackballs, once common in the early years, are now fairly uncommon. The Bandai Atmark, a Japanese console, had a trackball as standard for its gamepad, and the Atari 2600 had one as a peripheral, with a joystick as standard. Trackballs remain in use in pub golf machines (such as Golden Tee) to simulate swinging the club.
Trackballs are provided as the pointing device in some public internet access terminals. Unlike a mouse, a trackball can easily be built into the console - and, as such, cannot be ripped away or otherwise vandalised. Two examples are the Internet browsing consoles provided in some UK McDonalds outlets, and the BT Broadband Internet public phone boxes.
Some users of trackballs contend that because trackballs are stationary, they require less space for operation, and allow use in confined or cluttered areas such as a small desk
Some computer users prefer a trackball over the more common mouse for ergonomic reasons. There doesn't seem to be conclusive evidence of one being better than the other in terms of comfort. Users are recommended to test out different devices, and to maintain proper posture and scheduled breaks for comfort. 
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
- ^ Center for Disease Control web page about computer ergonomics
Categories: Articles to be expanded | FOLDOC sourced articles | Pointing devices | Game controllers | Computer hardware stubs