Explain the Trackball and Thumbwheel devices

 Joysticks

A joystick consists of a small, vertical lever (called the stick) mounted on a base that is used to steer the screen cursor around. Some joysticks are mounted on a keyboard. Most joysticks select screen positions with actual stick movement; while a few respond to the pressure on the stick. Figure 1.10 shows a movable joystick.

 The distance moved by the stick in any direction from its center position and the distance of the screen cursor movement in that direction are proportionate. Potentiometers mounted at the base of the joystick are used to measure the amount of movement, and the stick returns to the centre position when it is released due to the spring. One or more buttons can be programmed to act as input switches to signal certain actions once a screen position has been selected.