Explain the uses of thumbwheels
Two thumbwheels are usually required to control the screen cursor in its horizontal and vertical position respectively.
As the name implies, a trackball is a ball that can be rotated with the fingers or palm of the hand to produce screen-cursor movement. Potentiometers, attached to the ball, are used to measure the direction and amount of rotation. Trackballs are often mounted on keyboards or other devices such as the Z mouse as explained earlier.