Point and click! Without pointing devices like trackballs and mice, using a computer would be much more difficult. But how do these devices work?
Opto-mechanical trackballs like this use a blinking light to determine how the ball is being rolled. Pushing the trackball causes the two shafts and attached encoding wheels to spin. A pattern of blinking light is created as the LEDs shine through the holes in the encoding wheels. The light sensors opposite the LEDs detect the blinking light. The faster the wheels turn, the faster the light blinks!
The light sensors are connected to a microprocessor, which counts how many times the lights blink each second. The microprocessor then sends a series of electrical pulses to the computer through the connecting cord. These pulses describe how fast and in what direction the trackball is moving.
It only takes one LED and one light sensor to figure out how fast the trackball is rolling. However, two LEDs are needed to figure out the direction of trackball movement. The two LEDs at each encoding wheel each emit different kinds of light. By sensing which light appears first, the microprocessor can determine which way the trackball is rolling.
A trackball is like a computer mouse that has been turned on its back - instead of rolling the ball by moving the mouse along a mouse pad, you push the ball directly with your hands.
There's an interview with the creator, Douglas Engelbart, at Superkids. Mr. Engelbart grew up in Portland, Oregon, and attended Oregon State University, according to this biography.
But what about Optical mice?
If you'd like to learn about how optical mice work, using a camera instead of a ball, read about the Apple Pro Mouse.