Of Mice and Men
The Mouse is an input device used in a variety of functions. Chances are if you are viewing this page you probly have a mouse on your desktop right now. There are two difrent kinds of mouse. (I'm not going to say mice while dealing with hardware) They both do the same thing but in a diffrent fashion.
The first style, an optic mouse, uses a special mouse pad and photo receptors (light sensors) to sense the movement of the mouse. This animation will illustrate the point better.
As the mouse is dragged light beams bounce off the black lines and back to the photoreceptors in the mouse. So as the mouse crosses a line the bouncing beeams transmit this information to the computer. One plus to this type of mouse is that it has no moving parts to break down.
The second type. Which is far more common involves a ball and rollers or photo receptors. (those little light thingys again). In the second form the mouse ball turns a set of wheels which generate current in the circuts. By recieving signals from the difrent circuts and information as to which way the wheels are turning the computer can move the pointer around accordingly. While both work well most manufacturers follow the KISS method (Keep It Simple Stupid) and since photoreceptors are expensive and likely to break down they are not used as much as the traditional ball and roller.