Most modern slot machines are designed to look and feel like the old mechanical models, but they work on a complete different principle. The outcome of each pull is actually controlled by a central computer inside the machine, not by the motion of the reels. The computer uses step motors to turn each reel and stop it at the predetermined point. Step motors are driven by short digital pulses of electricity controlled by the computer, rather than the fluctuating electrical current that drives an ordinary electric motor. These pulses move the motor a set increment, or step, with great precision. But even though the computer tells the reels where to stop, the games are not pre-programmed to pay out at a certain time. A random number generator at the heart of the computer ensures that each pull has an equal shot at hitting the jackpot. Whenever the slot machine is turned on, the random number generator is spitting out whole numbers (typically between 1 and several billion) hundreds of times a second. The instant you pull the arm back or press the button, the computer records the next few numbers from the random number generator. Then it feeds these numbers through a simple program to determine where the reels should stop.   .