Perhaps the best way to illustrate how elevators work is to take a virtual trip to the surprisingly well-fortified WiseGEEK offices on the 65th floor of a modern office building. As you first enter the lobby, you'll notice a bank of elevators clustered together. What you're actually seeing is one set of doors and a call button. When you press that call button, a command is sent to a computer controlling all of the elevators. The computer determines which of the elevators is moving in a downward direction and is closest to the ground floor.
When the selected elevator car reaches the ground floor, the computer commands the electric motor at the top of the elevator shaft to stop gradually. Once the computer senses that the elevator's door and the lobby door are at the same level, it commands electrically powered arms to retract both doors. At this point, you may enter the elevator car itself. By pressing the button numbered "65," you have now sent an order to the computer controlling all of the elevators. The computer knows you want to reach the 65th floor, but what it actually does is use magnetic or optical sensors to count the number of dots on a long tape positioned on the side of the elevator shaft. When the car has passed enough dots, the computer orders the electric motor turning the winch to stop.
Elevators use steel cables and a powerful winch to lift or lower their cars. To reduce the amount of energy needed to overcome friction and gravity, elevators also use heavy steel plates as counterweights to the passenger cars. These counterweights are attached to the other side of the steel cable loops, and generally weigh about 40% as much as the maximum load limit of the passenger cars. When the passenger car goes up, an electric motor turns a winch and the counterweights fall towards the ground. You may feel a breeze if you stand in front of a cluster of elevators. This breeze is caused by the counterweights as they pass your floor.
Once the elevator's computer has sensed you've reached the 65th floor, electric motors open both the elevator and lobby doors. If you should happen to hesitate between the doors, mechanical and electrical sensors should prevent the doors from closing. If there is no elevator car behind a lobby door, the lobby door should not open. Elevators have a significant number of redundant systems and safety features built into them, including emergency brakes, call boxes and shock absorbers. The chances of elevators snapping all of their cables and plummeting to the ground are virtually zero.
Believe it or not, there is a reason why it sometimes takes a long time for elevators to reach your floor. The computer controlling all of the elevators is programmed to provide the most efficient service possible. Elevators tend to go all the way up before beginning a trip down. If you press a call button on the ground floor, the elevators going up will most likely ignore the call entirely. Only the elevators heading down will reach the ground floor in a timely fashion, and that's only after discharging passengers on all the other floors. This is also why you may have to take a trip up before you reach your lower destination.
Some modern elevators anticipate surges at various times of the day, such as shift changes or lunch breaks. Computers in charge of these elevators may send more cars to a specific floor at a specific time to handle the sudden demand. The elevators may head down to the parking garage level or up to a company cafeteria, based on a history of elevator usage. Future elevators may find other ways to accommodate the needs of passengers without the delays caused by the current "up or down" philosophy. Call buttons may also be replaced by more interactive devices which respond to voice commands or electronic badges worn by employees working on specific floors.
