An elevator moves things or people from one level to another, and is important to tall structures like skyscrapers. The car of an elevator, in which people ride, is attached to guard rails inside a tall, empty space called a shaft. It is moved by a steel cable that is attached to a large weight that counterbalances it. An electric motor raises and lowers the cable, changing the positions of the car and weight as the elevator moves from floor to floor. (Usually posted inside an elevator are numbers that indicate the carís weight limit; an elevator motor cannot do its job if a car is a lot heavier than the weight that balances it.) The first elevators in use were not especially safe because once in a while a cable would break, and a car, pulled by gravity, would come crashing down. Safety devices were soon added, though, to keep such disasters from occurring. (American inventor Elisha Otis invented the first "safety" elevator in 1853.) Additional ropes attached to cars and powerful metal "jaws" that grip guard rails keep elevators from falling if their main cables break. Other safety devices keep elevators from moving when their doors are still open and from traveling too fast. Automatic switches in the shaft allow an elevator to hurry past unwanted floors, or to slow and stop when a chosen floor is reached, unlocking its doors to admit and release passengers. Very long elevators are not always practical, so some buildings use one set of elevators to take passengers part way up the building and another set to service the upper floors.