ELEVATORS. The modern elevator is a direct evolution from the machine which Elisha G. Otis exhibited in 1853 at the World's Fair in the Crystal Palace, New York. Hoists of various kinds had been built before that time, but this was the first elevator wherein provision was made for stopping the fall of the car in the contingency of the breaking of the hoisting cables. During the next five years a number of machines were built similar to that exhibited, all being driven from line shafting. In 1859 the same inventor introduced an inde pendent reversible steam-engine directly con nected to the hoisting machinery, and from that date the era of the elevator as a separate insti tution of the age began. In 1871 the hydraulic elevator was introduced, soon to attain pre dominance in the elevator art and displacing the steam-engine. The year 1888 witnessed the first application of the electric motor to elevator ma chines, destined in turn to eclipse the hydraulic elevator. The first type of electric elevator ma chine, still in use to-day for low and moderately high buildings, consisted of an electric motor actuating a hoisting drum through the inter mediary of worm gearing. Although this ma chine has been developed to operate satisfactor ily at comparatively high speed, it could not satisfy the requirements imposed on the ele vator art with the advent of the skyscraper. Thus in 1903 a new type of electric elevator machine was developed, known as the 1:1 gear less traction machine, which has since Com pletely ousted the hydraulic machine from the field of high-rise, high-speed elevators. Another development in the electric elevator art is the so-called microdrive machine, first introduced in 1915. This machine is capable of accurately and automatically stopping an elevator platform level with the landing under any condition of loading. It is extensively used in all cases where heavy loads have to be wheeled on or off the elevator platform on trucks.
It is quite evident that the high state of de velopment of the electric elevator to-day could not have been accomplished without improve ments in the design of electric motors and con trolling devices. At the time of the introduc tion of the electric elevator in 1888 the design of the direct-current motor was already well advanced, while alternating-current motors were yet in their infancy. In the next decade the energy of designers was mostly bent upon the further development of the application of direct current, resulting in 1897 in the introduc tion of the direct-current magnet controller.
With this invention the direct-current electric elevator at once entered the field of high-speed elevator service and became a dangerous com petitor of the hydraulic elevator.
The electric power systems at that time were mostly direct current, but began gradually to make place for the more economic two- or three phase alternating-current systems. Along there with polyphase induction motors had been in troduced and began to be applied to elevators. The first attempts were not very promising. It would seem for a time that the polyphase alter nating-current elevator never would be suitable for high speed, owing to the fact that the motor operated only at a fixed single speed and to the inability to design mutable alternating-current magnets. To-day all of these difficulties have been overcome; polyphase induction motors are now easily built for two or more speeds as well as alternating-current magnets of sufficient power to operate controller and brake. In fact, the number of alternating-current installations to-day equals the number of direct-current in stallations and is doubtlessly destined to exceed the latter in the near future. Safety appli ances were developed hand in hand with the development of the various types of machines. Grips to arrest and stop a falling cage were first designed to operate upon the breaking of the hoisting rope only. These soon proved to be inadequate, since they remained inactive in runaway accidents from various causes not due to the parting of the ropes. This defect was removed with the introduction of centrifugal governors, which actuate the safety grips when the car speed exceeds a predetermined maxi mum. In 1890 steel began to be used for guide rails, which previously to that date consisted exclusively of wood. This brought about a new type of safeties. Other demands came with the increase in elevator speeds, necessitating the design of safety grips capable of arresting a falling car without shock or injury to the passengers. Other safety appliances gradually developed but which have now become part of the standard equipment are: automatic stop at the terminals of the travel; slack cable devices to prevent further motion of the machine in case the car is obstructed in its descent; door locks to prevent the starting of the cage as long as the door is open, and to prevent the opening of door unless the car is at rest at the landing.