Historical notes for N08-Geneva Wheel

 

By Daina Taimina

 

The Geneva wheel, also known as the Maltese cross or Geneva ratchet, is a cam-like mechanism that provides intermittent rotary motion and is widely used, in both low- and high-speed machinery.

 

  

     Clock mechanism in 18th century pocket watch       Watchmaker, woodcut by Jost Ammann, 1568

 

A watchmaker originally invented the Geneva mechanism. The watchmaker only put a limited number of slots in one of the rotating disks so that the system could only go through so many rotations. This prevented the spring on the watch from being wound too tight, thus giving the mechanism its other name, the Geneva Stop.

 

Spring-driven clocks appeared in Italy toward the end of the 15th century. Peter Henlein of Nuremberg, Bavaria, who qualified as a master locksmith in 1509, was building them in 1510. Watch making is one of the best-known industries in Switzerland and can be traced back to the arrival of French-Protestant refugees in Geneva in the 16th century. The development of the spring enabled watchmakers to build smaller and smaller timepieces. Such clocks also became more intricate and highly decorated, but they were not particularly accurate until 1660, when Robert Hooke invented the balance wheel and hairspring, made practicable by his friend Thomas Tompion. George Graham, who invented the deadbeat escapement, invented a cylinder escapement for watches in 1725.

 

Another well-known application of the Geneva wheel is a movie projector. Motion-picture photography is based on the phenomenon that the human brain can perceive an illusion of continuous movement from a succession of still images exposed at a rate above 15 frames per second. Although sequential posed pictures had been taken as early as 1860, photographers were not successful in capturing actual movement until 1877, when Eadweard Muybridge used 12 equally spaced cameras to demonstrate that all four hooves of a galloping horse leave the ground simultaneously at some time. In 1877-78 an associate of Muybridge devised a system of magnetic releases to trigger an expanded battery of 24 cameras.

 

The Muybridge pictures were widely published in still form. They were also made up as strips for a popular parlor toy, the Zoetrope "wheel of life," a rotating drum, which induced an illusion of movement from drawn or painted pictures. Although a contemporary observer of Muybridge's demonstration claimed to have seen "living, moving animals," such devices lacked several essential characteristics of true motion pictures.

 

A motion-picture camera must be able to advance the medium rapidly enough to permit at least 16 separate exposures per second, while at the same time bringing each frame to a full stop to record a sharp image. The principal technology that creates this intermittent movement is the Geneva wheel mechanism, in which a four-slotted star wheel, or "Maltese cross," converts the tension of the mainspring to the ticking of toothed gears. In 1882 Étienne-Jules Marey (1830-1904) employed a similar "clockwork train" type of intermittent movement in a photographic "gun" used to "shoot" birds in flight. Twelve shots per second could be recorded onto a circular glass plate.

 

Marey subsequently increased the frame rate, although for no more than about 30 images, and employed strips of sensitized paper (1887) and paper-backed celluloid (1889) instead of the fragile, bulky glass plates. The transparent material trade-named celluloid was first manufactured commercially in 1872. It was derived from collodion, nitrocellulose (gun cotton) dissolved in alcohol and dried. John Carbutt manufactured the first commercially successful celluloid photographic film in 1888, but it was too stiff for convenient use. By 1889 the George Eastman company had developed a roll film of celluloid coated with photographic emulsion for use in its Kodak still camera. This sturdy, flexible medium could transport a rapid succession of numerous images and was eventually adapted for motion pictures.

 

Thomas Edison (1847-1931) is often credited with the invention of the motion picture in 1889. However, the claim is disputable specifically because Edison's motion-picture operations were entrusted to an assistant, W.K.L. Dickson, and generally because there are several plausible pre-Edison claimants in England and France. Indeed, a U.S. Supreme Court decision of 1902 concluded that Edison had not invented the motion picture but had only combined the discoveries of others. His systems are important, nevertheless, because they prevailed commercially.

 

The heart of Edison's patent claim was the intermittent movement provided by a Maltese cross synchronized with a shutter. The October 1892 version of Edison's Kinetograph camera employed the format that is essentially still in use today. The film, made by Eastman according to Edison's specifications, was 35 millimeters (mm) in width. Two rows of sprocket holes, each with four holes per frame, ran the length of the film and were used to advance it. The image was 1 inch wide by 3/4 inch high.

 

At first Edison's motion pictures were not projected. One viewer at a time could watch a film by looking through the eyepiece of a peep-show cabinet known as the Kinetoscope. This device was mechanically derived from the zoetrope in that the film was advanced by continuous movement, and action was "stopped" by a very brief exposure. In the Zoetrope, a slit opposite the picture produced a stroboscopic effect. In the Kinetoscope the film traveled at the rate of 40 frames per second, and a slit in a 10-inch-diameter rotating shutter wheel afforded an exposure of 1/6,000th of a second. The film ran over spools and an electric bulb positioned directly beneath the film provided illumination. Its ends were spliced together to form a continuous loop, which was initially 25 to 30 feet long but later was lengthened to almost 50 feet. A direct-current motor powered by an Edison storage battery moved the film at a uniform rate.

 

The Kinetoscope launched the motion-picture industry, but its technical limitations made it unsuitable for projection. Films may run continuously when a great deal of light is not crucial, but a bright, enlarged picture requires that each frame be arrested and exposed intermittently as in the camera. The adaptation of the camera mechanism to the problem of projection seems obvious in retrospect but was frustrated in the US by Dickson's establishment of a frame rate well above that necessary for the perception of continuous motion.

 

After the Kinetoscope was introduced in Paris, Auguste and Louis Lumière produced a combination camera/projector, first demonstrated publicly in 1895 and called the cinématographe. The device used a triangular "eccentric" (intermittent) movement connected to a claw to engage the sprocket holes. As the film was stationary in the aperture for two-thirds of each cycle, the speed of 16 frames per second allowed an exposure of about 1/25th of second.

 

At this slower rate audiences could actually see the shutter blade crossing the screen, producing a "flicker" that had been absent from Edison's pictures. On the other hand, the hand-cranked cinématographe weighed less than 20 pounds (Edison's camera weighed 10 times as much). The Lumière units could therefore be transported much more easily and were carried around the world by photographers to shoot and screen their footage.


 

The first American projectors that employed intermittent movement were devised in 1895 by Thomas Armat (1866 – 1948), who used a Pitman arm or "beater" movement taken from a 1893 French camera.

 

The following year Armat agreed to allow Edison to produce the projectors in quantity and to market them as Edison Vitascopes. In 1897 Armat patented the first projector with a four-slot star and cam (as in the Edison camera).

 

 

 

 

 

References:

 

1.      Dale, Rodney Timekeeping, Oxford University Press, 1992

2.      Encyclopædia Britannica. 1997

3.      National Clock and Watch Museum

4.      Oledzki, A.: Research on Vibrations of Geneva-Wheel Drives in Film Projectors. Wissenschaftliche Zeitschrift der Technischen Hochshule Karl-Marx-Stadt. Jahrgang XIV, Heft 1 (in German). 1972.

5.      Oledzki, A.: Optimal Design of Geneva-Wheel for 35 and 70 mm Movie Projectors. Proceedings of the Second World Congress for the Theory of Machines and Mechanisms. Zakopane, Poland. 1969.