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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
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).
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.