The Pulse of Time

SUBJECT AREA: Horology

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Huygens was born into a cultured and privileged class. His father, Constantijn, was a poet and statesman who had wide interests. Constantijn exerted a strong influence on his son, who was educated at home until he reached the age of 16. Christiaan studied law and mathematics at Ley den University from 1645 to 1647, and continued his studies at the Collegium Arausiacum in Breda until 1649. He then lived at The Hague, where he had the means to devote his time entirely to study. In 1666 he became a Member of the Académie des Sciences in Paris and settled there until his return to The Hague in 1681. He also had a close relationship with the Royal Society and visited London on three occasions, meeting Newton on his last visit in 1689. Huygens had a wide range of interests and made significant contributions in mathematics, astronomy, optics and mechanics. He also made technical advances in optical instruments and horology.

Despite the efforts of Burgi there had been no significant improvement in the performance of ordinary clocks and watches from their inception to Huygens's time, as they were controlled by foliots or balances which had no natural period of oscillation. The pendulum appeared to offer a means of improvement as it had a natural period of oscillation that was almost independent of amplitude. Galileo Galilei had already pioneered the use of a freely suspended pendulum for timing events, but it was by no means obvious how it could be kept swinging and used to control a clock. Towards the end of his life Galileo described such a. mechanism to his son Vincenzio, who constructed a model after his father's death, although it was not completed when he himself died in 1642. This model appears to have been copied in Italy, but it had little influence on horology, partly because of the circumstances in which it was produced and possibly also because it differed radically from clocks of that period. The crucial event occurred on Christmas Day 1656 when Huygens, quite independently, succeeded in adapting an existing spring-driven table clock so that it was not only controlled by a pendulum but also kept it swinging. In the following year he was granted a privilege or patent for this clock, and several were made by the clockmaker Salomon Coster of The Hague. The use of the pendulum produced a dramatic improvement in timekeeping, reducing the daily error from minutes to seconds, but Huygens was aware that the pendulum was not truly isochronous. This error was magnified by the use of the existing verge escapement, which made the pendulum swing through a large arc. He overcame this defect very elegantly by fitting cheeks at the pendulum suspension point, progressively reducing the effective length of the pendulum as the amplitude increased. Initially the cheeks were shaped empirically, but he was later able to show that they should have a cycloidal shape. The cheeks were not adopted universally because they introduced other defects, and the problem was eventually solved more prosaically by way of new escapements which reduced the swing of the pendulum. Huygens's clocks had another innovatory feature: maintaining power, which kept the clock going while it was being wound.

Pendulums could not be used for portable timepieces, which continued to use balances despite their deficiencies. Robert Hooke was probably the first to apply a spring to the balance, but his efforts were not successful. From his work on the pendulum Huygens was well aware of the conditions necessary for isochronism in a vibrating system, and in January 1675, with a flash of inspiration, he realized that this could be achieved by controlling the oscillations of the balance with a spiral spring, an arrangement that is still used in mechanical watches. The first model was made for Huygens in Paris by the clockmaker Isaac Thuret, who attempted to appropriate the invention and patent it himself. Huygens had for many years been trying unsuccessfully to adapt the pendulum clock for use at sea (in order to determine longitude), and he hoped that a balance-spring timekeeper might be better suited for this purpose. However, he was disillusioned as its timekeeping proved to be much more susceptible to changes in temperature than that of the pendulum clock.

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SUBJECT AREA: Aerospace, Electronics and information technology, Weapons and armour

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Following education at Brechin High School, Watson-Watt entered University College, Dundee (then a part of the University of St Andrews), obtaining a BSc in engineering in 1912. From 1912 until 1921 he was Assistant to the Professor of Natural Philosophy at St Andrews, but during the First World War he also held various posts in the Meteorological Office. During. this time, in 1916 he proposed the use of cathode ray oscillographs for radio-direction-finding displays. He joined the newly formed Radio Research Station at Slough when it was opened in 1924, and 3 years later, when it amalgamated with the Radio Section of the National Physical Laboratory, he became Superintendent at Slough. At this time he proposed the name "ionosphere" for the ionized layer in the upper atmosphere. With E.V. Appleton and J.F.Herd he developed the "squegger" hard-valve transformer-coupled timebase and with the latter devised a direction-finding radio-goniometer.

In 1933 he was asked to investigate possible aircraft counter-measures. He soon showed that it was impossible to make the wished-for radio "death-ray", but had the idea of using the detection of reflected radio-waves as a means of monitoring the approach of enemy aircraft. With six assistants he developed this idea and constructed an experimental system of radar (RAdio Detection And Ranging) in which arrays of aerials were used to detect the reflected signals and deduce the bearing and height. To realize a practical system, in September 1936 he was appointed Director of the Bawdsey Research Station near Felixstowe and carried out operational studies of radar. The result was that within two years the East Coast of the British Isles was equipped with a network of radar transmitters and receivers working in the 7–14 metre band—the so-called "chain-home" system—which did so much to assist the efficient deployment of RAF Fighter Command against German bombing raids on Britain in the early years of the Second World War.

In 1938 he moved to the Air Ministry as Director of Communications Development, becoming Scientific Adviser to the Air Ministry and Ministry of Aircraft Production in 1940, then Deputy Chairman of the War Cabinet Radio Board in 1943. After the war he set up Sir Robert Watson-Watt \& Partners, an industrial consultant firm. He then spent some years in relative retirement in Canada, but returned to Scotland before his death.

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SUBJECT AREA: Medical technology, Photography, film and optics

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At the age of 19 Marey went to Paris to study medicine, becoming particularly interested in the problems of the circulation of the blood. In an early communication to the Académie des Sciences he described a much improved device for recording the pulse, the sphygmograph, in which the beats were recorded on a smoked plate. Most of his subsequent work was concerned with methods of recording movement: to study the movement of the horse, he used pneumatic sensors on each hoof to record traces on a smoked drum; this device became known as the Marey recording tambour. His attempts to study the wing movements of a bird in flight in the same way met with limited success since the recording system interfered with free movement. Reading in 1878 of Muybridge's work in America using sequence photography to study animal movement, Marey considered the use of photography himself. In 1882 he developed an idea first used by the astronomer Janssen: a camera in which a series of exposures could be made on a circular photographic plate. Marey's "photographic gun" was rifle shaped and could expose twelve pictures in approximately one second on a circular plate. With this device he was able to study wing movements of birds in free flight. The camera was limited in that it could record only a small number of images, and in the summer of 1882 he developed a new camera, when the French government gave him a grant to set up a physiological research station on land provided by the Parisian authorities near the Porte d'Auteuil. The new design used a fixed plate, on which a series of images were recorded through a rotating shutter. Looking rather like the results provided by a modern stroboscope flash device, the images were partially superimposed if the subject was slow moving, or separated if it was fast. His human subjects were dressed all in white and moved against a black background. An alternative was to dress the subject in black, with highly reflective strips and points along limbs and at joints, to produce a graphic record of the relationships of the parts of the body during action. A one-second-sweep timing clock was included in the scene to enable the precise interval between exposures to be assessed. The fixed-plate cameras were used with considerable success, but the number of individual records on each plate was still limited. With the appearance of Eastman's Kodak roll-film camera in France in September 1888, Marey designed a new camera to use the long rolls of paper film. He described the new apparatus to the Académie des Sciences on 8 October 1888, and three weeks later showed a band of images taken with it at the rate of 20 per second. This camera and its subsequent improvements were the first true cinematographic cameras. The arrival of Eastman's celluloid film late in 1889 made Marey's camera even more practical, and for over a decade the Physiological Research Station made hundreds of sequence studies of animals and humans in motion, at rates of up to 100 pictures per second. Marey pioneered the scientific study of movement using film cameras, introducing techniques of time-lapse, frame-by-frame and slow-motion analysis, macro-and micro-cinematography, superimposed timing clocks, studies of airflow using smoke streams, and other methods still in use in the 1990s. Appointed Professor of Natural History at the Collège de France in 1870, he headed the Institut Marey founded in 1898 to continue these studies. After Marey's death in 1904, the research continued under the direction of his associate Lucien Bull, who developed many new techniques, notably ultra-high-speed cinematography.

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See also: Demenÿ, Georges

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a system that enables Internet users to engage in text-based communication in real time. Messages posted via a chat system will be seen by every member of the participating group. It is a useful means for an organization to take the pulse of consumers to find out what they are thinking, and to generate unique content. Online chat can be particularly effective when there is a specific event occurring that is of interest to people, or when an expert can be made available to talk about a subject or product. To be productive, online chat needs to be well moderated, and is really only suited to small groups of people (2 to 20) at any one time.

SUBJECT AREA: Electronics and information technology, Recording, Telecommunications

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Black graduated from Worcester Polytechnic Institute, Massachusetts, in 1921 and joined the Western Electric Company laboratories (later the Bell Telephone Laboratories) in New York City. There he worked on a variety of electronic-communication problems. His major contribution was the discovery in 1927 that the application of negative feedback to an amplifier, whereby a fraction of the output signal is fed back to the input in the opposite phase, not only increases the stability of the amplifier but also has the effect of reducing the magnitude of any distortion introduced by it. This discovery has found wide application in the design of audio hi-fi amplifiers and various control systems, and has also given valuable insight into the way in which many animal control functions operate.

During the Second World War he developed a form of pulse code modulation (PCM) to provide a practicable, secure telephony system for the US Army Signal Corps. From 1963–6, after his retirement from the Bell Labs, he was Principal Research Scientist with General Precision Inc., Little Falls, New Jersey, following which he became an independent consultant in communications. At the time of his death he held over 300 patents.

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See also: Bode, Hendrik Wade; Nyquist, Harry

SUBJECT AREA: Horology, Telecommunications

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Alexander Bain was born into a crofting family in a remote part of Scotland. He was apprenticed to a watchmaker in Wick and during that time he was strongly influenced by a lecture on "Heat, sound and electricity" that he heard in nearby Thurso. This lecture induced him to take up a position in Clerkenwell in London, working as a journeyman clockmaker, where he was able to further his knowledge of electricity by attending lectures at the Adelaide Gallery and the Polytechnic Institution. His thoughts naturally turned to the application of electricity to clockmaking, and despite a bitter dispute with Charles Wheatstone over priority he was granted the first British patent for an electric clock. This patent, taken out on 11 January 1841, described a mechanism for an electric clock, in which an oscillating component of the clock operated a mechanical switch that initiated an electromagnetic pulse to maintain the regular, periodic motion. This principle was used in his master clock, produced in 1845. On 12 December of the same year, he patented a means of using electricity to control the operation of steam railway engines via a steam-valve. His earliest patent was particularly far-sighted and anticipated most of the developments in electrical horology that occurred during the nineteenth century. He proposed the use of electricity not only to drive clocks but also to distribute time over a distance by correcting the hands of mechanical clocks, synchronizing pendulums and using slave dials (here he was anticipated by Steinheil). However, he was less successful in putting these ideas into practice, and his electric clocks proved to be unreliable. Early electric clocks had two weaknesses: the battery; and the switching mechanism that fed the current to the electromagnets. Bain's earth battery, patented in 1843, overcame the first defect by providing a reasonably constant current to drive his clocks, but unlike Hipp he failed to produce a reliable switch.

The application of Bain's numerous patents for electric telegraphy was more successful, and he derived most of his income from these. They included a patent of 12 December 1843 for a form of fax machine, a chemical telegraph that could be used for the transmission of text and of images (facsimile). At the receiver, signals were passed through a moving band of paper impregnated with a solution of ammonium nitrate and potassium ferrocyanide. For text, Morse code signals were used, and because the system could respond to signals faster than those generated by hand, perforated paper tape was used to transmit the messages; in a trial between Paris and Lille, 282 words were transmitted in less than one minute. In 1865 the Abbé Caselli, a French engineer, introduced a commercial fax service between Paris and Lyons, based on Bain's device. Bain also used the idea of perforated tape to operate musical wind instruments automatically. Bain squandered a great deal of money on litigation, initially with Wheatstone and then with Morse in the USA. Although his inventions were acknowledged, Bain appears to have received no honours, but when towards the end of his life he fell upon hard times, influential persons in 1873 secured for him a Civil List Pension of £80 per annum and the Royal Society gave him £150.

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See also: Bakewell, Frederick C.

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SUBJECT AREA: Photography, film and optics

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Galileo began studying medicine at the University of Pisa but soon turned to his real interests, mathematics, mechanics and astronomy. He became Professor of Mathematics at Pisa at the age of 25 and three years later moved to Padua. In 1610 he transferred to Florence. While still a student he discovered the isochronous property of the pendulum, probably by timing with his pulse the swings of a hanging lamp during a religious ceremony in Pisa Cathedral. He later designed a pendulum-controlled clock, but it was not constructed until after his death, and then not successfully; the first successful pendulum clock was made by the Dutch scientist Christiaan Huygens in 1656. Around 1590 Galileo established the laws of motion of falling bodies, by timing rolling balls down inclined planes and not, as was once widely believed, by dropping different weights from the Leaning Tower of Pisa. These and other observations received definitive treatment in his Discorsi e dimostrazioni matematiche intorno a due nuove scienzi attenenti alla, meccanica (Dialogues Concerning Two New Sciences…) which was completed in 1634 and first printed in 1638. This work also included Galileo's proof that the path of a projectile was a parabola and, most importantly, the development of the concept of inertia.

In astronomy Galileo adopted the Copernican heliocentric theory of the universe while still in his twenties, but he lacked the evidence to promote it publicly. That evidence came with the invention of the telescope by the Dutch brothers Lippershey. Galileo heard of its invention in 1609 and had his own instrument constructed, with a convex object lens and concave eyepiece, a form which came to be known as the Galilean telescope. Galileo was the first to exploit the telescope successfully with a series of striking astronomical discoveries. He was also the first to publish the results of observations with the telescope, in his Sidereus nuncius (Starry Messenger) of 1610. All the discoveries told against the traditional view of the universe inherited from the ancient Greeks, and one in particular, that of the four satellites in orbit around Jupiter, supported the Copernican theory in that it showed that there could be another centre of motion in the universe besides the Earth: if Jupiter, why not the Sun? Galileo now felt confident enough to advocate the theory, but the advance of new ideas was opposed, not for the first or last time, by established opinion, personified in Galileo's time by the ecclesiastical authorities in Rome. Eventually he was forced to renounce the Copernican theory, at least in public, and turn to less contentious subjects such as the "two new sciences" of his last and most important work.

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