(spiral+press)

SUBJECT AREA: Horology

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John Arnold was apprenticed to his father, a watchmaker, and then worked as an itinerant journeyman in the Low Countries and, later, in England. He settled in London in 1762 and rapidly established his reputation at Court by presenting George III with a miniature repeating watch mounted in a ring. He later abandoned the security of the Court for a more precarious living developing his chronometers, with some financial assistance from the Board of Longitude. Symbolically, in 1771 he moved from the vicinity of the Court at St James's to John Adam Street, which was close to the premises of the Royal Society for the Encouragement of Arts, Manufactures \& Commerce.

By the time Arnold became interested in chronometry, Harrison had already demonstrated that longitude could be determined by means of a timekeeper, and the need was for a simpler instrument that could be sold at an affordable price for universal use at sea. Le Roy had shown that it was possible to dispense with a remontoire by using a detached escapement with an isochronous balance; Arnold was obviously thinking along the same lines, although he may not have been aware of Le Roy's work. By 1772 Arnold had developed his detached escapement, a pivoted detent which was quite different from that used on the European continent, and three years later he took out a patent for a compensation balance and a helical balance spring (Arnold used the spring in torsion and not in tension as Harrison had done). His compensation balance was similar in principle to that described by Le Roy and used riveted bimetallic strips to alter the radius of gyration of the balance by moving small weights radially. Although the helical balance spring was not completely isochronous it was a great improvement on the spiral spring, and in a later patent (1782) he showed how it could be made more truly isochronous by shaping the ends. In this form it was used universally in marine chronometers.

Although Arnold's chronometers performed well, their long-term stability was less satisfactory because of the deterioration of the oil on the pivot of the detent. In his patent of 1782 he eliminated this defect by replacing the pivot with a spring, producing the spring detent escapement. This was also done independendy at about the same time by Berthoud and Earnshaw, although Earnshaw claimed vehemently that Arnold had plagiarized his work. Ironically it was Earnshaw's design that was finally adopted, although he had merely replaced Arnold's pivoted detent with a spring, while Arnold had completely redesigned the escapement. Earnshaw also improved the compensation balance by fusing the steel to the brass to form the bimetallic element, and it was in this form that it began to be used universally for chronometers and high-grade watches.

As a result of the efforts of Arnold and Earnshaw, the marine chronometer emerged in what was essentially its final form by the end of the eighteenth century. The standardization of the design in England enabled it to be produced economically; whereas Larcum Kendall was paid £500 to copy Harrison's fourth timekeeper, Arnold was able to sell his chronometers for less than one-fifth of that amount. This combination of price and quality led to Britain's domination of the chronometer market during the nineteenth century.

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See also: Phillips, Edouard

SUBJECT AREA: Horology, Ports and shipping

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John Harrison was the son of a carpenter and was brought up to that trade. He was largely self-taught and learned mechanics from a copy of Nicholas Saunderson's lectures that had been lent to him. With the assistance of his younger brother, James, he built a series of unconventional clocks, mainly of wood. He was always concerned to reduce friction, without using oil, and this influenced the design of his "grasshopper" escapement. He also invented the "gridiron" compensation pendulum, which depended on the differential expansion of brass and steel. The excellent performance of his regulator clocks, which incorporated these devices, convinced him that they could also be used in a sea dock to compete for the longitude prize. In 1714 the Government had offered a prize of £20,000 for a method of determining longitude at sea to within half a degree after a voyage to the West Indies. In theory the longitude could be found by carrying an accurate timepiece that would indicate the time at a known longitude, but the requirements of the Act were very exacting. The timepiece would have to have a cumulative error of no more than two minutes after a voyage lasting six weeks.

In 1730 Harrison went to London with his proposal for a sea clock, supported by examples of his grasshopper escapement and his gridiron pendulum. His proposal received sufficient encouragement and financial support, from George Graham and others, to enable him to return to Barrow and construct his first sea clock, which he completed five years later. This was a large and complicated machine that was made out of brass but retained the wooden wheelwork and the grasshopper escapement of the regulator clocks. The two balances were interlinked to counteract the rolling of the vessel and were controlled by helical springs operating in tension. It was the first timepiece with a balance to have temperature compensation. The effect of temperature change on the timekeeping of a balance is more pronounced than it is for a pendulum, as two effects are involved: the change in the size of the balance; and the change in the elasticity of the balance spring. Harrison compensated for both effects by using a gridiron arrangement to alter the tension in the springs. This timekeeper performed creditably when it was tested on a voyage to Lisbon, and the Board of Longitude agreed to finance improved models. Harrison's second timekeeper dispensed with the use of wood and had the added refinement of a remontoire, but even before it was tested he had embarked on a third machine. The balance of this machine was controlled by a spiral spring whose effective length was altered by a bimetallic strip to compensate for changes in temperature. In 1753 Harrison commissioned a London watchmaker, John Jefferys, to make a watch for his own personal use, with a similar form of temperature compensation and a modified verge escapement that was intended to compensate for the lack of isochronism of the balance spring. The time-keeping of this watch was surprisingly good and Harrison proceeded to build a larger and more sophisticated version, with a remontoire. This timekeeper was completed in 1759 and its performance was so remarkable that Harrison decided to enter it for the longitude prize in place of his third machine. It was tested on two voyages to the West Indies and on both occasions it met the requirements of the Act, but the Board of Longitude withheld half the prize money until they had proof that the timekeeper could be duplicated. Copies were made by Harrison and by Larcum Kendall, but the Board still continued to prevaricate and Harrison received the full amount of the prize in 1773 only after George III had intervened on his behalf.

Although Harrison had shown that it was possible to construct a timepiece of sufficient accuracy to determine longitude at sea, his solution was too complex and costly to be produced in quantity. It had, for example, taken Larcum Kendall two years to produce his copy of Harrison's fourth timekeeper, but Harrison had overcome the psychological barrier and opened the door for others to produce chronometers in quantity at an affordable price. This was achieved before the end of the century by Arnold and Earnshaw, but they used an entirely different design that owed more to Le Roy than it did to Harrison and which only retained Harrison's maintaining power.

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See also: Burgi, Jost; Huygens, Christiaan

SUBJECT AREA: Broadcasting, Electronics and information technology

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Ives gained his PhD in physics from Johns Hopkins University, Baltimore, Maryland, and subsequently served in the US Signal Corps, eventually gaining experience in aerial photography. He then joined the Western Electric Engineering Department (later Bell Telephone Laboratories), c.1920 becoming leader of a group concerned with television-image transmission over telephone lines. In 1927, using a Nipkow disc, he demonstrated 50-line, 18 frames/sec pictures that could be displayed as either 2 in.×2 1/2 in. (5.1 cm×6.4 cm) images suitable for a "wirephone", or 2 ft ×2 1/2 ft (61 cm×76 cm) images for television viewing. Two years later, using a single-spiral disc and three separately modulated light sources, he was able to produce full-colour images.

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See also: Baird, John Logie; Jenkins, Charles Francis

SUBJECT AREA: Broadcasting, Electronics and information technology, Photography, film and optics

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During the early years of the motion picture industry, Jenkins made many innovations, including the development in 1894 of his own projector, the "Phantoscope", which was widely used for a number of years. In the same year he also suggested the possibility of electrically transmitting pictures over a distance, an interest that led to a lifetime of experimentation. As a result of his engineering contributions to the practical realization of moving pictures, in 1915 the National Motion Picture Board of Trade asked him to chair a committee charged with establishing technical standards for the industry. This in turn led to his proposing the creation of a professional society for those engineers in the industry, and the following year the Society of Motion Picture Engineers (later to become the Society of Motion Picture and Television Engineers) was formed, with Jenkins as its first President. Soon after this he began experiments with mechanical television, using both the Nipkow hole-spiral disc and a low-definition system of his own, based on rotating bevelled glass discs (his so-called "prismatic rings") and alkali-metal photocells. In the 1920s he gave many demonstrations of mechanical television, including a cable transmission of a crude silhouette of President Harding from Washington, DC, to Philadelphia in 1923 and a radio broadcast from Washington in 1928. The following year he formed the Jenkins Television Company to make television transmitters and receivers, but it soon went into debt and was acquired by the de Forest Company, from whom RCA later purchased the patents.

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See also: Baird, John Logie; Ives, Herbert Eugene; Zworykin, Vladimir Kosma

SUBJECT AREA: Horology

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Le Roy was born into a distinguished horological family: his father, Julien, was Clockmaker to the King. Pierre became Master in 1737 and continued to work with his father, taking over the business when his father died in 1759. However, he seems to have left the commercial side of the business to others so that he could concentrate on developing the marine chronometer. Unlike John Harrison, he believed that the solution lay in detaching the escapement from the balance, and in 1748 he submitted a proposal for the first detached escapement to the Académie des Sciences in Paris. He also differed from Harrison in his method of temperature compensation, which acted directly on the balance by altering its radius of gyration. This was achieved either by mounting thermometers on the balance or by using bimetallic strips which effectively reduced the diameter of the balance as the temperature rose (with refinements, this later became the standard method of temperature compensation in watches and chronometers). Le Roy had already discovered that for every spiral balance spring there was a particular length at which it would be isochronous, and this method of temperature compensation did not destroy that isochronism by altering the length, as other methods did. These innovations were incorporated in a chronometer with an improved detached escapement which he presented to Louis XV in 1766 and described in a memoir to the Académie des Sciences. This instrument contained the three essential elements of all subsequent chronometers: an isochronous balance spring, a detached escapement and a balance with temperature compensation. Its performance was similar to that of Harrison's fourth timepiece, and Le Roy was awarded prizes by the Académie des Sciences for the chronometer and for his memoir. However, his work was never fully appreciated in France, where he was over-shadowed by his rival Ferdinand Berthoud. When Berthoud was awarded the coveted title of Horloger de la Marine, Le Roy became disillusioned and shortly afterwards gave up chronometry and retired to the country.

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SUBJECT AREA: Civil engineering, Railways and locomotives

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Turner's first glasshouse commission was for the Palm House at the Botanic Gardens in Belfast, begun in 1839; this structure was designed by Charles Lanyon, Turner being responsible for the ironwork construction. The Belfast Palm House was followed in 1843 by the Palm House for the Royal Dublin Society, but the structure for which Turner is best known is the famous Palm House in the Royal Botanic Gardens at Kew Gardens in London. This was originally designed in 1844 by the architect Decimus Burton, but his concept was rejected and Turner was asked to design a new one. Burton tried again, basing his new design upon that of Turner but also incorporating features that made it more similar to the famous Great Conservatory by Paxton at Chatsworth. Finally, Turner was contracted to build the Palm Stove in collaboration with Burton. Completed in 1848, the Kew Palm House is the finest example of the glasshouses of that era. This remarkable structure is simple but impressive: it is 362 ft (110 m) long and is covered by 45,000 ft2 (4,180 m²) of greenish glass. Inside, in the central taller part, a decorative, cast-iron, spiral staircase gives access to an upper gallery, from where tall plants may be clearly viewed; the roof rises to 62 ft (19 m). The curving, glazed panels, set in ribs of wrought iron, rise from a low masonry wall. The ingenious method of construction of these ribs was patented by Turner in 1846. It consists of wrought-iron tie rods inserted into hollow cast-iron tubes; these can be tightened after the erection of the building is complete, so producing a stable, balanced structure not unlike the concept of a timber-trussed roof. The Palm Stove has only recently undergone extensive adaptation to modern needs.

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