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 free from flaws. About half the outside and half the inside surface is coated smoothly with tin foil, and the remainder of the glazed surface is painted with shellac varnish. A

wooden stopper closes the mouth of the jar, and through it a brass rod passes which terminates in a chain, or better still, three elastic brass springs, which make good contact with the inner coating. The rod terminates externally in a knob or screw terminal. The jar has a certain capacity C which is best expressed in microfarads or electrostatic units (see ), and is determined by the surface of the tin foil and thickness and quality of the glass. The jar can be charged so that a certain potential difference V, reckoned in volts, exists between the two coatings. If a certain critical potential is exceeded, the glass gives way under the electric strain and is pierced. The safe voltage for most glass jars is about 20,000 volts for glass th in. in thickness; this corresponds with an electric spark of about 7 millimetres in length. When the jar is charged, it is usually discharged through a metallic arc called the discharging tongs, and this discharge is in the form of an oscillatory current (see ). The energy stored up in the jar in joules is expressed by the value of CV2, where C is the capacity measured in farads and V the potential difference of the coatings in volts. If the capacity C is reckoned in microfarads then the energy storage is equal to CV2/2 × 106 joules or 0.737 CV2/2 × 106 foot-pounds. The size of jar commonly known as a quart size may have a capacity from th to th of a microfarad, and if charged to 20,000 volts stores up energy from a quarter to half a joule or from ths to ths of a foot-pound.

Leyden jars are now much employed for the production of the high frequency electric currents used in wireless telegraphy (see ). For this purpose they are made by Moscicki in the form of glass tubes partly coated by silver chemically deposited on the glass on the inner and outer surfaces. The tubes have walls thicker at the ends than in the middle, as the tendency to puncture the glass is greatest at the edges of the coatings. In other cases, Leyden jars or condensers take the form of sheets of mica or micanite or ebonite partly coated with tin foil or silver leaf on both sides; or a pile of sheets of alternate tin foil and mica may be built up, the tin foil sheets having lugs projecting out first on one side and then on the other. All the lugs on one side are connected together, and so also are all the lugs on the other side, and the two sets of tin foils separated by sheets of mica constitute the two metallic surfaces of the Leyden jar condenser. For the purposes of wireless telegraphy, when large condensers are required, the ordinary Leyden

jar occupies too much space in comparison with its electrical capacity, and hence the best form of condenser consists of a number of sheets of crown glass, each partly coated on both sides with tin foil. The tin foil sheets have lugs attached which project beyond the glass. The plates are placed in a vessel full of insulating oil which prevents the glow or brush discharge taking place over their edges. All the tin foils on one side of the glass plates are connected together and all the tin foils on the opposite sides, so as to construct a condenser of any required capacity. The box should be of glass or stoneware or other non-conducting material. When glass tubes are used it is better to employ tubes thicker at the ends than in the middle, as it has been found that when the safe voltage is exceeded and the glass gives way under electric strain, the piercing of the glass nearly always takes place at the edges of the tin foil.

Glass is still commonly used as a dielectric because of its cheapness, high dielectric strength or resistance to electric puncture, and its high dielectric constant (see ). It has been found, however, that very efficient condensers can be made with compressed air

as dielectric. If a number of metal plates separated by small distance pieces are enclosed in an iron box which is pumped full of air to a pressure, say, of 100 ℔. to 1 sq. in., the dielectric strength of the air is greatly increased, and the plates may therefore be brought very near to one another without causing a spark to pass under such voltage as would cause discharge in air at normal pressure. Condensers of this kind have been employed by R. A. Fessenden in wireless telegraphy, and they form a very excellent arrangement for standard condensers with which to compare the capacity of other Leyden jars. Owing to the variation in the value of the dielectric constant of glass with the temperature and with the frequency of the applied electromotive force, and also owing to electric glow discharge from the edges of the tin foil coatings, the capacity of an ordinary Leyden jar is not an absolutely fixed quantity, but its numerical value varies somewhat with the method by which it is measured, and with the other circumstances above mentioned. For the purpose of a standard condenser a number of concentric metal tubes may be arranged on an insulating stand, alternate tubes being connected together. One coating of the condenser is formed by one set of tubes and the other by the other set, the air between being the dielectric. Paraffin oil or any liquid dielectric of constant inductivity may replace the air.

See J. A. Fleming, Electric Wave Telegraphy (London, 1906); R. A. Fessenden, “Compressed Air for Condensers,” Electrician, 1905, 55, p. 795; Moscicki, “Construction of High Tension Condensers,” L’Éclairage électrique, 1904, 41, p. 14, or Engineering, 1904, p. 865.

 LEYS, HENDRIK, (1815–1869), Belgian painter, was born at Antwerp on the 18th of February 1815. He studied under Wappers at the Antwerp Academy. In 1833 he painted “Combat d’un grenadier et d’un cosaque,” and in the following year “Combat de Bourguignons et Flamands.” In 1835 he went to Paris where he was influenced by the Romantic movement. Examples of this period of his painting are “Massacre des échevins de Louvain,” “Mariage flamand,” “Le Roi des arbalétriers” and other works. Leys was an imitative painter in whose works may rapidly be detected the schools which he had been studying before he painted them. Thus after his visit to Holland in 1839 he reproduced many of the characteristics of the Dutch genre painters in such works as “Franz Floris se rendant à une fête” (1845) and “Service divin en Hollande” (1850). So too the methods of Quentin Matsys impressed themselves upon him after he had travelled in Germany in 1852. In 1862 Leys was created a baron. At the time of his death, which occurred in August 1869, he was engaged in decorating with fresco the large hall of the Antwerp Hôtel de Ville.  LEYTON, an urban district forming one of the north-eastern suburbs of London, England, in the Walthamstow (S.W.) parliamentary division of Essex. Pop. (1891) 63,106; (1901) 98,912. It lies on the east (left) bank of the Lea, along the flat open valley of which runs the boundary between Essex and the county of London. The church of St Mary, mainly a brick reconstruction, contains several interesting memorials; including one to William Bowyer the printer (d. 1737), erected by his son and namesake, more famous in the same trade. Here is also buried John Strype the historian and biographer (d. 1737), who held the position of curate and lecturer at this church. Leyton is in the main a residential as distinct from a manufacturing locality. Its name is properly Low Leyton, and the parish includes the district of Leytonstone to the east. Roman remains have been discovered here, but no identification with a Roman station by name has been made with certainty. The ground of the Essex County Cricket Club is at Leyton.  LHASA (,, “God’s ground”), the capital of Tibet. It lies in 29° 39′ N., 91° 5′ E., 11,830 ft. above sea-level. Owing to the inaccessibility of Tibet and the political and religious exclusiveness of the lamas, Lhasa was long closed to European travellers, all of whom during the latter half of the 19th century were stopped in their attempts to reach it. It was popularly known as the “Forbidden City.” But its chief features were known by the accounts of the earlier Romish missionaries who visited it and by the investigations, in modern times, of native Indian secret explorers, and others, and the British armed mission of 1904 (see ).

Site and General Aspect.—The city stands in a tolerably level plain, which is surrounded on all sides by hills. Along its