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 ELECTROSTATICS). 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 lb. 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. (J. A. F.)
[1] Park Benjamin, _The Intellectual Rise in Electricity_, p. 512.
LEYS, HENDRIK, BARON (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.