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LEYDEN JAR, or CONDENSER
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See also:LEYDEN See also:JAR, or See also:CONDENSER , an See also:electrical appliance consisting in one See also:form of a thin See also:glass jar partly coated inside and outside with See also:tin See also:foil, or in another of a number of glass plates similarly coated. When the two See also:metal surfaces are connected for a See also:short See also:time with the terminals of some source of electromotive force, such as an electric See also:machine, an See also:induction coil or a voltaic See also:battery, electric See also:energy is stored up in the condenser in the form of electric See also:strain in the glass, and can be recovered again in the form of an electric See also:discharge.
The earliest form of Leyden jar consisted of a glass vial or thin See also:Florence See also:flask, partly full of See also:water, having a metallic See also:nail inserted through the See also:cork which touched the water. The
See also:bottle was held in the See also:hand, and the nail presented harry See also:history.
to the See also:prime conductor of an electrical machine. If
the See also:person holding the bottle subsequently touched the nail, he experienced an electric See also:shock. This experiment was first made by E. G. von See also:Kleist of See also:Kammin in See also:Pomerania in 1745,1 and it was repeated in another form in 1746 by Cunaeus and P. See also:van See also:Musschenbroek, of the university of Leyden (See also:Leiden), whence the See also:term Leyden jar? J. H. Winkler discovered that an See also:iron See also:chain See also:wound See also:round the bottle could be substituted for the hand, and See also:Sir See also: See also:Benjamin See also:Franklin and Bevis devised independently the form of condenser known as a Franklin or Leyden See also:pane, which consists of a sheet of glass, partly coated on both sides with tin foil or silver leaf, a margin of glass all round being See also:left to insulate the two tin foils from each other. Franklin in 1747 and 1748 made numerous investigations on the Leyden jar, and devised a method of charging jars in See also:series as well as in parallel. In the former method, now commonly known as charging in cascade, the jars are insulated and the outside coating of one jar is connected to the inside coating of the next and so on for a whole series, the inside coating of the first jar and the outside coating of the last jar being the terminals of the condenser. For charging in parallel a number of jars are collected in a See also:box, and all the out-See also:side coatings are connected together metallically and all the inside coatings brought to one See also:common terminal. This arrangement is commonly called a battery of Leyden jars. To Franklin also we owe the important knowledge that the electric See also:charge resides really in the glass and not in the metal coatings, and that when a condenser has been charged the metallic coatings can be exchanged for fresh ones and yet the electric charge of the condenser remains. In its See also:modern form the Leyden jar consists of a wide-mouthed bottle of thin See also:English See also:flint glass of See also:uniform thickness, 1 See also:Park Benjamin, The Intellectual Rise in See also:Electricity. p. 512. 2 Ibid. p. 519. See also:free front flaws. About See also:half the outside and half the inside See also:surface is coated smoothly with tin foil, and the See also:remainder of the glazed surface is painted with shellac See also:varnish. A wooden stopper closes the mouth of the jar, and through it a See also:brass See also:rod passes which terminates in a chain, or better still, three elastic brass springs, which make See also:good contact with the inner coating. The rod terminates externally in a knob or See also:screw terminal. The jar has a certain capacity C which is best expressed in microfarads or electrostatic See also:units (see See also:ELECTROSTATICS), 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 See also: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 -Il6th 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 See also:tongs, and this discharge is in the form of an oscillatory current (see See also:ELECTROKINETICS). The energy stored up in the jar in joules is expressed by the value of CV', 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 X tos joules or 0.737 CV2/2 X rot See also:foot-pounds. The See also:size of jar commonly known as a quart size may have a capacity from Tooth to $--0th of a microfarad, and if charged to 20,000 volts stores up energy from a See also:quarter to half a See also:joule or from 1%ths to 8ths of a foot-See also:pound. Leyden jars are now much employed for the See also:production of the high frequency electric currents used in wireless telegraphy (see TELEGRAPHY, WIRELESS). 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 See also:outer surfaces. The tubes have walls thicker at the ends than in the See also: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 See also:mica or micanite or ebonite partly coated with tin foil or silver leaf on both sides; or a See also: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 See also:ordinary Leyden jar occupies too much space in comparison with its electrical capacity, and hence the best form of See also:con-denser consists of a number of sheets of See also: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 See also:vessel full of insulating oil which pre-vents the glow or See also:brush discharge taking See also: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 See also:dielectric because of its cheapness. high dielectric strength or resistance to electric See also:corn- puncture, and its high dielectric See also:constant (see ELECTRO- pressed See also:STATICS). It has been found, however, that very See also:air con- efficient condensers can be made with compressed air densers. 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 too lb. to t sq. in., the dielectric strength of the air is greatly increased, and the plates may there-fore be brought very near to one another without causing a spark 529 to pass under such voltage as would cause discharge in air at normal pressure. Condensers of this See also:kind have been employed by R. A. See also:Fessenden in wireless telegraphy, and they form a very excellent arrangement for See also: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. See also:Paraffin oil or any liquid dielectric of constant inductivity may replace the air. See A. See also:Fleming, Electric See also:Wave Telegraphy (London, 1906); R. A. J.essenden, " Compressed Air for Condensers," Electrician, 1905, 55, P. 795; Moscicki, " Construction of High Tension Condensers," L'Eclairage electrique, 1904, 41, p. 14, or See also:Engineering, 1904, p. 865. (J. A. Additional information and CommentsThere are no comments yet for this article.
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