Page:The American Cyclopædia (1879) Volume VI.djvu/517

 ELECTRICITY produced by a rearrangement of the atoms of the oxygen molecules in consequence of vibra- tions communicated by the electric force. Duration of Spark. The duration of the elec- tric spark was measured by Wheatstone in 1835 by means of a rotating mirror. Let m n. FIG. 14. Duration of Spark. fig. 14, represent a section of a mirror revolv- ing on an axis c. A spark at a will be seen from o in the position of a f when the surface of the mirror is in the plane e e', and in the position a" when the surface of the mirror is in the plane t t'. An instantaneous spark would appear as a point ; that it has duration is proved by its having the appearance of a line. Wheatstone estimated this duration to FIG. 15. Condenser. FIG. 16. Leyden Jar. De sr.Wcr of a second. Experiments afterward made by Feddersen, and published in 1858, showed that the spark when analyzed consist- ed of two parts, viz., a whitish streak and a red tail; and that, exclusive of the red tail, it was prolonged by increasing the charge, and also the area of the coating. A further and more careful analysis of the spark from a Ley- den jar, connected with a Ruhmkorff coil, has recently been made by Prof. 0. K Rood of Columbia college, in which he found the dura- tion varied with the substance of the electrodes, and also with the striking distance and the area of coating. By the use of nicely adjusted apparatus he succeeded in producing sparks from platinum electrodes at a distance of one millimetre, whose duration was ascertained to be a little more than 41 billionths of a second. An account of the experiments may be found m the "American Journal of Science" for September, 1869, September, 1871, and Sep- tember, 1872. Condensers. The condensation of electricity is usually eifected by separating two conductors by a plate of glass or other non-conductor, and connecting one of them with the earth in the manner shown in fig. 15. A plate of glass or other non-conductor em- ployed in this manner is called a dielectric. On bringing the insulated conductor in connec- tion with an electrified body, as the prime con- ductor it becomes charged with one kind of electricity, while the conductor on the other side of the plate becomes charged with the op- posite kind, drawn from the earth and held by the powerful attraction exerted through the non-conductor, or dielectric. Charges of enor- mous intensity maybe obtained by these means. The Leyden jar, fig. 16, said to have been in- FIG. 17. Puncture of Card. vented by Cuneus, a pupil of Musschenbroek of Leyden, in 1746, is constructed upon this principle. It consists of a glass jar coated with tin foil upon both inner and outer sides to the same height, space enough being left to avoid union of the opposite fluids across the ge. To the mouth is fitted a cover of baked wood, through which passes a rod carrying a chain which lies upon the inner coating, and having a knob upon its upper end by means of which connection between the outer and inner' coatings may be readily made by a jointed dis- charger provided with a glass handle. The Leyden jar is advantageously used in making xperiments requiring great intensity of charge. Its condensing capacity depends, within cer- tain limits, upon the thickness of the glass, it being in the inverse proportion of the dis- tance between the two coatings, and upon the xtent of their surface. Its power may there-