Page:The American Cyclopædia (1879) Volume VI.djvu/518

 510 ELECTRICITY ELECTRIC LIGHT fore be increased by placing a number of jars together on a sheet of tin foil or other good conductor, and connecting all the inner coat- ings by rods or chains passing from knob to knob. Such an apparatus is called a Leyden battery, and it may be charged, like a single jar, with the ordinary machine, by connecting one coating, it is not very material which, with the prime conductor, and the other with the earth or with the rubbers. In charging with the Holtz machine, one conductor is connected with the inner and the other with the outer coating. If a piece of card board is placed be- tween two points, as in fig. 17, and one of them, say the lower, is connected with the outer coating of a charged Leyden jar, on bringing the knob of the jar near the knob connected with the upper point, the fluid will leap across the spaces between the knobs as well as be- tween the points, but in passing from point to point it will pierce the card board. The hole will be found burred on both sides in conse- quence of the repulsion of the particles of paper, and its location is also nearer the nega- tive than the positive point, unless placed in an exhausted receiver, when it will be nearer the middle. By using a battery of several jars the fluid may be made to pierce a hole through a thin plate of glass. It may be re- marked that if the experiment fails the first time, subsequent ones with the same piece of glass will scarcely succeed as the fluid will almost invariably follow the course it first took. Gunpowder, turpentine, and combustible gases may be fired by means of the Leyden jar or battery, and metallic wires may be fused if they are small enough to offer sufficient resistance to the passage of the fluid. After a Leyden jar is discharged, if a short time is allowed to elapse, and the coatings be again brought near together, a second or a third smaller spark may be obtained. The electricity thus remain- ing after the first discharge is called the re- sidual charge, and, according to Faraday's ex- periments, it is caused by the gradual penetra- FIG. 18. Lane's Unit Jar. tion of the electric fluid into the substance of the glass dielectric, and accords with his opinion that conductors differ only in degree. The charge of a Leyden jar or battery may be esti- mated by means of an apparatus called a unit jar. Lane's unit jar or electrometer is represented in fig. 18. The jar or battery whose charge is to be measured is placed upon an insulated stand, and the inner coating connected with the prime conductor. If no connection were made be- tween the external coating and the earth, it is evident that the jar could not be charged. The connection is made through a small Leyden jar, c, whose inner coating is connected with the outer coating of the first jar or battery, while its outer coating is connected with a rod carrying a knob, which is brought near to the knob connected with the inner coating. When the small jar becomes sufficiently charged the fluid will leap across the space between the two knobs, which is called the striking distance, the amount being quite uniform for the same distance, and called the unit of measurement. By using the same instrument similarly adjusted a comparison of charges of different batteries, or of the same battery at different times, may be made. The physiological effects of the dis- charge of a Leyden jar are similar to those of lightning. A small jar of only a few inches surface is capable of giving a powerful shock when passed through the arms, and a large jar or a battery strongly charged is capable of giv- ing a shock sufficient to produce death. Fric- tional electricity is sometimes used in medicine, and it may often be employed with advantage, either by passing small repeated shocks by means of a small unit jar through the body or limbs, or the patient may be insulated and have sparks drawn from him ; but the galvanic battery, employed alone or with electro-mag- netic currents, may generally be used with more certain results. The subject will be no- ticed in the article MEDICAL ELECTRICITY. Among the numerous works that have been published on electricity are the following: ^Epinus, Tentamen Theories Electricitatis et Magnetismi (4to, St. Petersburg, 1759) ; Frank- lin, " Experiments and Observations on Elec- tricity, made at Philadelphia in America" (London, 1769); Hatiy, Exposition raisonnee de la theorie de V electricite et du magnetisme (Paris, 1787); Tiberius Cavallo, "A Complete Treatise on Electricity " (3 vols. 8vo, London, 1795); Faraday, " Experimental Researches in Electricity" (London, 1849-'55); Sir W. Snow Harris, "Electricity, Magnetism, and Galvan- ism" (London, 1867); Deschanel, ''Natural Philosophy " (New York, 1873) ; Ganot, " Phy- sics " (New York, 1873) ; Fleeming Jenkin, "Electricity and Magnetism" (New York, 1873); J. Clerk Maxwell, "Electricity and Magnetism " (London, 1873). ELECTRIC LIGHT, the light produced by a current of electricity in passing through a re- sisting medium, as" a gas or a small wire. Like solar light, it also produces the combina- tion of chlorine and hydrogen, acts chemically on chloride of silver in the photographic pro- cess, and when viewed through a slit in a screen by a prism it presents a spectrum crossed by very bright lines, differing in character with the substance of which the electrodes are formed, and with the gases through which