244 



ELECTRICITY. 



covered with silk throughout, and the average diam- 

 eter is 0.015 of an inch. 



The total weight of the wire is 606 Ibs., and the 

 resistance 33,560 B. A. units. The length of the 

 secondary coil is 50 inches, and the insulation through- 

 out is calculated to be 95 per cent, beyond that re- 

 quired. The secondary wire is insulated from the 

 primary by means of an ebonite tube of one-half an 

 inch in thickness and 8 feet in length. 



The length of the secondary coil is 54 inches, the 

 diameter is 19 inches, and without the internal ebonite 

 tube containing the primary wire and iron core it is a 

 cylinder 19 inches in diameter and 6 inches thick. > 



The condenser, made in the usual manner with 

 sheets of varnished paper and tinfoil, is arranged in 

 six parts, each containing 125 superficial feet, or T50 

 square feet of tinfoil in the whole. 



A large and substantially-made contact-breaker, 

 detached" from the great coil and worked by an inde- 

 pendent electro-magnet, was constructed and worked 

 very well with a comparatively moderate power of 

 10 or 20 large Bunsen's cells ; when, however, the 

 battery was increased to 30 or 40 cells, it became un- 

 manageable. 



The Bunsen's battery used in the experiments was 

 made with the largest porous cells that could be ob- 

 tained, and each cell contained about one pint of 

 nitric acid. 



Some experiments were tried with the battery ar- 

 ranged for intensity, and used with the complete con- 

 denser of 750 square feet of tinfoil, and 1,500 square 

 feet of paper. At first five cells were used, and these 

 gave a spark 12 inches in length. The number of 

 cells was gradually increased until 50 were in opera- 

 tion, when a spark from 28 to 29 inches in length was 

 obtained. 



Experiments were now tried to ascertain whether 

 any increase in the length of the spark could be ob- 

 tained by arranging theTbattery and the primary coil for 

 quantity, but no material advantage was obtained by 

 this arrangement : even where three groups of cells 

 were connected, a decrease in the length ol the spark 

 is observed when compared with the 45 or 50 cells ar- 

 ranged for intensity, the difference being as 20 to 28. 

 The spark obtained from the large coil is thick and 

 flame-like in its appearance, and therefore it will be 

 alluded to as the ''flaming spark." 



When the discharging point and circular plate are 

 brought within 6 or 7 inches of each other, the flam- 

 ing nature of the spark becomes still more apparent. 

 Two light-yellow flames, curving upward, appear 

 to connect the opposite poles. If a blast of air from 

 powerful bellows is directed against a flaming spark, 

 the flaming portion can be blown away and increased 

 in area, and thin wiry sparks are now seen darting 

 through it, sometimes in one continuous stream, at 

 another time divided into three or more sparks, all 

 following the direction in which the flame is blown. 



The flaming spark is very hot, and, if passed 

 through an asbestos (supported on an insulating 

 pillar), quickly causes the latter to become red-hot. 



The flaming spark has been ascribed by some ex- 

 perienced observers to the incandescence of the dust 

 in the air, and especially sodium chloride. 



To ascertain whether the " flaming spark " could 

 be obtained with a small number of cells, the large 

 Bunsen's battery was reduced to three cells, and it 

 was found that no appreciable spark could be pro- 

 duced when the whole primary wire was used with 

 less than five cells. 



By reducing the length of the primary wire, and 

 using the four divisions separately, with five cells the 

 spark was wiry, and varied from 4i inches to 6* 

 inches ; with ten cells it was wiry, and varied from 

 81 to 9* ; in the latter the spark was slightly flaming. 

 With fifteen cells the spark was slightly flamino-, and 

 varied from 10 inches to 114 inches. With 20 cells a 

 flaming spark varying from Hi inches to 12i inches 

 was obtained. 

 When the two wires from the secondary coil are 



placed in water no spark is perceptible, even when 

 the wire was brought very close together, until they 

 touched. 



If the negative wire is passed through a cork, on 

 which a glass tube (a lamp-glass) is fixed containing 

 a depth of 5 inches of water, and the positive wire is 

 brought within half an inch of the surface of the 

 water in the tube, it becomes red-hot, and if drawn 

 further away from the surface the upper part of the 

 tube is filled with a peculiar glow or light abounding 

 in Stokes's rays. 



The experiments with the vacuum-tube, and espe- 

 cially Gassiot's cascade, are, as might be expected, 

 very beautiful. When a coal-gas vacuum-tube of 

 considerable diameter, and conveying the full dis- 

 charge from the secondary coil, is supported over a 

 powerful electro-magnet axially, the discharge is con- 

 densed and heat is produced. 



If placed equatorially^ the heat increases greatly, 

 and when the discharge is condensed, and impinges 

 upon the sides of the glass tube, it becomes too hot 

 to touch, and if the experiment was continued too 

 long the tube would crack. 



The enormous quantity of electricity of high ten- 

 sion which the coil evolves, when connected with a 

 battery^ of forty cells, is shown by the rapidity with 

 which it will charge a Ley den battery. 



Under favorable circumstances, three contacts with 

 the mercurial break will charge forty square feet of 

 glass. 



On one occasion a series of twelve large Leyden 

 jars arranged in cascade were discharged the noise 

 was great ; and each time the spark (which was yeiy 

 condensed and brilliant) struck the metallic disk, 

 and the latter emitted a ringing sound, as if it had 

 received a sharp blow from a small hammer. 



The discharges were made from a point to a metal- 

 lic disk ; and, when the former was positive, the dense 

 spark measured from 18i to 18f inches, and fell to 8i 

 inches when the metallic plate was positive and the 

 point negative. 



Variations of the Leyden-jar experiments were 

 tried by connecting the coil-work by a quantity bat- 

 tery of 25 + 25 cells with six Leyden jars arranged in 

 cascade, and the spark obtained measured 81 inches. 



The same six jars connected with the coil, when 

 the fifty cells were arranged continuously for inten- 

 sity, gave a spark of 12 inches of very great density 

 and brilliancy. 



The Duration of an Electric Discharge. 

 The American Journal of Science for Sep- 

 tember contains a valuable paper, by Prof. 

 Ogden N. Eood, of Columbia College, on the 

 "Nature and Duration of the Discharge of a 

 Leyden Jar connected with an Induction Coil," 

 originally read before the National Academy 

 of Sciences. The following are the more im- 

 portant portions of the paper : 



In the year 1835, Wheatstone published in the 

 Philosophical Transactions, Part II., page 583, an ac- 

 count of his celebrated experiment on the duration 

 of the discharge of a Leyden jar charged by a com- 

 mon frictional machine. The light from the spark 

 was received directly on a plane mirror revolving at 

 rates between 30 and 800 times in a second ; the mirror 

 was driven by a set of multiplying wheels connected 

 by strings. This apparatus was constructed by Mr. 

 Saxton, of Washington, who at that time was residing 

 in London. The eye of the observer was placed near 

 the mirror, and as the image of the snark was not 

 sensibly drawn out by the rotation ot the mirror, 

 Wheatstone concluded that its duration was less than 

 the one millionth of a second, a result which was 

 accepted by the scientific world for about a quarter 

 of a century, passing unquestioned till the publica- 

 tion in 1858 of the first of an admirable series of in- 

 vestigations of this subject by Feddersen (Pogg. 



