ELECTRICITY, COMMON. 



ELECTRICITY, ATMOSPHERIC. 



tioaally raised ; but in general the resistance to conduction U maul- 

 tated by Ute erolution of heat, the measure of which U inversely u 

 the conducting power. Harrta contrived a kind of air thermometer with 

 largo bulb, acroes which could be placed wire* of equal length and 

 thicknew. and he found that on discharging equal quantities of elec- 

 tricity through these wine in .uooesrion. he was enabled to assign 

 numerical values to them, the smallest number* being given to the beet 

 conductors, or thoee which emitted the leut heat. Thus copper and 

 silver were each represented by 6, gold by 9, rino 18, pUtinum SO, 

 iron 30, tin M, lead 78. By alloying the metal* with each other the 

 conducting power WM in some caMi greatly reduced : thus, an alloy of 



3 parU gold and 1 part copper, gave the conducting power of 25, while 

 gold 1 and copper 3, gare only 15. Gold 3 and silver 1 = 25, tin 1 and 

 lead 1 a 64, tin 1 and copper 8 = 11, and braes = 18. When different 

 quantities of electricity were transmitted through the aame wire, it 

 was found that the increase in temperature was as the square of the 

 quantity ; so that if the thermometer with a given charge rose 10, 



4 times that charge were required to raise it twice that amount or 20. 

 Thin wires of silver, steel, platinum, copper, *c., can be readily fused 

 and dispersed by sending a strong charge through them. The amount 

 of such charge as measured by the unit jar is equally powerful, whether 

 diffused over a large or a small surface. The intensity of the charge, 

 as expressed by the quantity of electricity passing through a given 

 space in a given time, U the same in the wire, although the intensity 

 of the charge on equal surfaces of the jar may vary. It must be 

 remembered that although in the charge we have chiefly to deal with 

 surfaces, yet in the discharge, all the particles, (that is, the whole 

 thickness of the conducting wire,) ore concerned in the result. In the 

 discharge by disruption, the particles of the di-electric gradually be- 

 come more and more highly polarised or excited, the tension on one 

 or more particles becomes so great as to exceed the limits of resistance, 

 the opposite induced forces cease to balance each other, and the dis- 

 charge passes along the line of least resistance, accompanied by light, 

 heat, and noise; while portions of the solid conductors become de- 

 tached and give characteristic colours to the spark. By this transference 

 of metallic particles from one conductor to another, particles of silver 

 may be precipitated on copper, and even mode to penetrate its sub- 

 stance ; and there are oases in which gold has been made to penetrate 

 a plate of silver, and appear on the opposite side where the sparks 

 passed. For some grand examples, however, of disruptive discharge 

 we must refer to LIOHTHDTO. 



We have seen that in air of whatever density (unless so rare as to 

 conduct), the same amount of charge produces the same extent of con- 

 duction, other things being equal. The distance through which the 

 discharge of equal quantities of electricity, or what is called the 

 ttrikimj dittantr, takes places, varies inversely as the pressure. A 

 double pressure doubles the number of aerial particles in the same 

 space, and double the amount of insulating matter is required to be 

 polarised; for example, if in air, at common pressure the striking 

 distance be two inches, at double that pressure it would be one inch ; 

 halve the pressure and it would be 4 inches, at one-fourth the pressure 

 8 inches, and so on until in ranio the striking distance would be un- 

 limited. When the density of the air remains constant, the striking 

 distance Tories as the intensity of the charge. Thus, if the striking 

 distance be with a certain charge 1 inch, at double that charge it will 

 be 2 inches, at treble that charge 3 inches. But with equal charges 

 the striking distance varies in different gases, irrespective of their 

 relative density, so that each gas has a specific insulating power ; thus 

 hydrochloric acid has twice the insulating power of atmospheric air, 

 and three times that of hydrogen of equal elasticity. When the dis- 

 charge takes places between a good conductor presenting a small 

 surface, and a bad one of larger surface, there is a rapid but inter- 

 mitting succession of discharges to the particles of air around ; and the 

 sparks thus diluted form a bruth, which has a quivering kind of motion, 

 and is attended by a subdued roaring noise : its root is brighter than 

 the rays. The phenomena of the brush vary in different gases, the 

 most beautiful effects being produced in nitrogen. The largest brush 

 is produced from a surface charged with vitreous electricity : when a 

 point is held to a surface charged resinously, a star or point of light is 

 produced instead of a brush. When the charge is feeble, discharge is 

 sometimes effected by a quiet glom instead of the noisy brush, and con- 

 tvetioB then takes place, that U, a current of air conveys the charge to 

 a distance, which current has sufficient force to give motion to electrical 

 toys arranged for that purpose. 



The duration of a flash of lightning, or of the spark discharge of a 

 Leyden jar is so instantaneous, that the most rapid motions which 

 we can give to machinery appear to be rest as compared with it. Thus, 

 if we print tin- words AT KKST in large letters on a disk of card- 

 board, and cause this to spin rapidly round upon on axis through 

 its centre, the words will of course disappear ; but if wo allow the 

 flash of a Leyden jar to illuminate the disk, the words may be read 

 with perfect ease, since the light has come and gone before the disk 

 had time to move through any appreciable space. In this way 

 \Vhcatstone has shown that the light of the electric discharge lasts lens 

 than the millionth of a second. Suppose a small wheel of dull metal to 

 contain 100 bright equidistant rays, and to revolve 10 times per second 

 or once in the Ath of a second. The appearance of these radii, as seen 

 by the reflected light of an electric spark, will differ according to it* 



duration. If the time be infinitely short, the reflexion during 

 a second will give the appearance of 100 fixed luminous rays. If it 

 lift gfeth of a second, the whole circle will be luminous, since the 

 impression of each ray upon the eye will remain until that of the 

 succeeding ray is produced. For a duration of J, Jrd, Jth, |th, tat., 

 of ninth of a second, corresponding illuminated segments will be seen, 

 and Jnd, (da, Khs.or jths of the circle will appear deprived of light, that 

 is, there will be alternate bright and dark spaces corresponding to thoee 

 quantities. By increasing the size of the wheel, the scale of these 

 measures may be augmented, as may also its subdivision by increasing 

 the velocity, or by multiplying the number of the spokes. By a modi- 

 fication of this apparatus, called the Ckronotcope, WheaUtone was able 

 to measure the velocity of the discharge of a Leyden jar through an 

 insulated copper wire, and he estimated it 288,000 miles per second. 

 (' Phil. Trans.,' 1884.) The copper wire was about half a mile in length, 

 and was broken at three points, one within a few feet of the inner 

 coating of a Leyden jir, a second in the middle of the wire, and a 

 third near the outer coating of the jar, and the wire was so contorted 

 that these three breaks were arranged side by side on an insulsted 

 disk or fjiark board, so that the three sparks could be seen cinml- 

 toneously. When the jar was discharged through this half mile of 

 wire the three sparks appeared to be simultaneous, but when seen by 

 reflexion in a small steel mirror rapidly revolving on an axis parallel 

 to its surface the sparks did not appear as points of light in the same 

 horizontal line, but gave the appearance of three bright lines of equal 

 length ; the two outer lines were found to begin and end within the 

 same horizontal space, but the middle one coming a little Inter than 

 the others, the angular position of the mirror advanced somewhat 

 before the middle spark made its appearance. Now, as the velocity of 

 the mirror was known, and the amount of angular deviation of the 

 central spark could be easily ascertained, the retardation of the dis- 

 charge by the copper wire, or the velocity with which it moves 

 through it, can be estimated. Professor Miller, who in his ' El< i 

 of Chemistry,' part 1, gives a succinct but masterly sketch of modern 

 electrical science, remarks that this experiment " affords a convincing 

 proof of simultaneous action and reaction in the operations <>t 

 tricity, and of its existence as a duplicate force : at the same moment 

 that a positive influence leaves the inner coating, an equal amount of 

 negative influence leaves the outer coating, and these two neutralise 

 each other at the central point of the conductor, after the lapse of an 

 extremely minute but still appreciable interval of time. It appears 

 from this experiment that Franklin's theory, though in many coses a 

 simple and convenient mode of explaining facts, is not the true repre- 

 sentation of the phenomena. The theory of two fluids, or rather of 

 two forces, acting in opposite directions, seems by this experiment to 

 be demonstrated." It must be remarked, however, that the velocity of 

 the electric discharge varies with the intensity of the charge and the 

 nature of the conducting medium. 



ELECTRICITY, ATMOSPHERIC. The similarity of lightning to 

 the spark obtained by friction from an electrical apparatus was 

 observed by the earliest experimenters in electricity ; and in one of 

 Franklin's letters, written apparently before the year 1 750, the points 

 of resemblance ore distinctly stated. The first fruit of this discovery 

 was the employment of thunder-rods for the protection of buildings 

 and ships ; and rods or wires projecting above the tops of edifices were 

 soon extensively used by philosophers for the purpose of enabling 

 them to ascertain the nature and intensity of the electricity in the 

 atmosphere. Such means are not unattended by danger ; and science 

 has to record the death of Professor Richman, during a thunder-storm, 

 while attending to the indications of the electrometer connected with 

 an apparatus of that kind. [LIGHTNING.] 



Franklin in America, M. de Romas in France, and Cavallo in 

 England, each employed, for the purpose of bringing electricity from 

 the atmosphere to the surface of the earth, a kite made of silk stretched 

 on a frame, from the upper part of which projected a piece of pointed 

 metal, and from which proceeded along the string a slender metallic 

 wire. M. Buffon, M. Lemonnier, and others, planted vertically in tlio 

 ground poles, from 30 to 40 feet in height, carrying at the top a 

 pointed piece of tin or iron, from which descended a metallic wire. 

 M. Me'zeas in France, Mr. Ronayne in Ireland, and Mr. Crosse in 

 England, employed long wires in horizontal positions, which were 

 insulated by being stretched between two gloss pillars, each on the top 

 of a pole planted in the ground. The wire used by Mr. Crosse was 1800 

 feet long, and was above 100 feet from the ground. MM. Becquerel 

 and Breschet examined the electric state of the air in the upper regions 

 by means of a cord covered with tinsel about 90 yards long, one end of 

 which was placed on the cap of a gold leaf electroscope, and the 

 was attached to a metal arrow which was shot into the air. The gold 

 leaves were seen to diverge in ]>ro|iortiou to the ascent of the 

 (Becuuerel,' TraiU' <1" I'KI.-ctriciUY (. iv.t 



Tin' numerous experiments made by Cavallo serve to prove tli 

 electric fluid always exists in the atmomhere, but in vn-y ilill.n-nt 

 quantities at different time*, and tliat. * alnnnlaiit in tho 



higher regions than near the earth. The same philosopher found, also, 

 that it is more intense in frosty than in warm weather, and that fogs 

 are accompanied by a great quantity of electricity, exospt when they 

 become rain ; in this oase little electricity is perceptible, the nun con- 

 ducting to the earth the electricity of the air above. In high winds, 



