492 STRIKING DISTANCE. [243. would exist in the same space, and the polarity would therefore be transmitted through double the quantity of insulating matter: —so that, if a given charge in air of ordinary density pass as a spark at 2 centimetres, at double the usual pressure the striking distance would be reduced to I centimetre; at a pressure of onehalf it would be increased to 4 centim.; at one quarter, to 8 centim.; and so on, until in vacuo theoretically it would pass through an unlimited distance. Experiment, however, has shown that a certain portion of matter, though it may be attenuated to an extent almost beyond the limits of calculation, is necessary for the transmission of the electric discharge (312). If the density of the air continue to be constant, it is found that the striking distance varies directly as the tension of the charge. For example if with a certain charge the striking distance be I centimetre, a double charge will discharge itself through 2 centim., and a three-fold charge through 3 centim. (Harris.) For equal quantities of electricity the striking distance is inversely as the extent of charged surface; so that, when a single jar is charged with a quantity of electricity sufficient to produce a discharge at 6mm., on employing two similar jars with the same quantity, the striking distance is reduced to 3mm., and with three similar jars to 2mm. For equal charges, the striking distance, however, varies in different gases, independently of their relative density, so that each gas has a specific insulating power. Hydrochloric acid has FIG. 199. AA twice the insulating power of common air, and three times that of hydrogen of equal pressure. This is in striking contrast to the equality of inductive capacity (237) in all gases. This inequality of insulating power was proved by Faraday by opening to the same charge two separate paths, one of them through air, the other through a receiver filled with the gas which was to form the subject of the experi ment, as shown in fig. 199. The distances be tween the balls were varied until the discharge took place with equal facility in both receivers; the same charge was thus found to traverse double the distance in air that it did in hydrochloric acid gas. Rarefaction of air, whether effected by heat or by mechanical means, equally favours the electric discharge. A jar may consequently be discharged through several centimetres of a common flame, in which the air is rarefied by heat to nearly five times its ordinary bulk, the temperature of an alcohol flame, ac 243.] DISSIPATION OF CHARGE. 493 cording to Becquerel's experiments, being nearly 2200° (1204° C.). A flame also acts by its pointed form in dissipating a charge with great rapidity, and its proximity should be avoided in exact experiments. Dissipation of the electric charge in dry air, according to Matteucci, is not increased by agitation of the air. Further, if the gases are all perfectly dry, and at the same temperature and pressure, the dissipation of the charge takes place with equal rapidity in air, in carbonic anhydride, and in hydrogen. In a moist atmosphere the loss of charge increases directly, cæteris paribus, as the amount of moisture. As the temperature rises, the dissipation of the charge increases in rapidity, the loss of the charge being twice as rapid at 18° as at o° C. If the density of the air be reduced, the tension of the charge which an insulated body will retain is reduced also, but the dissipation of the charge is very much diminished. Matteucci found, when an electroscope, feebly charged, was placed in a receiver, exhausted till the pressure was reduced to 3mm. of mercury (-inch), that the divergence remained unaltered after a lapse of two days. The form and size of the spark depend upon the shape of the discharging surfaces almost as much as upon the intensity of the charge. Between the rounded parts of the prime conductor and a large uninsulated metallic ball dense brilliant sparks pass; whilst if the same ball be presented to a wire which projects 8 or 10 centimetres from the conductor, and which terminates in a ball 25mm. or an inch in diameter, a long, forked, and often branching spark, resembling a miniature flash of lightning, will be obtained. When disruptive discharge occurs between a good conductor of limited surface and a bad one which exposes a larger surface, an intermitting and dilute spark or brush passes, which, when it occurs in air, consists of a rapid succession of discharges to the particles of air around: such a brush has a bright root with pale ramifications, attended with a quivering motion and a subdued roaring noise. Brushes of this kind are well seen when, the machine being in powerful action, the conductor is made to discharge itself into the air by means of a blunt rod which projects from it. The brush is largest from a positively charged surface, such as the prime conductor of the machine. From a negatively charged surface this discharge occurs at a lower tension, and more resembles a bright point or star of light. The formation of brushes is facilitated by rarefying the air around the charged points. 494 ELECTRIC BRUSH-LICHTENBERG'S FIGURES. : [243. Some remarkable differences have been observed between the positive and the negative spark for a charge of equal tension, the striking distance, between a good conductor positively charged and an inferior conductor, is greater in air than from the same conductor negatively charged, as may be seen in using the electrophorus. The greater facility with which positive electricity traverses the air may also be shown in the following manner :-Colour a card with vermilion ; unscrew the balls, a, b, from the discharger, fig. 197, and place the points on opposite sides of the card, one about half an inch (12mm.) above the other; discharge a large jar through the card. It will be perforated opposite the wire attached to the negative coating, and an irregular dark line of reduced mercury will be found extending on the positive side to the point of the positive wire. If the experiment be made in vacuo, the perforation will be formed midway between the two wires. The distinction between positive and negative electricity is also beautifully shown by what are termed Lichtenberg's figures, which may be obtained as follows:-Dry a glass plate, and draw lines on it with the knob of a positively charged jar, then sift over the plate a mixture of sulphur and minium in fine powder: on inverting the plate the minium will fall off and leave traces of the lines in sulphur. If the experiment be made with a jar negatively charged, the minium will adhere to the traces, whilst the sulphur will fall off. The explanation is very simple: by the friction in sifting, the sulphur becomes negatively, the red lead positively electric, and thus the sulphur attaches itself to the positively electrified lines upon the glass, and the minium to the negatively electrified lines, in accordance with the usual law of electric attraction. The experiment may also be varied in the following way :-Take two circular trays of tin-plate half an inch (12mm.) deep and 12 or 14 inches (30 or 35 centimetres) in diameter, fill them with melted resin, and allow them to cool; cause sparks of positive electricity to fall in 8 or 10 places upon one plate, and sparks of negative electricity in like manner over the other; on sifting a little brickdust over the two plates, the dry powder will assume the appearance of brushes over the plate electrified positively, and of oval or circular patches upon the negatively excited plate. Other remarkable differences between the sparks from positive and negative surfaces will be mentioned when noticing the modified discharges through exhausted tubes (312). The colour, light, and sound of the electric spark and brush vary. in different gases (106), the brush being larger and more beautiful in nitrogen than in any other gas, and its colour is purple or bluish. The sparks in oxygen are whiter than in air, but less brilliant. In hydrogen they are of a fine crimson colour. In coal-gas they are sometimes green and sometimes red; occasionally both colours are seen in different portions of the same spark. In carbonic anhydride the sparks resemble those taken in air, but they are more irregular and pass more freely. (244) c. Convection.—With a feebler charge the sonorous brush is replaced by a quiet glow, attended in this case with a continuous dispersion of the charge. The process of disruptive discharge thus gradually passes into the third method-viz., that by convection. When the glow is produced, a current of air, the particles of which are individually charged, passes from the charging surface. The course of this current may be exhibited by its 245.] CONVECTION-SOURCES OF ELECTRICITY. 495 action on the flame of a taper, which will often be extinguished if brought near an electrified point which is connected with the machine in action; and light models may be set in motion by it. If the production of the current from the point be prevented, as by sheltering the pointed wire in a varnished glass tube, the brush or glow may be converted into a series of small sparks. These currents may take place in liquid dielectrics as well as in gaseous ones. Let a piece of sealing-wax be fixed on the end of a wire and attached to the conductor of a machine in action; if it be softened by the application of the flame of a spirit-lamp, it will be thrown off in filaments towards a sheet of paper held near it. Solid insulated particles may also be the medium of convective discharge, as is seen when pith-balls or other light substances are attracted and repelled by electrified objects; and in delicate, experiments even the particles of dust floating in the atmosphere are not without effect in charging or discharging the apparatus employed. The process of convection assumes considerable importance in the phenomena of voltaic electricity, where it is intimately connected with chemical decomposition. (281 et seq.) (245) Other Sources of Electricity.-Hitherto we have limited our attention to cases in which electricity is excited by the friction of dissimilar substances, and it may here be remarked that Peclet by a careful series of experiments found that the quantity of electricity developed was the same whether sliding friction was employed, as in the ordinary mode of exciting the electrical machine, or whether it was a rolling friction, in which the rubber was pressed against the cylinder and allowed to roll upon its axis as the machine was worked. The development of electricity by friction is, however, but a special case of a much more general law, for it has been found that, whenever molecular equilibrium is disturbed, a concomitant development of electricity takes place. The following instances will exhibit the variety of circumstances under which this observation has been made. The mere compression of many crystallized bodies is attended by electric action: a rhombohedron of Iceland spar, if compressed by the fingers, exhibits this peculiarity. It is also found that all bodies that have been pressed together, if properly insulated, offer signs of electricity on being separated; although the effect is most easily observed between a good conductor and a bad one. The two bodies are always in opposite states. Even where two disks of the same substance are pressed together, if one be a little warmer than the other, distinct excitement is produced, the warmer disk becoming nega 496 ELECTRIC EXCITEMENT BY HEAT. [245. tively electrified; the charge, cæteris paribus, increases in all cases directly as the pressure to which they are subjected. Fracture is likewise attended with electric disturbance; the freshly broken surfaces of roll sulphur often exhibit this effect to an extent sufficient to produce divergence of the leaves of the electroscope when the fragments are placed upon the cap of the instrument. The sudden rending asunder of the laminæ of a film of mica in a dark room, is usually attended with a pale electrical light, and the separated portions in this case exhibit opposite electrical states. A melted substance in the act of solidifying, sometimes exhibits electric excitement. If sulphur be allowed to solidify in a glass vessel, it becomes negatively excited, whilst the glass is rendered positively electrical; ice also is frequently electric; and the same thing has been observed of chocolate as it becomes solid. These results are probably due to friction occasioned by the contraction or expansion of the solid mass in the mould, from which it detaches itself by this change of bulk. In some instances simple elevation or depression of temperature causes electric excitement. These effects are most distinctly seen in crystallized non-conductors which are not symmetrical in form, being produced in bodies which are hemihedral. Tourmaline, boracite, and the crystals of tartaric acid, offer the best examples of this description. The tourmaline, for instance, commonly assumes the form of a three-sided prism, the edges of which are replaced by two narrow planes. The extremities of the crystal are b/s S FIG. 200. 2 I a formed by the three faces of the rhombohedron. No. 1, fig. 200, shows the end of the crystal which becomes positive by heat; No. 2, the opposite end of the crystal which becomes negative. If a crystal of tourmaline be gently heated, it becomes powerfully electrical whilst the temperature is rising, one extremity, termed the analogous pole, becoming positive, the other extremity, or antilogous pole, becoming negative.* When the temperature becomes stationary, the electric excitement ceases: as the crystal *The crystal must not be too strongly heated, about 302° (150° C.) being the best point; if heated very strongly, 752° (400° C.) or beyond, the tourma line becomes a conductor for a time: it resumes its insulating power on cooling, but is rendered hygroscopic till after it has been washed and dried at 302 (150° C.)-Gaugain. |