When the knob of the insulated jar is touched, a spark is got, and if the finger be then removed to the outer coating, another spark, but of the opposite name, is obtained, and the knob is again prepared to give a spark, and this alternating process may be con tinued till the jar is emptied. When the inner coating is touched, the outer coating becomes insulated, and thus the potential always shifts to the insulated coating with an opposite name to what it had before. Each spark obtained by the finger in going from the one to the other consumes so much of the energy of the charge, and so the potential is gradually lowered. When the jar is discharged by the tongs, the charge of the dielectric glass is thrown into the dielectric air. The particles of the glass, though more easily electrified than those of air, having a higher specific inductive capacity, offer a much greater resistance to discharge than those of air. At the same stage of polarifica tion, the air gives way, while the glass still keeps polarified. Hence a jar with glass only a fraction of an inch in thickness can give rise to an air-spark of several inches; besides this, the charge in the glass is somewhat uniformly distributed. In the air, with the tongs, the force of the charge is concentrated on a certain region of it, and the breaking down of the conductive resistance of the air is more easily effected. The feeble residual spark from the jar, after the first main discharge, is due to what is called electric absorption. Somehow, the E. given to a dielectric is not immediately available when a circuit is offered, the dielectric taking some time to recover itself. This is. observable in all solid dielectrics, but no trace of such action is found in air.
The sparking or striking distance of the jar indicates the potential of the charge. The quantity may be measured by the turns of the charging machine. It is found that when the same quantity is given to two jars, one double the other in point of covered surface, the striking distance of the large jar is only half that of the small jar; and that to charge• the large one so as to obtain the same length of spark, twice the quantity must be given. If two jars be taken of the same size, and one of them be charged, we find that, on con necting their outside coatings, a spark passes when their knobs are brought together, and that, when now the double jar is discharged, the spark is only half as long as was. got from the single jar discharged directly. The quantity discharged finally in the double jar was the same as in the single jar, but the potential was half. The spark occurring at the participation of the charge accounts for the loss of potential.
For great power, large surfaces are necessary. This can be obtained either by con structing a large jar, or by uniting several small jars together so as to act as one. The latter method is preferable, as we can vary the surface according to the number of jars employed. A combination Of sniall jars united together as one is called an electric battery. In a very convenient form of electric battery the knobs of each jar communi cate with a large central one by means of arms of brass moving on hinges, and the outer coatings are put in conducting connection, by being placed on an insulated stool cov ered with tinfoil. The interior coatings are conveniently charged by a long projecting arm from the central knob, and the exterior ones by connecting the stool with the knob of the unit jar, or by a wire with Any jar can be thrown out of action by throwing back its arm.
By discharging the Leyden jar or electric battery through particular channels, we obtain some beautiful illustrations of the power of electricity. When the discharge is erect ed through thin wires of gold or platinum, the heat accompanying its passage is so great as to dissipate them in vapor. The expansion of the air caused by the spark is shown by the electric mortar. This is a wooden mortar with two wires entering air-tight at the opposite sides of the breach, with a small wooden ball fitting closely in the muzzle. The
spark passing between these wires in discharge causes a sufficient expansion of the air within the mortar to drive the ball to some distance off. When the discharge is made through gunpowder, it tosses the grains violently about, but causes no ignition; when, however, it is retarded by introducing an imperfect conductor, such as a wet string, into the circuit, the gunpowder is fired. When the discharge is made through glass by two points pressing against its opposite surfaces, a small hole is drilled into the glass.
Velocity of Electrical Discharge.—The velocity of E. is found to vary with the nature of the circuit to the extent, indeed, of its inductive embarrassment (see TELEGRAPH). Thus, in air-lines of telegraph it is greater than in sea-cables. Wheatstone was the first to determine the velocity of E. in an insulated copper wire stretched in air. He did this by the device of a revolving mirror. Any one who takes a mirror in his hand and makes it revolve, sees that objects are apparently displaced by it, and'that the reflected image describes an angle the double of that of the mirror. If, while the small mirror rotates at 50 turns a second, the image of a spark should show a displacement of 90°, we know that the mirror has moved through 45°, and the time during which this takes place is .gier = of a second. If the duration of the spark, then, had been of a second, we should have seen its image move through 90°. The eye, however, during this time would not have been able to discern any difference between the beginning and the end of the spark, so that the 90° would have appeared as one arc of light. Examined in this way, however, the spark of a machine and of a Leyden jar were seen as if the mirror had been at rest. He arranged a Leyden jar circuit of half a mile with three breaks in it, two near the coatings, and one in the middle of the half-mile, and had these breaks placed nearly side by side, so that the sparks at them, when discharge took place, could be seen together in the revolving mirror. He found that all three sparks had a duration of a second, and that the middle spark occurred so far behind the other two as o to indicate a velocity of 194,000 m. per second in the wire.
Electric Theories.—There are two theories which have played an important part in the history of the science—the two-fluid theory of Dufay, and the one-fluid theory of Franklin. According to the former, matter is pervaded with two highly elastic impon derable electric fluids—one, the vitreous; the other, the resinous. These are supposed to repel themselves, but attract each other. Neutral bodies give no evidence of their presence, for they are there neutralized the one by the other; but when by friction or other operation the fluids are separated, each body observes the attractions and repul sions of the fluid it happens to have. According to the latter, there is only one electric fluid which repels itself, but attracts matter. Friction determines a gain of the fluid to the positive, and a loss to the negative body. Faraday's theory of electric induction by contiguous molecules appears to be gaining ground. "It explains satisfactorily how conductors and non-conductors are alike in kind; how the charge on the conductor can only reside at the boundary of the conductor and non-conductor, or—which is the same thing—the surface of the conductor; how the charge resides in the dielectric; how the polarity of the galvanic circuit is effected; how a battery current originates in and effects chemical decomposition; and how the velocity of discharge is dependent on the conformation of the circuit" (Electricity, Chambers's Educational Course, 1867). Prof. Clerk Maxwell's classical work, Electricity and Magnetism (1873) gives to Faraday's views a mathematical significance and comprehensiveness hardly contemplated by the great philosopher himself.