Experiments with the induction that we experiment with a coil like the one shown in the figure about 1 ft. long and nearly 6 in. in diameter, which yields readily sparks of from 4 to 5 in. with a battery of 6 Bunsen cells. After connecting the battery wires, and setting the commutator so as to complete the contact, let us place the movable rods within an inch of each other. An uninterrupted rush of sparks is transmitted between the points of the rods. The spdrks are not the clear single sparks. of the electric machine, but seem to be made up of several sparks occurring at the same instant, which are white and crooked. These are enveloped in a luminous haze, which, on closer examination, wears the appearance of a congregation of the spiral sparks, the convolutions of which are iu constant rotation. This hazy spark can be blown away by the breath, and sepa rated from the white spark, which cannot be so removed. As the rods are withdrawn from each other, it disappears, and when they stand above 3 in. apart, the spark resem bles in every respect the forked single spark of a powerful electric machine. When the points are withdrawn beyond striking distance, electric brushes still play betivden them, which become visible in a darkened room. If the hand be brought near the rod connected with the exterior end of the coil, sharp stinging sparks, 2 or 3 in. in length, are got. connected with the inner end does not yield them so readily, and this is the same whether it be the positive or negative pole. When a gold-leaf electroscope is brought near, the leaves part energetically from each other; and when a spark is received by it from one of the rods, it remains permanently charged. When, however, the knob of the electroscope is brought into actual contact with either of the rods, this action ceases, because the induced currents, inverse and neutralize each other. When the knob touches, both currents affect the leaves equally; but when it is at some distance, the direct current alone has tension enough to act. Each pole of the induction coil is the seat of two opposite electricities, alternating with each other, alike in quantity, but differing in tension, and this accounts for the resemblances and differences between the coil and machine electricities. When the poles are put in connection with the coat a Leyden jar, the sparks passing between the points are much more brilliant, and Ithe sharp snap of the simple spark grows into a loud report. The Leyden jar effects a condensation of the, electricity of each direct current, and each spark discharge takes place in shorter time, and consequently with greater intensity. The condensed spark punctures paper and the like with great facility, but it is of very low heating power. The uncondenscd spark, more particularly the hazy spark, got when the poles are near each other, kindles paper, gunpowder, coal-gas, and other combustibles with readiness and certainty. It is from this property of its spark that the induction coil is of so great use in mining operations. The two ends of the wires coming from the coil are fixed
near each other without touching, and are imbedded in a charge of gunpowder at a safe distance from the operator. The wires are insulated by gutta-percha, and when the induced current is sent through them, sparks pass through the gunpowder between the ends of the wires, and set fire to it. When several charges have to be ignited simul taneously, the spark of the coil-machine is considered much more reliable than the action of a powerful galvanic battery in heating thin wires connecting the ends of the battery wires. See GALVANTSNI. The power of the direct induced current of even large induction coils to deflect the magnetic needle, and to effect chemical decomposition, is very insignificant. This shows that it is very, much inferior to the inducing current in quantity, however much it may be superior in tension. The physiological effect, on the other hand, is tremendous, and the experimenter must take care not to allow any part of his body to form the medium of communication between the poles, as the shock so got might be dangerous, if not fatal.
When the induced current is made to pass through nearly vacuous spaces, a very splendid effect is produced The electric egg is employed to display this. It consists of a glass vessel in the shape of an egg, with an open neck above, and another below. Brass fittings are attached to these. The lower opening is fitted with a stop cock, and can be screwed to the plate of an air-pump. A brass rod and ball rise a short way into the egg. The fittings above are intended to allow of a rod ending in a ball passing up and down air-tight, so that the two balls can be conveniently set at different distances. When the egg i•exhausted, and the wires from the coil are attached, the one above, and the other below, a luminous glow extends between the balls, which is wide in the middle, and contracts at either extremity. When the exhaustion has reached one.twelftli of an inch, as shown by the gauge of the air-pump, black bands are seen to lie horizontally in the light, so as to wear the appearance of stratification. These occur more readily when a drop or two of turpentine, alcohol, or ether have been introduced into the egg. The cause of the stratification is as yet a matter of speculation, The ball which forms the negative pole is enveloped in a covering of blue light. The glow, which is of a beautiful mauve tint, appears to proceed from the positive ball, and reaches nearly to the negative' ball, from which it is separated by a well-marked non-luminous space. By means of the commutator, these appearances at the balls can be instantly transposed. Serving the same purpose as the electric egg, there is a great variety of vacuous tubes hermetically sealed and ready for use at any time. These have been first filled with particular gases, and then exhausted, and they exhibit lights of various tints, according to the gas con: tamed by them.