“So,” adds the writer, “the duration (of spark) increases in proportion with the number of the jars. It increases also with the striking distance, but is independent of the diameter of the balls or globes between which the spark strikes.”
Many examples might be given of the spark discharge of the electric current. This form is seen in the blue line extending between the knob of a machine and the hand, and the duration of a spark with a jar charged with an induction coil is stated by Professor Rood to vary, but the brightest portion with a jar of 114 square inches only existed for the 175th billionth of a second, and with a smaller surface was much shorter. Such a spark may be conducted to a plate of gunpowder and will not ignite it, because the time of the duration of the “fire” is not sufficiently long; the powder will be scattered, but not ignited. If, however, a partly non-conducting medium be interposed between the jar and the powder, so that the spark be retarded a little, the gunpowder will be fired.
While speaking of electric discharge we may remark upon the beautiful effect of lightning. These discharges are sometimes miles long, and by the return stroke from the cloud may kill a person a long way from the actual discharge. This phenomenon was illustrated by Viscount Mahon in 1779, in a very interesting book on the principles of electricity.
There are different ways in which the electric discharge shows itself. We have spoken about the spark discharge which, however, is found to present very different appearances in varying conditions. Professor Faraday proved that the colour of the electric sparks showed in air, when obtained with brass balls, the intense light and blue colour so familiar to all. In nitrogen they are even bluer. In oxygen again the sparks are much brighter than in air, but not so brilliant. In hydrogen they become crimson, but the sound is almost inaudible because of the physical character of the gas. In carbonic acid gas they are almost the same as in atmospheric air, only more irregular. In dry muriatic gas they are nearly white and very bright. In coal gas the colours vary—sometimes being green and sometimes red. Occasionally the same spark will be red and green at different extremities, and even black portions have been observed. The density and pressure of the atmosphere has been proved to exercise considerable influence upon the spark discharge.
The “Brush” discharge is shown in “a series of intermittent discharges which appear continuous.” This discharge assumes the shape of a fan. “It is accompanied with a low chattering sound,” which is the result of the separate and continuous discharges, and Faraday also demonstrated that its effects varied according to the medium in which it was exhibited.
The effect of the air pressure on electricity may be observed in the following way:—
If we pass a spark through rarefied air by an apparatus known as the Electric Egg, we may obtain many curious effects. The “egg” consists of a glass globe, through which enter two rods with a knob upon each inside end. The upper rod is moveable, and held in its place by a “cap” like the lower rod. There is a stop-cock in the lower cap, so that the egg may be fastened to any plate or stand. When the egg is filled with air, the electric spark passed into the glass globe has the usual appearance, but as the air is gradually rarefied by an air-pump, the spark assumes beautiful forms and colours. As the exhaustion continues, however, we shall find the spark decreasing in brilliancy, and finally the spark will cease to be visible. It thus is shown that the colour of the spark depends upon the gaseous medium and on the material of the conductors, and when the electric spark is faint this medium can be observed, for nitrogen will produce a blue tinge and carbonic acid a green; hydrogen gives us a red, as already remarked. By multiplying the number of eggs and plates of glass, and placing discs of tinfoil in various shapes at certain distances, many beautiful figures may be observed when the spark is set free.
Professor Tyndall at the Royal Institution showed a very pretty experiment. He took a funnel with a very fine bore, and permitted sand to flow from it as it will in the hour-glass. When he permitted the electric current to come in contact with the sand, however, it, instead of falling vertically to the table, spread out fan-like, each grain repelling and being repelled by its neighbour with an effect very beautiful to see. Luminous effects have frequently been produced by passing an electric spark through various bodies. For instance, a lump of sugar can be made quite brilliant in the dark by passing electricity through it, and there are other substances similarly affected. Even eggs and some fruits are thus made phosphorescent. The illumination of the “diamond” covered Leyden jar is familiar to all who ever attended a lecture on Electricity.
The various effects of the electric discharge need not here be described. We have witnessed the results of lightning, but even in our laboratory many pretty little experiments can be made, such as the perforation of a card by the electric discharge. The chemical effects are various. Decomposition of water is effected by electricity, and the discharge can also be, and has been utilized for military purposes, such as employed by Professor Abel in his fuse, and in his apparatus for firing mines. Experiments at Chatham and elsewhere have been very successful in the application of electricity to modern warfare.
We will illustrate one or two of these. A thick card should be placed, as in the illustration (fig. 216), between two insulated points, and to the lower portion of the apparatus a chain be attached, held in the hand, and wound round the Leyden jar. If then the knob of the jar and the knob above the upper point be brought together, the spark will pass through the card.