1 1,152,000

definite duration would appear to be drawn out into an elongated streak. Such an elongation was found to be visible when a Leyden jar was discharged through a copper wire half a mile long; and when the circuit was interrupted at three points, one in the middle and one at each end of this wire, three sparks were obtained, which, viewed in the mirror, showed a lateral displacement, indicating (with the particular rate of rotation employed) that the middle spark took place of a second later than those at the ends. Wheatstone argued from this a velocity of 288,000 miles per second. But Faraday showed that the apparent rate of propagation of a quantity of electricity must be affected by the capacity of the conductor; and he even predicted that since a submerged insulated cable acts like a Leyden jar (see Art. 274), and has to be charged before the potential at the distant end can rise, it retards the apparent flow of electricity through it. Professor Fleeming Jenkin says of one of the Atlantic cables, that, after contact with the battery is made at one end, no effect can be detected at the other for two-tenths of a second, and that then the received current gradually increases, until about three seconds afterwards it reaches its maximum,

and then dies away. This retardation is proportional to the square of the length of the cable as well as to its capacity and to its resistance; hence it becomes very serious on long cables, as it reduces the speed of signalling. There is in fact no definite assignable "velocity of electricity."

A very simple experiment will enable the student to realise the excessively short duration of the spark of a Leyden jar. Let a round disc of cardboard painted with black and white sectors be rotated very rapidly so as to look by ordinary light like a mere gray surface. When this is illuminated by the spark of a Leyden jar it appears to be standing absolutely still, however rapidly it may be turning. A flash of lightning is equally in

stantaneous; it is utterly impossible to determine at which end the flash begins.1

297. Electric Dust-figures.-Electricity may creep slowly over the surface of bad conductors.

Lichtenberg devised an ingenious way of investigating the distribution of electricity by means of certain dust-figures. The experiment is very easy. Take a charged Leyden jar and write with the knob or a dry sheet of glass.

of it upon a cake of shellac Then sift, through a bit of

muslin, over the cake of shellac a mixture of powdered red lead and sulphur (vermilion and lycopodium powder answer equally well). The powders in this process rub against one another, the red lead becoming +, the sulphur Hence the sulphur will be attracted to those parts where there is + electrification on the disc, and settles down in curious branching yellow streaks like

1 Sometimes the flash seems to strike downwards from the clouds sometimes upwards from the earth. This is an optical illusion, resulting from the unequal sensitiveness to light of different portions of the retina of the eye.

those shown in Fig. 109. little red heaps and patches where the electrification is negative. Fig. 110 shows the general appearance of the Lichtenberg's figure produced by holding the knob of

The red lead settles down in

the Leyden jar at the centre of a shellac plate that has previously been rubbed with flannel, the negative electrification being attracted upon all sides toward the central positive charge.

Powdered tourmaline, warmed and then sifted over a sheet of glass previously electrified irregularly, will show similar figures, though not so well defined.

Breath-figures can be made by electrifying a coin or other piece of metal laid upon a sheet of dry glass, and then breathing upon the glass where the coin lay, revealing a faint image of it on the surface of the glass.

298. Production of Ozone.-Whenever an electric machine is worked a peculiar odour is perceived. This was formerly thought to be evidence of the existence

of an electric "effluvium" or fluid; it is now known to be due to the presence of ozone, a modified form of oxygen gas, which differs from oxygen in being denser, more active chemically, and in having a characteristic smell. The discharge of the Holtz-machine and that of the induction coil are particularly favourable to the production of this substance.

299. Dissipation of Charge.—However well insulated a charged conductor may be, and however dry the surrounding air, it nevertheless slowly loses its charge, and in a few days will be found to be completely discharged. The rate of loss of charge is, however, not uniform. It is approximately proportional to the difference of potential between the body and the earth. Hence the rate of loss is greater at first than afterwards, and is greater for highly charged bodies than for those feebly charged. The law of dissipation of charge therefore resembles Newton's law of cooling, according to which the rate of cooling of a hot body is proportional to the difference of temperature between it and the surrounding objects. If the potential of the body be measured at equal intervals of time it will be found to have diminished in a decreasing geometric series; or the logarithms of the potentials at equal intervals of time will differ by equal amounts.

This may be represented by the following equation :

where V. represents the original potential and Vt the potential. after an interval t. Here e stands for the number 2.71828... (the base of the natural logarithms), and p stands for the "coefficient of leakage," which depends upon the temperature, pressure, and humidity of the air.

The rate of loss is, however, greater at negatively electrified surfaces than at positive.

300. Positive and Negative Electrification.— The student will not have failed to notice throughout

this Lesson frequent differences between the behaviour of positive and negative electrification. The striking dissimilarity in the Lichtenberg's figures, the displacement of the perforation - point in Lullin's experiment, the unequal tendency to dissipation at surfaces, the remarkable differences in the various forms of brush and glow discharge, are all points that claim attention. Gassiot described the appearance in vacuum tubes as of a force emanating from the negative pole. Crookes's experiments in high vacua show molecules to be violently discharged from the negative electrode, the vanes of a little fly enclosed in such tubes being moved from the side struck by the negative discharge. Holtz found that when funnel-like partitions were fixed in a vacuum tube the resistance is much less when the open mouths of the funnels face the negative electrode. These matters are yet quite unaccounted for by any existing theory of electricity.

The author of these Lessons is disposed to take the following view on this point:-If electricity be really one and not two, either the so-called positive or the negative electrification must be a state in which there is more electricity than in the surrounding space, and the other must be a state in which there is less. The student was told, in Art. 6, that in the present state of the science we do not know for certain whether "positive" electrification is really an excess of electricity or a defect. Now some of the phenomena alluded to in this Article seem to indicate that the so-called "negative" electrification really is the state of excess. In particular, the fact that the rate of dissipation of charge is greater for negative electrification than for positive, points this way; because the law of loss of charge is the exact counterpart of the law of the loss of heat, in which it is quite certain that, for equal differences of temperature between a body and its surroundings, the rate of loss of heat is greater at higher temperatures than at lower; or the body that is really hotter loses its heat fastest.

LESSON XXIV.-Atmospheric Electricity.

301. The phenomena of atmospheric electricity are of two kinds. There are the well-known electrical phenomena of thunderstorms; and there are the phenomena