Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/76

Rh G6 ELECTRICITY [DISRUPTIVE DISCUAKUK. to get. When the spark passes, pressure is suddenly trans mitted through the fluid in all directions, and if it be in closed in a tube the tube is generally broken, even when the spark is by no means long. When the surface of the liquid is free, a considerable portion is usually projected into the air, The convective discharge is very marked in liquids, If two small balls connected with the electrodes of a Holtz s machine in action be dipped in paraffin oil at a small distance apart, the whole liquid is thrown into violent motion by the convection currents, runs up the wires which lead to the balls, and spouts off in little jets. There is also a distinct heaping up of the liquid between the balls, and if one of them be gradually withdrawn from the liquid, for a centimetre or so it raises a column after it, which adheres until the machine is stopped. It is very probable that other effects due to the alteration of the apparent surface tension, owing to the difference of electrical stress in the air and oil, are present in these phenomena, but this is hardly the place to discuss the matter. The electric discharge passes with great facility through card-board and other bodies of loose texture. In all pr &amp;gt; .bability the air in such cases has quite as much to do with the resulting effects as the solid body. Lullin s A curious experiment of this kind is often made. Two points experi- are arranged so as to touch the opposite sides of a piece of card-board, nieut. If the points be opposite each other, the discharge passes straight through, leaving in the case of small charges a liny hole with burnt edges. If, however, the points be not opposite each other, the perforation occurs in the neighbourhood of the negative point. The peculiarity is no doubt connected with those differences between positive and negative discharges in air which we have several times noticed above. In fact, it is found that in an exhausted receiver the card is pierced at a spot very nearly equidistant from the two points. Dis- In other cases the main part of the dielectric strength charge in depends on the solid material. The power of such bodies sohds. to sus t a i n the electrical tension is often very considerable. Yet there is a limit at which they give way. A thickness of 6 centimetres of glass has been pierced by means of a powerful induction coil. In such experiments special precautions have to be taken to prevent the spark from gliding over the surface of the glass instead of going through; this is managed in some cases by embedding the glass along with the terminals of the coil in an electrical cement of considerable insulating power; in ordinary experiments, however, it is in general sufficient to place a drop of olive oil round one of the terminals where it abuts on the glass. The appearance of the perforations depends considerably on the quantity of electricity that passes in the discharge. In some cases the glass cracks or even breaks in pieces. In some large blocks we have seen a perforation in the form of several independent threads, each of which had a sort of beaded structure, which may possibly be in some way ana logous to the stratifications in vacuum tubes. Surface Discharge along the Surface of a Body, Dust Figures, electri- and Dust Images. The class of phenomena referred to ncation. un d er this head are remarkable for the methods by which they are usually demonstrated. They were at one time much studied on account of the light they were supposed to throw on the nature of the so-called electric fluid or fluids. Though no longer regarded in this light, they have reference to an extremely important and comparatively little studied subject, viz., the distribution of electricity over the surface of non-conductors. It is easy to see that the demonstration of surface electrification on insulators is beset with difficulties of a peculiar kind. A very con venient method is to project on the surface a powder elec trified in a known way ; this powder clings to the parts oppositely electrified to itself, and avoids those similarly electrified, so that the state of the surface is seen at once. Lycopodium seed and powdered resin have been used in this way ; they are sifted through linen cloth, the lyco- podium becoming thereby weakly positive, and the powdered resin strongly negative. If the lycopodium be used, it covers both positive and negatively electrified patches, only the latter more thickly than the former. The powdered resin, on the other hand, covers the positive and avoids the negative regions. The most effective powder, however, is a mixture of flowers of sulphur 1 and red lead. In the process of sifting, the red lead powder becomes positively and the sulphur negatively electrified, and the powders separate themselves. The sulphur colours positive regions yellow, and the red lead colours negative regions red. The result is very striking ; and the test is found to be very delicate. The dust figures of Lichtenberg are one of the best Lichte known instances of the kind of experiment indicated kerg s above. A sharp-pointed needle is placed perpendicular to ^S lires a non-conducting plate, with its point very near to or in contact with the plate. A Leyden jar is discharged into the needle, and the plate is then tested with the powder. If the electricity communicated to the needle was positive, a widely extending patch is seen on the plate, consisting of a dense nucleus, from which branches radiate in all directions. If negative electricity was used, the patch is much smaller and has a sharp circular boundary entirely devoid of branches. This difference between the positive and negative figures seems to dopend on the presence of the air ; for the difference tends to disappear v/hen the experiment is conducted in vacuo. Riess explains it by the negative electrification of the plate caused by the fric tion of the water vapour, &c., driven along the surface by the explosion which accompanies the disruptive discharge at the point. This electrification would favour the spread of a positive, but hinder that of a negative discharge. There is, in all probability, a connection between this phenomenon and the peculiarities of positive and negative brush and other discharge in air ; lUess, indeed, suggests an explanation of the latter somewhat similar to the above. There is another class of figures, to which Riess gives the name of electric images, of which the following may be taken as a type. A signet or other engraved piece of metal is placed on a plat - of insulating material, and steadily electrified by means of a dry pile or otherwise positively or negatively for half an hour or so. When the metal is removed and the plate dusted, an exact figure of the stamp appears, consisting of a red or yellow background on which the engraved lines stand out free from dust. There is no difference between positive and negative electricity here as far as form is con cerned, and the colour of the figure indicates charge on the plate opposite to that on the metal. The phenomenon appears to be due simply to the electrification of parts of the non-conducting surface opposite the metal. Another class of phenomena, to which Riess gives the name Figur secondary, depend, not on the electrification of the surface, but on of Ka permanent alterations produced by the discharge, whether in the sten. form of spark or otherwise. Sometimes these are directly visible to the eye or touch, e.g., the roughening and discoloration which mark the path of the spark over a polished glass surface. In some cases they are chemical alterations, which may be shown by means of the proper reagents, e. g., the separation of the potash in the spark traces on glass. In certain cases they become evident on breathing upon the glass; of this description are the images of Kars- ten. A piece of mirror glass is placed on an uninsulated inetal plate, and on the glass is placed a coin or medal. Sparks are taken for some time between the coin and an electric machine, and then the glass plate is removed and breathed upon. A representation of the coin then appears on the g lass, often complete to the smallest detail. The reader who is interested in these matters, historically or otherwise, will find a variety of information, with directions how to find more, in Riess s Reibungsclcctricit&t, Bd. ii. 739 sqq. Electromagnctism and Electrodynamics? Mention has already been made of the discovery of Oer sted, that the electric current exerts a definite action on a magnetic needle placed in its neighbourhood. This dis- c 1 First used by Villarsy in 1788. 2 Throughout this section the reader is supposed to be familiar with the experimental laws of magnetism (see art. MACNKTISM). If he desires fully to understand the mathematical developments that occur here and there, an occasional reference to the analysis used in the theory of magnetism will also be necessary, if he is net already I aimllar with it.