Elementary Text-book of Physics/Ch. IV Part VII
CHAPTER VII.
LUMINOUS EFFECTS OF THE CURRENT
318. The Electric Arc.—If the terminals of an electric circuit, in which is an electromotive force of forty or more volts, be formed of carbon rods, a brilliant and permanent luminous are will appear between the ends of the rods if they be touched together and then withdrawn a short distance from each other. The temperature of the arc is so high that the most refractory substances melt or are dissipated when placed in it. The carbon forming the positive terminal is hotter than the other. Both the carbons are gradually oxidized, the loss of the positive terminal being about twice as great as that of the negative. The arc is, however, not due to combustion, since it can be formed in a vacuum.
The current passing in the arc is, in ordinary cases, not greater than ten amperes, while the measurements of the resistance of the arc show that it is altogether too small to account for this current when the original electromotive force is taken into account. This fact has been explained by Edlund and others on the hypothesis that there is a counter electromotive force set up in the arc, which diminishes the effective electromotive force of the circuit. The measurements of Lang show that this counter electromotive force in an arc formed between carbon points is about thirty-six volts, and in one formed between metal points about twenty-three volts.
319. The Spark, Brush, and Glow Discharges.—When a conductor is charged to a high potential and brought near another conductor which is joined to ground, a spark or a series of sparks will pass from one to the other. This phenomenon and others associated with it are most readily studied by the use of an electrical machine or an induction coil, between the electrodes of which a great difference of potential can be easily produced. If the spark be examined with the spectroscope, its spectrum is found to be characterized by lines which are due to the metals composing the electrodes, and to the medium between them.
The passage of the spark through air or any dielectric is attended with a sharp report, and if the dielectric be solid, it is perforated or ruptured. If the electrodes be separated by a considerable distance, the path of the spark is usually a zigzag one. It is probable that this is due to irregularities in the dielectric, due to the presence of dust particles.
With proper adjustment of the electrodes, the discharge may sometimes be made to take the form of a long brush springing from the positive electrode, with a single trunk which branches and becomes invisible before reaching the negative electrode. Accompanying this is usually a number of small and irregular brushes starting from the negative electrode.
Another form of discharge consists of a pale luminous glow covering part of the surface of one or both electrodes. If a small conducting body be interposed between the electrodes when the glow is established, a portion of the glow will be cut off, marking out a region on the electrode which is the projection of the intervening conductor by the lines of electrical force. This phenomenon is called the electrical shadow.
The difference of potential required to set up a spark between two slightly convex metallic surfaces, separated by a stratum of air 0.125 centimetre thick, has been shown by Thomson to be about 5500 volts. The difference of potential which produces the sparks between the electrodes of an electrical machine, which are sometimes fifty or sixty centimetres long, must therefore be very great. The quantity of electricity which passes during the discharge is, however, exceedingly small, on account of the great resistance of the medium through which the discharge takes place.Faraday showed that many of the phenomena of the discharge depend to some extent upon the medium in which it occurs. The differences in color and in the facility with which various forms of the discharge were set up in the gases upon which he experimented were especially noticeable.
It was proved by Franklin that the lightning flash is an electrical discharge between a cloud and the earth or another cloud at a different electrical potential. The differences of potential to which such discharges are due must be enormous, and the heat developed by the discharge shows that the quantity of electricity which passes in it is considerable.
Slowly moving fire-balls are sometimes seen, which last for a considerable time and disappear with a loud report and with all the attendant phenomena of a lightning discharge. It is probable that they are glow discharges which appear just before the difference of potential between the cloud and the earth becomes sufficiently great to give rise to a lightning flash.
320. The Electrical Discharge in Rarefied Gases.—If the air between the electrodes of an electrical machine be heated, it is found that the discharge takes place with greater facility and that the spark which can be obtained is longer than befuie. Similar phenomena appear if the air about the electrodes be rarefied by means of an air-pump. After the rarefaction has reached a certain point the discharge ceases to pass as a spark, and becomes apparently continuous. The arrangement in which this discharge is studied consists of a glass tube into which are sealed two platinum or, preferably, aluminium wires to serve as electrodes, and from which the air is removed to any required degree of exhaustion by an air-pump. Such an arrangement is usually called a vacuum-tube.
As the exhaustion proceeds there appears about the negative electrode in the tube a bright glow, separated from the electrode by a small non-luminous region. The body of the tube is filled with a faint rosy light, which in many cases breaks up into a succession of bright and dark layers transverse to the direction of the discharge. The discharge in this case is called the stratified discharge. A vacuum-tube m which the exhaustion is such that the phenomena are those here described is often called a Geissler tube. As the exhaustion is raised still higher, the rosy light in the tube fades out, the non-luminous space around the negative electrode becomes very much greater, and the phenomena in the tube become exceedingly interesting. They were discovered and have been carefully studied by Crookes, and the vacuum-tubes in which they appear are hence called Crookes' tubes. They may be most conveniently described by assuming that the molecules of gas in the tube break into their constituent ions in the region near the negative electrode, and that the negative ions are repelled from that electrode. The stream of negative ions may be called the cathode discharge. This view receives some support from the fact that the relations of current and resistance in the tube are such as to indicate a counter electromotive force at the negative electrode.
The region occupied by the discharge from the negative electrode may be recognized by a faint blue light, which was not visible in the former condition of the tube. At every point on the wall of the tube to which this discharge extends occurs a brilliant phosphorescent glow, the color of which depends on the nature of the glass. The discharge seems to be independent of the position of the positive electrode, and to take place in nearly straight lines, which start normally from the negative electrode. If two negative electrodes be fixed in the tube, the discharge from one seems to be deflected by the other, and two discharges which meet at right angles seem to deflect one another.
If the discharge from a flat electrode be made to fall upon a body which can be moved, such as a glass film, or the vane of a light wheel, mechanical motions will be set up.
If the negative electrode be made in the form of a spherical cup, and a strip of platinum-foil be placed at its centre, the foil will become heated to redness when the discharge is set up.
There is no evidence that two discharges in the same direction act directly on each other, but a magnet brought near the outside of the tube will deflect a discharge as if it were an electrical current.
The explanation of these phenomena was indicated by Crookes, and Spottiswoode and Moulton. The particular form of it here given was developed by J. J. Thomson. It is assumed that they are due to the presence of the gas left in the tube after the exhaustion has been brought to an end. The mean free path of the molecules in the tube is much greater than that at ordinary densities, and they can accordingly move through long distances in the tube before their motion is checked by collisions. It is assumed that the molecules of gas in the tube are dissociated near the negative electrode, and that their negative ions are repelled from it. The phenomena which have been described are then due to the collision of these ions with other bodies or with the wall of the tube, or to their mutual electrical repulsions and to the action between a moving quantity of electricity and a magnet.
The experiments of Spottiswoode and Moulton, who showed that the same phenomena appeared at lower exhaustions, if the intensity of the discharge were increased, are in favor of this explanation. So is also the fact that the Orookes phenomena appear with a maximum intensity at a certain period during the exhaustion of the tube, while if the exhaustion be carried as far as possible, by the help of chemical means, they cease altogether and no current passes in the tube. The connection of these phenomena with the action of the radiometer (§ 223) is also at once apparent.
321. The Röntgen Radiance.—It was discovered by Hertz that the cathode discharge will pass through a thin strip of alumioium-foil placed in its path within the tube. In 1894 Lenard constructed a tube in which a part of the glass wall was replaced by aluminium-foil, and found that when the cathode discharge was directed upon the aluminium-foil a series of phenomena was obtained outside the tube, which he ascribed to the cathode discharge which passed through the aluminium. He found that similar effects could be produced outside a tube in which there was no aluminium window, and so concluded that the cathode discharge could pass through glass; he also showed that it could pass through other substances with varying degrees of facility. Among the effects ascribed by Lenard to this discharge were the production of fluorescence in many fluorescent substances, the production of photographic action in ordinary photographic dry-plates, and the penetration of the discharge through bodies by an amount dependent upon their densities, it being less as the densities are greater. The discharge was deflected when brought into a magnetic field.
In 1895 Röntgen discovered that effects in some degree similar to those investigated by Lenard could be obtained from any highly exhausted vacuum tube. The results of his researches and of those of many other physicists who have investigated the same action may be described as follows: Wherever the cathode discharge falls upon certain substances, the most important of which, as yet known, are platinum and glass, an action is set up known as the Röntgen radiance. This radiance excites fluorescence in many fluorescent substances and acts upon the photographic plate. It proceeds in straight lines and its intensity varies inversely with the square of the distance; it is not affected by the presence of a magnetic field; in these respects it apparently differs from the action investigated by Lenard. It penetrates all substances and is partly obstructed by all substances, the obstruction being greater as the density of the substance is greater. It is apparently capable of true reflection to a very small degree. No indubitable evidence has as yet been given that it can be refracted, or that it exhibits the phenomena of interference, diffraction, or polarization. When it falls upon an electrified body the charge on the body gradually disappears, the effect being to render the air or other gas surrounding the body a conductor.
No satisfactory theory of the Röntgen radiance can as yet be given. It has been variously ascribed to the mechanical movement of the molecules of the residual gas in the tube in which it originates or of the walls of the tube, to transverse vibrations in the ether of a wave length much shorter than those of the shortest waves of light hitherto known, and to longitudinal vibrations in the ether. The first explanation is supported by certain facts known with regard to the changes that go on in the tube as the discharge is kept up through it, but it is otherwise unsatisfactory. Most of the facts known are consistent with the theory of short transverse vibrations, but no explanation of their origin is given. The theory of longitudinal vibrations has been to some extent developed by Jaumann; he assumes that the characteristic factor of the dielectric, which we have called the dielectric constant, is not really constant, but variable, and a function of the electromotive force. He then shows that on this assumption the electrical discharge in rarefied gases may set up longitudinal waves, and that these waves possess many, if not all, of the properties of the cathode discharge. Since the properties of the cathode discharge and of the Rontgen radiance are not the same, we cannot conclude that the latter are explicable by longitudinal waves, though there is as yet no evidence to the contrary.