Popular Science Monthly/Volume 35/August 1889/Electrical Waves
SINCE the time when Maxwell occupied himself with the theory of electricity, perhaps even since the time of Faraday, it has been generally accepted by most physicists that electricity is a phenomenon resulting from oscillations of the luminiferous ether. However, with the exception of a few experiments on inductive capacities, etc., instigated by Maxwell's "electro-magnetic theory of light," no direct experimental verifications of this hypothesis had been made until the latter part of last year, when Prof. Hertz, of Carlsruhe, Germany, commenced a series of experiments on the interference of electrical waves. In all, six articles have been published — two in Band 31 and four in Band 34 of the "Annalen der Physik und Chemie." The earlier articles are of a qualitative character, while the later are quantitative. The former are of less interest than the latter, because the phenomena are less striking and are not so decisive as a proof. They are substantially as follow: The secondary electrodes of a large Ruhmkorff coil consist of two brass rods whose ends are surmounted with brass balls. The two rods are in the same straight line, and separated from each other by a short airspace of about seven millimetres in length. This is the general form of discharger in a Ruhmkorff. From either of these electrodes is led a wire, which connects with a rectangularly bent wire, which, however, is not completely closed, but is cut in some portion, and each of its ends surmounted by brass balls.
If, now, the Ruhmkorff be excited, the following phenomena result: If the point of contact between the conducting wire and the rectangle be moved along the latter, it will be found that, for most places, a spark passes between the balls of the rectangle, which varies in intensity, and at one place entirely disappears. This place, if we suppose the opening in the rectangle to be in the middle of one end and both balls to be of the same size, is in the middle of the other end. If, now, while no spark is passing in the rectangle, an insulated conductor be brought into connection with either ball, the sparks again appear. These, again, may be caused to disappear by moving the point of contact toward the manipulated terminal. The same effect would also be produced if, instead of changing the point of contact, an equal insulated conductor were touched to the other ball.
The length, resistance, and quality of the conducting wire have no influence upon the sparks; neither does the resistance or material of the rectangle affect it noticeably: e. g., one half of the rectangle being made of thick copper wire and the other of very fine German-silver wire did not alter the phenomena. Another conductor being brought in contact with the joint between the conducting wire and the rectangle has no influence.
The size of the rectangle has a great influence upon the size and length of the spark between its terminals; the larger giving, within certain limits, always the longer spark.
The air distance of the Ruhmkorff discharger is of great importance; under five and more than fifteen millimetres proved to be infelicitous.
Hertz's explanation of these phenomena is the following: At the moment when a discharge takes place between the terminals of a Ruhmkorff coil, in the whole circuit, and in all conductors in contact with it, powerful wave disturbances are agitated, which follow each other in such infinitesimal portions of time that the time which is required to travel with enormous velocity even a short wire is appreciable. These waves, arriving through the conducting wire at the rectangle, divide and traverse simultaneously both branches. If both sides are electrically symmetrical, the two wave-branches arrive at the balls of the rectangle in exactly the same phase, but oppositely directed, and interfere; there can then be, of course, no spark. If, however, they are not symmetrical, as when the contact is not in the middle, they do not interfere totally, but a spark passes. As the contact moves around the rectangle, the spark at its terminals will be less or more powerful as the interference is more or less total.
The electrical symmetry depends not alone upon the length of the wire, but upon its self-induction coefficient and its capacity. The formula which expresses the relations is one from Lorenz ("Annalen der Physik und Chemie," vii, p. 161):
where T = time of oscillation of the electrical wave, P = the self-induction of the conductor concerned, C = its electrostatic capacity, and A = velocity of electrical propagation, which is assumed to be that of light. It will thus be seen that each conductor has its own proper time of electrical oscillation and wave-length.
If, now, the capacity of one side of the rectangle be increased, the time of oscillation of the waves on that side will be also increased. This will increase the wave-length, and equilibrium can be established by adding the same capacity to the other side, or by changing the point of contact.
For the reason that the only variables in the time of oscillation are the self-induction and the capacity, the resistance and material of the rectangle have no influence on the phenomena. Because the capacity of each half of the rectangle is chiefly that of the balls at its terminals, the employing of fine wire for one half can produce no noticeable effect.
That the size of the rectangle should have such an influence is to be expected up to certain limits — that is, until the total length of the sides is one wave-length or a multiple of the same. Then the waves could be made to arrive at the terminals in opposite phases, and would give the largest sparks.
Were this the only proof which Hertz could give of interference, a great deal of doubt might be cast upon its conclusiveness. Would not one naturally expect that, if both sides of the rectangle were of the same length and had the same capacity, the potential on both balls would be the same, and no discharge could take place; or, when of different capacities, the charging and discharging following each other so rapidly that the same quantity of electricity would tend to pass through a section of each side of the rectangle, and would thus necessitate a discharge?
But Hertz's quantitative experiments are more satisfactory. In order to understand them, a few preliminary phenomena must be described. These relate to what he calls the principle of "resonance." As any sound resonator, having its own proper wave-length, can be set in vibration by a vibrating body of the same or multiple time of vibration, so we might suppose that any electrical conductor could be set in vibration by a neighboring electrical wave disturbance of proper time of oscillation. This supposition is verified by experiment.
The apparatus and arrangement are very similar to those in the previous experiment. However, instead of the two outer brass balls on the Ruhmkorff discharger, two hollow zinc spheres of thirty centimetres diameter were substituted, and these were movable along the rods. As these constitute the electrical ends of the discharger, the same may be altered in length by the total diameter of each by simply letting the rods project into the cavity of the spheres. The time of oscillation of the waves in the Ruhmkorff can thus be altered. The brass balls of the rectangle were provided with a micrometer adjustment, so that the length of spark which passed might be measured. The connecting wire was in these experiments dispensed with, and the rectangle was mounted on insulators in front of the Ruhmkorff discharger.
With this arrangement Hertz carried out a complete set of observations, in each of which the effect of a regular series of changes in one of the variables was investigated — e. g., the time of oscillation of the primary discharger would be regularly increased by changing the capacity or self-induction, and for each change the length of spark in the rectangle would be measured. One series in detail will suffice for our purpose.
Suppose, at the beginning of the experiment, that the time of oscillation of the rectangle is smaller than that of the Ruhmkorff discharger, and the spark is one millimetre long. If now we hang two hooks of wire on each ball of the rectangle, the capacity is increased, and we get a spark of three millimetres. Add two more equal hooks, and the spark is five millimetres. Add two more, and it falls off to three millimetres again. If this process be continued, the spark will alternately reach a maximum and minimum, and the natural inference is that the time of oscillation of the rectangle is nearest that of the Ruhmkorff discharger when the spark in the former is at a maximum.
Perhaps it is most striking to place the micrometer at the maximum spark distance, and then, by constantly changing the capacity of either conductor, cause the spark to disappear and reappear. Should small spheres be used, instead of wires, for changing the capacity, we would then have a direct means of determining the wave-length.
These sets of experiments led Hertz to conclude that the principle of resonance is as true for electrical waves as for sound waves, and he employs it for his quantitative work.
The arrangement of apparatus is as follows: To the outer ends of the Ruhmkorff discharger are attached two plates, whose planes are vertical and embrace the line of direction of the discharger. Back of one of these is mounted on an insulated stand a similar plate of the same size. A wire leads from the inner central edge of this to a point on a level but just back of the air-space of the discharger. It then turns in a curve to a point about thirty centimetres directly over the discharger, and then continues in a straight horizontal line some sixty metres. The end is left free, and, if now the Ruhmkorff be excited, a series of stationary electrical waves will be formed in the wire. To detect these we employ the principle of resonance. A wire whose time of oscillation has been determined and found to be nearly equal to that of the primary conductor is bent into a circle, and the ends are brought close together. This is then brought close to the long wire, and held so that its plane embraces the latter. A fine display of sparks will be seen to accompany the Ruhmkorff discharge.
If this proof circuit be approached to the extreme end of the long wire, no sparks will be seen. The wire has at its end, in fact, a node the same as a stopped organ-pipe has. As the air in the pipe is undisturbed, so the potential of the wire end is unchangeable. As we recede from the end, the sparks grow longer, but finally disappear again. Here is another node. We measure the distance between the two and cut the wire so that its total length shall be a multiple of this length, and then we proceed to find all the nodes, and mark them by paper riders. If we measure each of these distances and take the mean, or measure the whole length of the wire and divide by the number of nodes, we have a value for the wave-length of the conductor. In Hertz's experiment this value was 2·8 metres. From this value, and the time of oscillation reckoned from the self-induction and capacity, he gets the velocity of propagation of electrical disturbances as two hundred thousand kilometres per second. This result Hertz prints in bold-faced type, and puts it as a climax of all his work. This is truly wonderful. If we consider that the calculated value of the time of oscillation depends upon the assumption that the velocity of electrical wave propagation is the same as that of light (three hundred thousand kilometres per second), and this circuitous calculation of the same thing gives two hundred thousand kilometres per second, we can hardly give Hertz the credit of extremely accurate work. However, Hertz has made a great advance in physical science. Since Weber introduced the absolute system of units, no great advance has been made. Physicists have busied themselves in measuring the various constants, in refining and perfecting the methods of measurement, or in applying principles already known to technical and practical purposes. Hertz, however, has opened a new and unexplored field, which must eventually bring us into a closer acquaintance with the mysteries which we are daily manipulating.
This series of experiments has excited a great deal of attention in English physical circles. Prof. Fitzgerald, of this department of the British Association, laid great emphasis, at the last meeting, on the advance which had been made. Oliver Heaviside has justified his patronymic by publishing a complex mass of mathematical formulae on the subject. He considers that the waves of Hertz are of a much more complex nature than the experiments would leave one to infer.
When we remember the effect which electricity has upon the plane of polarized light, it would seem that Hertz's wave-lengths are of an entirely different order from what they should he. How can electrical wave-lengths of one metre be in any way associated with light-waves of less than one billionth of a millimetre? Whatever we have known of the wave lengths of the ether, in radiant heat and light, has always been of that infinitesimal order. Still, should the velocity of propagation of electrical waves be much greater than has been supposed, then with these large wave-lengths the times of oscillation could be of the same order as those of light.
Hertz, however, has a system of stationary waves, and it would seem that no direct calculations could give a correct value for the time of oscillation. This can be shown by moving a long trough of water. By holding one end in the hand, suitable impulses can be given so as to produce any desired wave-lengths. Should Hertz be wrong in his conclusions, still the impulse which he has given in this direction is sure to fructify. It is possible that induction may be found to be a phenomenon of pure wave-motion, and that it can be likened directly to radiation. Could we then carry the comparison still further, and say that a conductor is an opaque medium; that a dielectric is transparent — then we would likely soon be constructing electrical lenses, would be detecting electrical refraction, diffraction, and possibly be constructing an electrical spectrum. Doubtless, if not this, some similar thing will develop, and no young physicist need then say that all the things in physics have already been discovered and measured.
- Read before the Mathematical Physical Club of Boston and Cambridge, December 17, 1888.