RESEARCHES IN RADIOTELEGRAPHY."

(With 2 plates.)

By Prof. J. A. FLEMING, M. A., D. Sc., F. R. S.

Radiotelegraphy, popularly called wireless telegraphy, has outlived the tentative achievements of its precocious infancy and obtained for itself a settled but important position amongst our means of communication.

This stage, however, has only been reached after a long struggle with experimental difficulties and much labor in analyzing the processes involved. As many of these matters are of general scientific interest, it is proposed, during the present hour, briefly to summarize the results of some recent research.

You are doubtless all aware that every radiotelegraphic station comprises three elements. There is, first, the external organ called the air wire or antenna, by which the electromagnetic waves are radiated and absorbed. This antenna consists of one or more wires extending up into the air, either vertically or sloping, or partly vertical and partly horizontal. These wires are insulated at the upper ends and may be arranged fan fashion, or may form one or more nearly closed loops, placed in a vertical position. The antenna is, so to speak, the mouth or ear of the station, by which it speaks through the ether, or by which it hears the etherial whispers coming to it from other stations. The ether waves are produced by very rapid electric currents moving to and fro in the antenna wires, and these, like the vibrations of a violin string, or the aerial oscillations in an organ pipe, set up a periodic disturbance in the surrounding medium, which in the electrical case consists of alternating electric and magnetic forces taking place at each point in space around the antenna.

There are, then, appliances in the station collectively called the transmitter, which have for their function to create these powerful electric oscillations in the antenna, and to control them so as to send out short or long trains of ether waves in accordance with the dot or dash signals of the Morse alphabet. Lastly, there is the receiving

a Lecture before the Royal Institution of Great Britain, Friday, June 4, 1909. Reprinted by permission from pamphlet copy published by the Royal Institution.

apparatus, which, when connected to the antenna, serves to detect the presence in it of the very feeble oscillations which are being generated in the antenna by the powerful oscillations in the antenna of some far-distant sending station. It is usual to employ the same antenna at any one station both for sending and receiving, and to switch it over from the transmitter to the receiver according as we wish to send or receive messages, although methods have been described and are being developed for using the antenna simultaneously for both purposes.

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By way of preface let me illustrate by a few experiments the manner in which these electric oscillations are set up in the air wire, and the nature of the effects produced by them in the surrounding space. We have here a very long wire which, for the purpose of keeping it within a small compass, is coiled upon an ebonite tube. Two such spirals, H, and H2, are placed side by side and connected at the bottom through two other small coils of wire S (see fig. 1). In contiguity to these last two coils of wire are two others, P, which are in series with a condenser or battery of Leyden jars, C, and a spark gap. If we charge the condenser by an induction coil, I, and let it discharge across the gap, we produce rapidly succeeding trains of electric oscillations in the condenser circuit, and these induce other currents in the open or helix circuit of similar kind. The result is that electricity rushes up and down the spiral wires, which we may consider to represent two very long air wires or antennæ. We have therefore, alternately, free charges of electricity at the top ends of the wires and electric currents passing to and fro across the middle point. We may compare this movement of electricity in the helix to the oscillations of a liquid in a U-tube when it is disturbed. In the electrical case we have at each spark discharge 20 or 30 electrical swings or oscillations separated by relatively long intervals of silence, the intervals between two swings in the train being about one four-hundredth-thousandth of a second, while the interval between the groups or trains of swings is about one-fiftieth of a second.

FIG. 1.

Such electrical oscillations in the wire produce two effects in external space, called, respectively, electric and magnetic force. In the case of a simple vertical air wire the magnetic force is distributed along concentric circular lines embracing the wire while the electric force is distributed along certain looped lines in the plane of the wire. If, however, we employ a close-wound spiral antenna, as in our experiment, the positions of the electric and magnetic forces are interchanged as compared with those of the single vertical wire.

As the currents in the air wire reverse their direction the magnetic and electric effects in the external space also reverse, but not everywhere at the same moment. The magnetic and electric forces are affections or states of the ether, and in virtue of the inertia and elasticity of the medium they are propagated from point to point with a finite velocity which is the same as that of light. We can then explore the field near the antenna and obtain an approximate idea of its nature and intensity by the use of a Neon vacuum tube, which glows when held in the electric field with greater or less brilliancy. At certain intervals of distance in the space the magnetic and electric forces reverse direction in the same way at the same instant, and this distance is called a wave length.

In the case of a straight air wire, the magnitude of the forces at considerable distances varies inversely as the distance from the antenna, and the antenna radiates equally in all directions. If, however, we employ a U-shaped antenna, as in the present experiment, the currents being in opposite directions in the two branches, then along a median line transverse to their common plane their actions will neutralize each other, and the radiation will be symmetrical only with respect to the plane of the antenna. In constructing an antenna intended to radiate in all directions, it is necessary to connect the lower end to a large plate of metal or network of wires either sunk in the earth or placed just above the surface. In the former case, this plate is called an earth plate, and in the latter a balancing capacity. It is necessary that this balancing capacity, if insulated, should be of sufficient size to take up all the electricity which rushes out of the antenna at each oscillation without sensible rise in potential. If we are only employing an antenna of moderate capacity for short distance signaling, then an insulated balancing capacity would not be of unwieldly dimensions and may be constructed of a number of wires stretched out or laid on the ground or insulated a little way above it. When, however, we have to employ a very large antenna of great capacity for long distance work, then the provision of a suitable balancing capacity would involve constructive difficulties which are best obviated by making the earth itself the balancing capacity-in other words, by connecting the base of the antenna to an extensive network of wires or large metal plates buried in the ground. It has been asserted that the direct earth connection damps out the free oscillations in the antenna more quickly than would be the case if an insulated balancing capacity is employed. Although this may be true to a certain extent, we have to set against it the fact that the use of an insulated balancing capacity is out of the question in many cases-as on board ship, where a connection to the hull of the vessel is always made. Also for any but small antennæ the necessary insulated balancing capacity may be somewhat large, and it is

in every way better to put it below ground, in other words, to employ an earth plate and compensate for any slight earth damping by an antenna of rather larger capacity.

This matter is, however, only part of a much larger question, viz, the function of the earth in radiotelegraphy. It is well known that the nature of the earth's soil or surface between the sending and receiving stations has a great effect upon electric waves passing over it. Various imperfect explanations were given of this action in early days, but the basis for a better knowledge has been laid by the experimental researches of Admiral Sir Henry Jackson and the theoretical discussions of M. Brylinski and Doctor Zenneck. To follow their explanations it must be borne in mind that high-frequency electric currents, as used in radiotelegraphy, are confined chiefly to the surface of conductors by means of which they are conducted. Such a current does not distribute itself uniformly over the whole cross section of a wire carrying it, but is confined to a thin skin or surface layer. This can be proved by the following experiment: We take a copper wire spiral or loop and make it part of a circuit in which a high-frequency current exists. If we measure in any way the current in that circuit we find it has a certain value. If we substitute for the copper wire an iron wire of the same size, we find that the current in the circuit is then much less. This can be discovered by placing near the circuit in question another testing circuit comprising an inductance and a capacity and some means for testing the amplitude of the oscillations set up in this secondary circuit. This decrease is not due to the mere fact that the iron has a greater resistance than copper, but to the fact that the iron is magnetizable, and such magnetization absorbs energy owing to so-called hysteresis. If, however, we dip the iron for a moment into molten zinc and deposit on it a thin surface layer of zinc, or galvanize it, we find it then becomes almost as good as a solid copper wire for conveying highfrequency currents. On the other hand, if we burn off the zinc from a piece of galvanized-iron wire, we render it a worse conductor for high-frequency oscillations. This experiment proves that such oscillations are conveyed by a thin surface layer of the conductor. In the case of a copper wire for oscillations having a frequency of one million, the current penetrates about one-third of a millimeter, and in the case of an iron wire, about one-fortieth of a millimeter into the metal.

For nonmagnetic substances the depth to which a current of a given frequency penetrates into a conductor is greater in proportion as the conductivity of the material is less. Hence high frequency currents penetrate farther into carbon than into metal. Accordingly a much thicker layer of carbon than of zinc would be needed to shield the iron spiral in our last experiment. The same thing happens in

the case of an electric wave propagated over a terrestrial surface. If the surface is a very good conductor the wave hardly penetrates into it, but glides over the surface. If it is a poor conductor the wave penetrates into it to a greater extent, and the worse the conductivity the deeper the penetration.

The materials of which the earth's crust is composed, with some exceptions, owe their electric conductivity chiefly to the presence of water in them. They are called electrolytic conductors. Substances like marble and slate when free from iron oxide are fairly good insulators. Dry sand or hard dry rocks are poor conductors, but wet sand and moist earth are fairly good conductors. Sea water, owing to the salt in it, is a much better conductor than fresh water. The following table gives some figures, which, however, are only approximate, for the specific resistance of various terrestrial materials in ohms per meter cube. It will be seen that dry sand or soils are of very high specific resistance, and damp or wet sand or clay fairly low.

TABLE I.—Approximate conductivity and dielectric constant of various terrestrial materials.

If our earth's surface had a conductivity equal say to that of copper, then the electric radiation from an antenna would glide over the surface without penetration. In the case of the actual earth there is, however, considerable penetration of the wave into the surface, and therefore absorption of energy by it.

Brylinski and also Zenneck have calculated the depth to which electric waves of such frequency as are used in radiotelegraphy penetrate into the sea or terrestrial strata of various conductivities. For mathematical reasons it is customary to define it by stating the depth in meters or centimeters at which the wave amplitude is reduced to 1/=0.367 of its amplitude at the surface. I have represented in a diagram some of Zenneck's results calculated for waves of 1,000 feet in length, and for terrestrial surface materials of various kinds, conductivities, and dielectric constants (see fig. 2). You will see that in the case of sea water an electric wave traveling over it penetrates only to the depth of a meter or two, whereas in