MULTIPLEX TELEPHONY AND TELEGRAPHY BY MEANS OF ELECTRIC WAVES GUIDED BY WIRES.1 [With 1 plate.] DR. GEORGE O. SQUIER, Major Signal Corps, United States Army. I. INTRODUCTION. Electrical transmission of intelligence, so vital to the progress of civilization, has taken a development at present into telephony and telegraphy over metallic wires; and telegraphy, and, to a limited extent, telephony, through the medium of the ether by means of electric waves. During the past 12 years the achievements of wireless telegraphy have been truly marvelous. From an engineering viewpoint, the wonder of it all is, that, with the transmitting energy being radiated out over the surface of the earth in all directions, enough of this energy is delivered at a single point on the circumference of a circle, of which the transmitting antenna is approximately the center, to operate successfully suitable receiving devices by which the electromagnetic waves are translated into intelligence. The "plant efficiency" for electrical energy in the best types of wireless stations yet produced is so low that there can be no comparison between it and the least efficient transmission of energy by conducting wires. The limits of audibility, being physiological functions, are well known to vary considerably, but they may be taken to be in the neighborhood of 16 complete cycles per second as the lower limit and 15,000 to 20,000 cycles per second as the upper limit. If, therefore, there are impressed upon a wire circuit for transmitting intelligence harmonic electromotive forces of frequencies between 0 and 16 cycles per second, or, again, above 15,000 to 20,000 cycles per second, it would seem certain that whatever effects such electric-wave frequencies produced upon metallic lines, the present apparatus employed in operating them could not translate these effects into audible signals. 1 A paper presented at the twenty-eighth annual convention of the American Institute of Electrical Engineers, Chicago, Ill., June 26-30, 1911. Copyright, 1911, by A. I. E. E. Reprinted by permission from Proceedings of the Institute for May, 1911, pp. 857-905. There are, therefore, two possible solutions to the problem of multiplex telephony and telegraphy upon this principle by electric waves, based upon the unalterable characteristics of the human ear, viz, by employing (1) electric waves of infra sound frequencies, and (2) those of ultra sound frequencies. One great difficulty in designing generators of infra sound frequencies is in securing a pure sine wave, as otherwise any harmonic of the fundamental would appear within the range of audition. Furthermore, the range of frequencies is restricted, and the physical dimensions of the tuning elements for such low frequencies would have a tendency to become unwieldy. The electromagnetic spectrum at present extends from about four to eight periods per second, such as are employed upon ocean cables, to the shortest waves of ultra-violet light. In this whole range of frequencies there are two distinct intervals which have not as yet been used, viz, frequencies from about 3x1012 of the extreme infrared to 5x101o, which is the freqeuncy of the shortest electric waves yet produced by electrical apparatus, and from about 80,000 to 100,000 cycles per second to about 15,000 to 20,000 cycles per second. The upper limit of this latter interval represents about the lowest frequencies yet employed for long-distance wireless telegraphy. Within the past few years generators have been developed in the United States giving an output of 2 kilowatts and above at a frequency of 100,000 cycles per second, and also capable of being operated satisfactorily at as low a frequency as 20,000 cycles per second. Furthermore, these machines give a practically pure sine wave. The necessary conditions for telephony by electric waves guided by wires are an uninterrupted source of sustained oscillations and some form of receiving device which is quantitative in its action. In the experiments described in multiplex telephony and telegraphy it has been necessary and sufficient to combine the present engineering practice of wire telephony and telegraphy with the engineering practice of wireless telephony and telegraphy. The frequencies involved in telephony over wires do not exceed 1,800 to 2,000, and for such frequencies the telephonic currents are fairly well distributed throughout the cross section of the conductor. As the frequency is increased the so-called "skin effect" becomes noticeable, and the energy is more and more transmitted in the ether surrounding the conductor. It has been found possible to superimpose, upon the ordinary telephonic wire circuits now commercially used, electric waves of ultra sound frequencies without producing any harmful effects upon the operation of the existing telephonic service. Fortunately, therefore, the experiments described below are constructive and additive, rather than destructive and supplantive. Electric waves of ultra sound frequencies are guided by means of wires of an existing commercial installation and are made the vehicle for the transmission of additional telephonic and telegraphic messages. APPARATUS AND EQUIPMENT. Under a special appropriation granted to the Signal Corps by Congress in the army appropriation act of 1909, a small research laboratory has been established at the Bureau of Standards, in the suburbs of the city of Washington. This laboratory is equipped with the latest forms of apparatus now employed in the wireless telephone and telegraph art, and also with the standard types of telephone and telegraph apparatus now used upon wire circuits. The small construction laboratory of the United States Signal Corps is located at 1710 Pennsylvanue Avenue and is also equipped with the usual types and forms of apparatus used in transmitting intelligence by electrical means. Each of these laboratories is supplied with a wireless telephone and telegraph installation with suitable antennæ. In addition, these two laboratories are connected by a standard telephone cable line about 7 miles in length, which was employed in the experiments described below. THE 100,000-CYCLE GENERATOR.1 The high-frequency alternator, which is shown complete with driving motor and switchboard in the accompanying illustrations, is a special form of the inductor type designed for a frequency of 100,000 cycles with an output of 2 kilowatts, making it adapted for use in wireless telephony or telegraphy (pl. 1). Driving motor. The motor is a shunt-wound 10-horsepower machine with a normal speed of 1,250 revolutions per minute. It is connected by a chain drive to an intermediate shaft which runs at a speed of 2,000 revolutions per minute. The intermediate shaft drives the flexible shaft of the alternator through a De Laval turbine gearing, having a ratio of 10 to 1. The flexible shaft and inductor thus revolve at a speed of 20,000 revolutions per minute. Field coils.-The field coils, mounted on the stationary iron frame of the alternator, surround the periphery of the inductor. The magnetic flux produced by these coils passes through the laminated armature and armature coils, the air gap, and the inductor. This flux is periodically decreased by the nonmagnetic sections of phosphor-bronze embedded radially in the inductor at its periphery. Armature coils.-The armature or stators are ring-shaped and are made of laminated iron. Six hundred slots are cut on the radial face of each; a quadruple silk-covered copper wire, 0.016 inch (0.4 1 Alexanderson, Trans. Amer. Inst. Electr. Eng., vol. 28, p. 399, 1909. millimeter) in diameter, is wound in a continuous wave up and down the successive slots. The peripheries of the armature frames are threaded to screw into the iron frame of the alternator. By means of a graduated scale on the alternator frame, the armatures can be readily adjusted for any desired air gap. Inductor. The inductor or rotor has 300 teeth on each side of its periphery, spaced 0.125 inch (3.17 millimeters) between centers. The spaces between the teeth are filled with U-shaped phosphorbronze wires, securely anchored, so as to withstand the centrifugal force of 80 pounds (36.3 kilograms) exerted by each. Since each tooth of the inductor gives a complete cycle, 100,000 cycles per second are developed at 20,000 revolutions per minute. The diameter of the disk being 1 foot (0.30 meter), the peripheral speed is 1,047 feet (319 meters) per second, or 700 miles (1,127 kilometers) per hour, at which rate it would roll from the United States to Europe in four hours. By careful design and selection of material, a factor of safety of 6.7 is obtained in the disk, although the centrifugal force at its periphery is 68,000 times the weight of the metal there. Bearings.-The generator has two sets of bearings, as shown in the illustrations, the outer set being the main bearings which support the weight of the revolving parts. These bearings are self-aligning and are fitted with special sleeves, which are ground to coincide with longitudinal corrugations of the shaft, thus taking up the end thrust. A pump maintains a continuous stream of oil through these bearings, thus allowing the machine to be run continuously at full speed without troublesome heating. The middle bearings normally do not touch the shaft, but take up excessive end thrust and prevent excessive radial vibration of the flexible shaft. An auxiliary bearing or guide is placed midway between the gear box and the end bearing. Its function is to limit the vibration of that portion of the shaft. Critical periods.-In starting the machine, severe vibration occurs at two distinct critical speeds, one at about 1,700 and the other at about 9,000 revolutions per minute. The middle bearings prevent this vibration from becoming dangerous. Voltage. With the normal air gap between the armatures and revolving disk of 0.015 inch (0.38 millimeter), the potential developed is 150 volts with the armatures connected in series. It is possible, however, to decrease the air gap to 0.004 inch (0.10 millimeter) for short runs, which gives a corresponding increase in voltage up to nearly 300 volts. It is considered inadvisable, however, to run with this small air gap for any considerable length of time. The machine is intended to be used with a condenser, the capacity reactance of which balances the armature induction reactance, which |