by Nikola Tesla
Telegraph and Telegraph Age
October 16, 1927
The transmission of power without wires is not a theory or a mere possibility, as it appears to most people, but a fact demonstrated by me in experiments which have extended for years. Nor did the idea present itself to me all of a sudden, but was the result of a very slow and gradual development and a logical consequence of my investigations which were earnestly undertaken in 1893 when I gave the world the first outline of my system of broadcasting wireless energy for all purposes. In several demonstrative lectures before scientific societies during the preceding three years, I showed that it was not necessary to use two wires in transmitting electrical energy, but that one only might be employed equally well. My experiments with currents of high frequencies were the first ever performed in public and elicited the keenest interest on account of the possibilities they opened up and striking character of the phenomena. Few of the experts familiar with the up-to-date appliances will appreciate the difficulty of my task with the elementary devices I had then at command, as accurate adjustments for resonance had to be made in every experiment.
The transmission of energy through a single conductor without return having been found practicable it occurred to me that possibly even that one wire might be dispensed with and the earth used to convey the energy from the transmitter to the receiver.
High Frequency Dynamo and Tesla Coil
Manifestly, currents such as were ordinarily employed in the arts and industries were unsuitable and I had to devise special generators and transformers for furnishing impulses of the requisite quality. First, I perfected high frequency dynamos which were of two types, one with a direct current field excitation and the other in which the magnet was energized by alternating currents of different phase, producing a rotating magnetic field. Both of these have found employment in connection with my broadcasting wireless system. In the first machine I exhibited an efficiency of ninety per cent. was attained, but it was necessary to run it in hydrogen or rarefied air to minimize the otherwise prohibitive windage loss and deafening noise.
In order to overcome the inherent limitations of such machines I next concentrated my efforts on the perfection of a peculiar transformer consisting of several tuned circuits in inductive relation which received the primary energy from oscillatory discharges of condensers. This apparatus, originally identified with my name and considered by the leading scientific men my best achievement, is now used in every wireless transmitter and receiver throughout the world. It has enabled me to obtain currents of any desired frequency, electromotive force and volume, and to produce a great variety of electrical, chemical, thermal, light and other effects, Roentgen, cathodic and other rays of transcending intensities. I have employed it in my investigations of the constitution of matter and radioactivity, published from 1896 to 1898 in the Electrical Review in which it was demonstrated, prior to the discovery of Radium by Mme. Sklodowska and Pierre Curie, that radio-activity is a common property of matter and that such bodies emit small particles of various sizes and great velocities, a view which was received with incredulity but finally recognized as true. It has been put to innumerable uses and proved in the hands of others a veritable lamp of Aladdin. As I think of my earliest coils, which were nothing more than scientific toys, the subsequent development appears to me like a dream.
The "Magnifying Transmitter" and Earth Resonance
While I was perfectly convinced, from the outset, that success would be ultimately achieved, it was not until by slow improvement I evolved the so-called "Magnifying Transmitter" that I obtained convincing evidence of the feasibility of wireless power transmission on a vast scale for all industrial purposes.
The chief discovery, which satisfied me thoroughly as to the practicability of my plan, was made in 1899 at Colorado Springs, where I carried on tests with a generator of fifteen hundred kilowatt capacity and ascertained that under certain conditions the current was capable of passing across the entire globe and returning from the antipodes to its origin with undiminished strength. It was a result so unbelievable that the revelation at first almost stunned me. I saw in a flash that by properly organized apparatus at sending and receiving stations, power virtually in unlimited amounts could be conveyed through the earth at any distance, limited only by the physical dimensions of the globe, with an efficiency as high as ninety-nine and one-half per cent.
The mode of propagation of the currents from my transmitter through the terrestrial globe is most extraordinary considering the spread of the electrification of the surface. The wave starts with a theoretically infinite speed, slowing down first very quickly and afterward at a lesser rate until the distance is about six thousand miles, when it proceeds with the speed of light. From there on it again increases in speed, slowly at first, and then more rapidly, reaching the antipode with approximately infinite velocity. The law of motion can be expressed by stating that the waves on the terrestrial surface sweep in equal intervals of time over equal area, but it must be understood that the current penetrates deep into the earth and the effects produced on the receivers are the same as if the whole flow was confined to the earth's axis joining the transmitter with the antipode. The mean surface speed is thus about 471,200 kilometers per second--fifty-seven per cent. greater than that of the so-called Hertz waves--which should propagate with the velocity of light if they exist. The same constant was found by the noted American astronomer, Capt. J.T.T. See, in his mathematical investigations, for the smallest particles of the ether which he fittingly designates as "etherons." But while in the light of his theory this speed is a physical reality, the spread of the currents at the terrestrial surface is much like the passage of the moon's shadow over the globe.
It will be difficult for most people engaged in practical pursuits to measure or even to form an adequate conception of the intensity of inspiration and force I derive from that part of my work which has passed into history. I have every reason to consider myself one of the most fortunate men, for I experience incessantly a feeling of inexpressible satisfaction that my alternating system is universally employed in the transmission and distribution of heat, light and power and that also my wireless system, in all its essential features, is used throughout the world for conveying intelligence. But my pioneer efforts in this later field are still grossly misunderstood.
Short Wave Broadcasting and "Beam" Transmission
Nothing illustrates this better than the recent demonstrations of a number of experts with very short waves, which have created the impression that power will be eventually transmitted by such means. In reality, experiments of this kind are the very denial of the possibility of economic transmission of energy. I have investigated this special subject experimentally during a great number of years, using sometimes waves as short as one millimeter, and have found even these unsuitable for such a purpose, not to say that their production is inseparable from great waste.
In order to secure good results by this method it would be necessary to employ radiations of a wave length incomparably smaller than the dimensions of the reflector, as radiant heat, light, infra-red or ultra-violet rays. Notwithstanding my repeated explanations experts do not seem to realize that no concentration of energy such as I attain in my wireless power system can or will ever be achieved through the instrumentality of reflectors, for in transmitting energy in this manner the receiver can collect only an amount proportionate to the area exposed to the rays, while in my system it draws the energy from an immense reservoir in ever so much greater quantity. Similar considerations apply to directional transmission by short reflected waves or "beams." If we could produce economically electric vibrations of a frequency approximating that of radiant heat waves, efficient reflectors without appreciable dispersion, and prevent absorption--then such a mode of transmitting energy might become of great importance. But attempts to accomplish this purpose with relatively very low frequencies are sure to prove futile. More than twenty-five years ago my efforts to transmit large amounts of power through the atmosphere resulted in the development of an invention of great promise, which has since been called the "Death Ray," and attributed to Dr. Grindell Mathews, an ingenious and skillful English electrician. The underlying idea was to render the air conducting by suitable ionizing radiations and to convey high tension currents along the path of the rays. Experiments, conducted on a large scale, showed that with pressures of many millions of volts virtually unlimited quantities of energy can be projected to a small distance, as a few hundred feet, which might be satisfactory if the process were more economical and the apparatus less expensive. Since that time I have made important improvements and discovered a