Tesla's Experiments with Alternating Currents of High Frequency - Review by E. Raverot

By E. Raverot

The phenomena produced by alternating currents of great frequency and very high potentials were exhibited in public for the first time in France, on Friday, Feb. 19, by the discoverer. The event was one of great importance, as the first experimental presentation before the physicists and electricians of France of an entirely new field of research. Physics, after having created the scientific industries, now receives from them in return very valuable means of investigation which assure the investigator a rich harvest of discoveries. When we consider the researches of Planté with the secondary battery, and witness the discoveries of Tesla with his powerful induction coils, the proof of this assertion becomes evident, and the field of future developments in store for experimental physics grows brighter.

In the light of the experiments of Mr. Tesla we can understand more clearly many of the mysterious phenomena of the radiant state of matter pointed out by Prof. Crookes, of phosphorescence and the molecular bombardment; they lead us to clearer ideas of several contested points in the theory of electricity.

The limited time did not permit Mr. Tesla to dwell as much as he would have wished upon the theory and the arrangements of the experiments, and we must content ourselves here with giving simply the personal impressions of a charmed listener who will endeavor to state what he saw or what he believed he saw.

In order to produce the currents of the very great frequency and very high potential necessary for the experiments, recourse was had to two methods. In the one he employs a multipolar alternator of his own design, giving directly a frequency of 15,000 to 25,000 alternations per second. In this machine (which is the same as that used in his lecture in New York**) the field magnets are fixed and consist of a forged iron ring 81 centimetres in outside diameter and about 2½ centimetres in breadth, having 384 polar projections, between which are wound in a zigzag form two wires, to constitute the exciting circuit for the magnets. The inside diameter of this stationary field is about 76 centimetres. The movable armature is made of a disc, on the surface of which are located the windings, attached laterally by strong pins. The speed of the machine varies between 2,000 and 3,000 revolutions per minute; it can generate as much as 10 ampères. The potential of the machine is regulated by means of a condenser. The current from it supplies the primary circuit of an induction coil of special construction, of which we will speak later on.

The other method used by Mr. Tesla to obtain the enormous frequencies of 300,000 to 400,000 alternations per second and the exceedingly high potentials of over half a million volts, is based on the employment of the disruptive discharge of a condenser in combination with induction coils of peculiar construction. Fig. 1 shows this arrangement of apparatus, which was already described in his former papers. Fig. 2 shows a diagram of the disposition of the apparatus and connections which, however, Mr. Tesla did not stop to describe; we give it here as we remember it, and if there are any errors they are on our part. From the connections shown it seems to us to represent the general disposition of the apparatus used in the experiments which we witnessed.

On this diagram, A represents a Siemens alternator which generated the initial energy of the primary circuit I of the first induction coil of the transformers. In the secondary circuit i of this transformer are connected symmetrically condensers C and C¹, the disruptive discharge of which passes with the desired frequency through the primary circuit J J¹ of the second transformer, the secondary circuit of which terminates at the poles E and E¹, where the currents of high frequency and enormous potentials may then be led off. G and F are discharge knobs intended, apparently, to regulate the frequency of the disruptive discharges. We call attention to the function of these adding, however, that our impressions as to their function were obtained by a single inspection of the diagram and connections, but that we do not guarantee that our views are correct. The same is said with regard to Fig. 3, which shows the transformer used by Mr. Tesla, which was drawn from memory. Regarding this transformer Mr. Tesla stated that the length of its secondary wire is only from 60 to 100 metres. It gives as high as half a million volts when its primary circuit is excited by the disruptive discharge of the condensers, with a frequency of 800,000 to 400,000 alterations per second. The transformer ratio is one to six and the number of windings of the primary coil is 300. The entire coils are placed in a tank filled with oil. This is indispensable under the condition of such high voltages and such great frequencies.†

The principle of this method is based oh the phenomenon of resonance which requires very delicate and laborious regulation and adjustment. Mr. Tesla dwelt repeatedly on this difficulty, and illustrated the same with experiments, which could have been shown also by calculation; the chief obstacle in this is the uncertain knowledge of the effective capacity of bodies separated by air, that is, the effect of convection or of absorption.

The second method shows the important part which condensers take in all the experiments of Mr. Tesla, the electrostatic function of which shows clearly the nature of the observed phenomena.

Mr. Tesla also pointed out the analogy of the effects produced by his induction coils for high frequency with those produced by the Wimshurst machine. He can produce phenomena and sparks with his apparatus absolutely identical with those which are ordinarily observed with such influence machines. It seems, in fact, that many of the experiments shown ought to succeed equally well under similar conditions with the discharges from his coils and with those from an electrostatic machine. If, for instance, one pole of a small radiometer of Crookes be connected, the radiometer is put in motion, at least when the conditions of resonance of the circuit are favorable. At the lecture the radiometer, when attached by only one pole, was at first immovable, but started to move when a plate of a certain capacity was connected to the second electrode of the radiometer bulb.

The operation of the radiometer of Crookes, attributed to the unequal molecular bombardment produced on the two different surfaces of the fans, is analogous to the unipolar effects produced at each one of the two poles of the high tension transformer of Mr. Tesla. One can understand now the statement of Mr. Tesla in his former publications, that he utilizes in his experiments the "molecular bombardment" and "the condensing action." We recall also a statement of Mr. Tesla in his former publications that "every body, whether a conductor or susceptible of conducting a current of high tension, can, if it be inclosed in a properly exhausted tube, become luminous or incandescent when it is connected directly to the pole of a secondary circuit of the source of energy excited by induction."

Such are in effect the conditions of operation of Mr. Tesla's unipolar incandescent lamp. At the end of the conducting wire is placed a knob of conducting matter which is placed in a bulb from which the air is exhausted, like in the ordinary incandescent lamp; this knob becomes illuminated with a brilliant light when the wire is connected to one pole of the high frequency transformer. The lamp is shown in Fig. 4. The knob is subjected to the molecular bombardment of a rapidly varying electrostatic field. The nature of the phenomenon is made evident when the intensity of this field is increased by the operator bringing his hand near the bulb, his body communicating through the ground with the other pole of the transformer. Another remarkable and very useful quality of the unipolar lamp of this inventor is that of being able to vary the amount of light at will by simply changing the electrostatic capacity of the system. Mr. Tesla utilizes this property in a curious manner by employing as a plate condenser a lamp shade in the form of a reflector as shown in Fig. 5.

This recalls us to the experiment of Prof. Crookes‡ shown in the course of his researches, of the passage of an electric discharge in a high vacuum. A small piece of charcoal placed at C (Fig. 6) in a bulb exhausted to the extreme vacuum of one ten-millionth part of an atmosphere is brought to incandescence under the influence of molecular currents.

In the course of his investigations Mr. Tesla has varied the dimensions of the bulbs of the lamps between great extremes. The relative size of the incandescent knob, and the degree of rarefaction in the bulb, are conditions which require very careful examination and determination. He presented to the audience numerous facts showing the complexity of his investigations and the great amount of patience which is required.

Not being able to go at greater length into this subject we must content ourselves with pointing out merely the chief experiments by which he showed the very great influence of the relative proportions of the parts and their relation to the high frequency transformer. In connecting the poles to bare wires or even to wires covered with an insulating substance, and interposing a plate of very high insulating material (in order to avoid direct discharges), the wires will be seen to become radiant along their entire length, with a small crackling light, resulting from the direct or outside bombardment on the whole surface of the wire. Fig. 9 shows the appearance of two concentric circles showing this phenomenon in one of its finest forms. Another similar phenomenon is shown in Fig. 8, in which a very fine wire, invisible by itself, radiates brilliantly when connected to the high frequency transformer.

Under slightly different conditions a conductor of a certain capacity in the form of a metallic plate was connected to one pole, and there then appeared on the other pole a true flame, as shown in Fig. 9. This experiment suggested to Mr. Tesla some very original thoughts on the constitution of ordinary flames, the nature of which might be connected in some way with the phenomenon of molecular bombardment produced by the rapid variations of chemical energy and the equalization of the affinities.

The discharge in the form of a flame is one of the five typical forms of the illuminating discharges which Mr. Tesla brought out in his original paper.

The discharges vary in form, depending on the frequency, the capacity and the potential; if, for instance, the poles of the high frequency transformer terminate in two poles, the discharge takes the form of a spark depending on the dimensions of the electrodes of the exciter. In this connection one of the most curious experiments shown is that indicated in Fig. 10. The discharge passes at first in the form of a simple spark or of an arc between two discs, one of which is made of metallic cloth. The interposition between these of an insulated plate of ebonite, instead of simply obstructing the passage of the arc, increases the condensing action and substitutes for the arc a form of discharge which fills all the space between the discs; the heating action on the ebonite manifests itself to a great degree.

During the lecture Mr. Tesla took a ball of copper in each hand, and by means of them touched the two poles of the high frequency transformer; he then received through his body the full potential of 70,000 volts developed in this transformer of 9,000 ohms. It certainly is not one of the least curious effects of enormous frequency that it renders harmless to the human body currents the danger of which is only too well known when the frequency is low. The investigator remarked to the audience that when he first tried this experiment he felt like a man about to jump from the Brooklyn Bridge. In the whole course of his experiments Mr. Tesla was not afraid to hold in his hands the lighting apparatus and to use his body as a conductor. The action of the body is due as much to its induction effects resulting from the approach of a conducting surface as to its conductivity. The discharges of excessive frequencies do not, in fact, require two-wire conductors for their transmission. If, for instance, one pole is connected to one end of the exhausted tube shown in Fig. 11 the tube will be illuminated, but it will be brighter at that end to which the wire is connected, and will gradually grow less bright toward the other end. It suffices, however, merely to connect to the other end of the tube a conducting or semiconducting body having a sufficiently good conducting surface, in order to make the illumination equal throughout the whole tube. The body which is connected acts as a real diffuser through which the discharge is disseminated into the surrounding air, which therefore replaces the return conductor. The same effect is produced with a lamp having two carbons instead of one, as shown in Fig. 12. If one pole only is connected to one terminal of the transformer, it becomes highly incandescent while the second one is only slightly illuminated. An interesting feature to be noted here is the shadow thrown in the bulb by the second carbon and its support. It suffices, however, merely to connect to the second carbon a body having the proper capacity, in order that both may burn equally bright. This experiment was one of those in which the light produced was of the greatest intensity; it was like that of one of the ordinary incandescent lamps with carbon filaments when forced to a very high candle power. Mr Tesla furthermore pointed out that the carbon knobs of these lamps can also be illuminated entirely by induction, a phenomenon which he pointed out in his earlier papers. They are in this case furnished with condensing armatures placed on the outside and inside of the bulb. Such an arrangement obviates the necessity of introducing wires through the bulbs and therefore permits them to be hermetically sealed without a break in the continuity of the bulb. The energy is transmitted entirely in this case through the glass. The induction is, perhaps, in this case the cause of the lowering of the efficiency of these lamps, which otherwise ought to be very high. Fig. 13 shows an arrangement of two concentric bulbs as made by Mr. Tesla in order to diminish the action of the air in contact with the bulb. The space intermediate between the two bulbs is exhausted to a less degree than that in the inner bulb and appears to act like an interposed conductor. This arrangement, however, increases the temperature of the interior bulb.

The question of the life of the substances subjected to the molecular bombardment in the bulbs is one of the features which require very careful investigation in order to render such lamps practical. In the beginning of his researches Mr. Tesla used for the incandescent knobs carbon prepared like that for the filaments of the ordinary incandescent lamps, and treated with hydrocarbons. He obtains more satisfactory results now with a variety of pure carbon prepared by Mr. Atchison and known by the name of carburendum, as nearly as we can remember from the pronunciation.

In the course of his investigations he was led to the very interesting and particularly fascinating study of phosphorescent bodies. With his high frequencies and exceedingly high potentials the phosphorescence in the path of the discharge acquires a degree heretofore unknown. Figs. 14, 15 and 16 give but a poor idea of the fine appearance of some Crookes tubes containing calcium sulphide (bright yellow), yttria (green) and rubies (red), resembling strawberries, etc. Mr. Tesla also showed numerous glass tubes having their internal surface covered with phosphorescent matter. In the latter, the phosphorescence of the glass adds its light to that of the matter in the tube; in a large number of cases the action is particularly intense at the equator of the bulb, as shown in Fig. 17. We call attention to the illumination of the glass tube, in spite of the thin layer of metal deposited on its surface, which, as might at first appear, ought to obscure the light.

The great variety and number of experiments shown and mentioned by Mr. Tesla showed the activity with which he has pursued this captivating problem of alternating currents of very high frequency. He is now studying the question as to how far such effects may be utilized and conducted to a distance. The great difficulty which the insulation of such conductors presents is evident; their surface, whether bare or covered with any insulation whatsoever, remains exposed to the molecular bombardment. He spoke of a wire having the ordinary protection of a continuous insulating covering which is surrounded by an armature, which acts as a condenser. A unipolar lamp will burn with a different intensity when it is connected to the conductor itself or to the exterior metallic cover. Mr. Tesla expects to get good results from a system of static screens formed by a metallic protection in short lengths around the wire.

In this connection he made an allusion to some hopes he had regarding the possibility of transatlantic telephony, which, however, we did not quite understand. As to the transportation of energy to a distance, by currents of such great frequency, he says that there will be no necessity felt for such a transportation, as one would then know how to obtain the energy and motive force in any point or place in the universe. To illustrate this he mentioned that the radiometer of Crookes operates anywhere, in the light as well as in invisible heat, and according to his views this machine is one of the most inefficient known. Under the conditions in which these unipolar lamps are lighted, there is nothing to hinder one from gathering the energy in a form suitable to drive a motor. Fig. 18 shows such an apparatus in the form of a motor turning from the impulses of discharges of high frequency traversing a solenoid wound around the ends of a bundle of iron wires. The movable part is a copper disc unsymmetrically situated relatively to the bundle of wires. The general analogy of the form of this with the gyroscopic apparatus of Messrs. de Fonvielle and Lontin (and others) appears very curious. Does it not indicate the identity of the action of currents whose frequencies are very different from each other? As an incidental experiment the rapid heating of a piece of iron subjected to the action of the discharge currents was pointed out.

Mr. Tesla furthermore described a phenomenon capable of application to submarine telegraphy, by showing the extreme sensitiveness of the directing action of a magnet (even if very long and very weak) on the discharge in a unipolar lamp as shown in Fig. 19, the electrical processes presenting the subject of the light of the future in quite a different aspect than the chemical processes, and Mr. Tesla showed on several occasions the striking experiment which in his opinion shows the ideal method of lighting. His experiment showed that the energy of the electrostatic field produced by the rapid and powerful alternations of a high tension transformer is capable of being utilized. This is, so to speak, his typical experiment. A metallic plate about three metres in length and one-third of a metre in breadth was suspended about two and one-half metres above the lecturer's platform, as shown in Fig. 20. The electrostatic field was generated in the space in which the lecturer stood, between the plates and the floor, which were respectively connected to the two poles of the transformer. Exhausted tubes of glass more than a metre long, held in the hand of Mr. Tesla in this field, were brilliantly illuminated throughout their entire length as shown in the figure. The light was intense, being almost vivid, and resembling in its tint the light from the moon. One might say it appeared like a luminous sword in the hand of an archangel representing justice. The appearance was very fine and the effect very grand.

But the experiments of Mr. Tesla have, in our opinion, quite a different bearing, as we are tempted to see equally well in the phenomena of the electrostatic field as in the illumination of the unipolar suns of the inventor the experimental confirmation of our own views on the electrostatic nature of light.*

One sees from this lecture the deep interest which the works and discoveries of Mr. Tesla have inspired among physicists since the first appearance of his publication, and it is with great satisfaction that we are able to express the feeling of admiration which his experiments have inspired in us.

* La Lumière Electrique.

** See The Electrical World, July 11, 1891.

† This reminds us of the first great induction coil of M. Jean, which was insulated with the essence of turpentine. (Du Moncel, "Notice sur la Bobine d'Induction," p. 40.

‡ "L'Electricité et son Trajet," par William Crookes. La Lumière Electrique, vol. XXXIX., p. 337.