New Books - Light and Other High-Frequency Phenomena

Introduction. - Mr. Tesla introduces his lecture about as follows: - In our study of the physical universe we find that our sense of sight is superior to all other senses, because it enables us to penetrate much deeper into the fathomless depths of Nature's mysterious activities. The activity by means of which Nature reveals herself to us through our organ of sight, the activity which we call light, deserves our closest attention. What is light? It is an electromagnetic activity, according to most advanced views; hence the importance of investigating those forms of electromagnetic activities which approach the phenomena of light as near as experimental possibilities of the present day will permit.

The introduction might lead one to expect that Mr. Tesla intends to describe a novel class of high-frequency electrical oscillations for the purpose of illustrating and giving new evidence in favor of the electromagnetic theory of light. In that case one would perhaps feel somewhat disappointed to find that Mr. Tesla's experiments are simply Hertz's experiments in a modified form, a form by which it can be shown that in the first place the Hertzian oscillations enable us to transmit a considerable amount of energy along a single wire, and in the second place that novel vacuum tube effects can be produced by rapid alternations of the electromagnetic field.

But this feeling of disappointment, if it should occur at all, would be soon dispelled and quickly give way to a feeling of admiration for the patient work of an enthusiastic inventor who has brought all the ingenuity of his remarkable engineering skill to bear upon the problem before him, and that problem is contained in the question: - What immediate benefits can modern electrotechnics derive from Hertzian oscillations? This problem Mr. Tesla handles with extraordinary ability.

Method of Conversion. - To produce the oscillations Mr. Tesla connects a direct or alternating current dynamo to an oscillatory transformer. The primary of this transformer, consisting of a small number of turns of stranded wire, corresponds to the Hertzian exciter, and the secondary, consisting of an appropriate number of turns of wire surrounding the primary, corresponds to the Hertzian resonator. Hertz worked with oscillations of about 10⁸ periods per second. Mr. Tesla is satisfied with a frequency of 10⁶ or even smaller, hence his primary and secondary coils have a much larger self-induction than that of the Hertzian exciters and oscillators. By a suitable arrangement of the ratio of the turns in the primary and secondary, the oscillatory transformer can be an up or a down transformer. The primary spark space is similarly located as in the Hertzian experiments. Capacity is generally added to both the primary and the secondary coils.

Precautions which must be observed in order to render the Primary Impulses very Powerful. - No arc formation should be allowed in the primary spark space. Four different devices for this purpose are described, the purpose of each being to blow out the arc as soon as it begins to form.

1. Spark space is placed between the poles of a sufficiently powerful magnet.

2. Current of hot air sweeps continually through the spark space (especially when dielectrically weak air is required between the discharge knobs).

3. Spark space is placed in a partially exhausted glass tube, the tube being placed between the poles of a strong magnet to prevent arc formation.

4. Two objections against a spark space through a gaseous medium are pointed out: (a) the discharges are not sufficiently disruptive; (b) there is too much dissipation. To improve both, a liquid dielectric was tried. A hollow pulley was partially filled with oil (the quality of oil not mentioned); the pulley being rotated rapidly, the oil was thrown out centrifugally and compressed against the rim; discharge through oil improved the disruptive character of the primary sparks and diminished losses. These discharges were effective for producing great spark length, but not for luminous effects, since for these, according to Mr. Tesla, a harmonically varying electromagnetic force is preferable.

The best form of generator is then discussed; it is pointed out that the most convenient method is to use a low-tension dyna-motor, to transform direct currents into low frequency alternating currents of low voltage. The low-voltage alternating current thus obtained is then transformed into high tension alternating by a stationary transformer, and this high-tension current is then employed to feed the oscillatory transformer.

Phenomena produced by Electrostatic Force. - In the introduction to these phenomena, Mr. Tesla makes the following statement concerning electrostatic force: "It is the force which governs the motion of the atoms, which causes them to collide and develop the life-sustaining energy of heat and light, and which causes them to aggregate in an infinite variety of ways according to Nature's fanciful designs, and to form all these wondrous structures we perceive around us. ..."

Evidently Mr. Tesla makes even a broader claim for the electrostatic force than Professor J. J. Thomson does for Faraday's tubes of electrostatic induction. (Compare also H. von Helmholtz's lecture on Faraday.¹)

Experiment 1. - One pole of the oscillatory transformer is touched with one hand, and the other hand approaches the other pole. A brush discharge passes from the free hand to the free pole. The frequency of the current is said to be a million, and the electromotive force = 2 x 10⁵ volts. Mr. Tesla calculates that, with four times the frequency, and a potential of 3 x 10⁶ volts, his whole body would be surrounded by a flame, but the current would do him no harm, because the body offers a large crosssection to the high-frequency current, and the current in its passage through the body distributes itself uniformly, so that the current density is small at any point. This interpretation takes it for granted that Hertzian oscillations pass through the human body just the same as an ordinary direct current would pass through a good conductor.

Experiment 2. - Illustrates difference between discharges at high and low frequency. Two circular brass plates, each connected to a pole of an oscillatory transformer, are placed at a distance of 12 inches. At high frequency the luminosity of the discharge between the plates is steady and perfectly uniform; at low frequency, the discharge is in the form of numerous, quivering, and noisy spark filaments. Mr. Tesla admits the impossibility of disruption at high-frequency; he observes the powerful chemical action of the high-frequency discharge; he seems, however, to fail to recognize in the difference of the two discharges a confirmation of the generally accepted dissociation theory of electrical conduction through gases.

Experiment 3. - An incandescent lamp bulb with a single straight carbon filament is attached to one pole of the oscillatory transformer. An apparently unipolar discharge takes place. (The luminosity in the immediate neighborhood of the external surface of the lamp is evidently due to the varying condenser charges of that surface. If that surface has any sharp points, the current density there will be large, and the point will be heated.²) A bulb drawn out into a point is (therefore) heated and is ultimately broken at that point. Mr. Tesla ascribes this heating to molecular bombardment which, according to his opinion, will take place in heterogeneous media like gases, because the molecules or atoms in such bodies are free to move under the impulse of electrical forces. Mr. Tesla does not seem to be aware of the fact that nobody has succeeded yet in electrifying the molecules of a perfect gas. Several experiments are then described in support of the bombardment theory, but they do not seem to be convincing.

On Currents or Dynamic Electricity Phenomena. - In these experiments it is shown that electrical energy can be transmitted along a single open-circuited wire. The modus operandi is the following: - One end of a wire is attached to one of the poles of the secondary of an oscillatory transformer; the other end is attached to a large plate. An adjustable coil is also inserted in this circuit. The plate gives this open circuit its principal capacity, the adjustable coil, which the reviewer calls the inertia coil, enables the experimenter to regulate its self-induction or inertia. When the frequency is very high, the capacity of the plate can be dispensed with, the capacity the remaining part of the circuit being sufficient to bring the circuit in resonance with the impressed electromotive force.

Two methods are then employed to demonstrate the flow in the open circuit and its dependence on the constants of the circuit; namely, capacity, self-induction, and frequency.

Method I. - The circuit of several turns of wire surrounding the auxiliary inertia coil is closed by a 50-volt, 16-C.P. lamp, and the flow studied by the intensity of the glow of this lamp.

Method II. - The flow in the open circuit is transformed in various ways, and various simple devices are then employed to cause a disc to rotate under the electromagnetic or electrodynamic action of the oscillatory currents thus employed. The first method points out the possibility of a distribution of oscillatory currents for electric lighting; the second method demonstrates in a certain way the feasibility of employing such currents for operating electromotors. The difficulties which would necessarily attend such an employment of oscillatory currents might have been discussed a little more fully, in order to prevent a misconception on the part of those who are not familiar with the subject. To show the effects of impedance (a wave distribution of potential energy along a short, stout wire), Mr. Tesla employs Lecher's well-known and generally adopted method of producing very rapid Hertzian oscillations. He devotes, then, twelve pages to a discussion of electrical resonance, and particularly of the bearing of electrical resonance upon the problem of transmitting signals all over the earth by causing the electrical charge of the earth to oscillate.

This discussion is evidently introduced to relieve somewhat the serious character of the rest of the lecture, and seems to bear the same relation to the rest of the lecture as the amusing gravediggers' dialogue in "Hamlet" bears to the rest of the play. In the reviewer's opinion, Mr. Tesla does not wish to be taken au serieux in this part of his lecture.

Vacuum Tube Effects produced by Hertzian Oscillations.

I. Incandescence of a Solid. - It was observed a long time ago that the electrodes of a Geissler tube, especially the cathode, become very hot, although the current through the tube may be exceedingly small. Mr. Crookes, it seems, was the first to suggest that this heating was due to molecular bombardment. Mr. Tesla shows that by means of this property of the vacuum-tube discharge carbon filaments and carbon buttons, when enclosed in the vacuum of an incandescent lamp, can be rendered incandescent by an exceedingly small current, provided that the potential of these carbons is very high and the frequency considerable; the higher the frequency, the slower will be the process of disintegration of the carbons. He also ascribes this heating to molecular bombardment, but his arguments in favor of this view are open to the same objections which were brought, long ago, against the molecular bombardment theory of Crookes. Some of the heating in such tubes is very probably due to molecular, or rather atomic, bombardment, but according to the splendid investigations of E. Wiedemann, Schuster, Elster and Geitel, J. J. Thomson, Giese, and others, the chemical process going on in such tubes owing to the action of the electrical forces will account easily for a very large part of the heat. It should also be remembered that the spectrum of an incandescent gas is very rich in ultra-violet light, which part of the luminous energy of the gas never leaves the bulb, being absorbed by the same and transformed into heat. To ascribe all the various heat phenomena in a vacuum tube simply to molecular bombardment means to turn a sharp corner, in order to get out of the way of difficulties instead of forcing the difficulties themselves to get out of the way. The same criticism applies to Mr. Tesla's view of the cause which produces phosphorescence and fluorescence in vacuum tubes. It is, of course, a well-known fact that mechanical shocks can produce phosphorescence and fluorescence; but it is equally well known that no agent is as powerful as ultra-violet light and purely chemical processes to produce these phenomena. Why, then, refer these phenomena in vacuum tubes to molecular bombardment and to nothing else?

It must be observed, however, that the value of Mr. Tesla's work is, of course, independent of the view which he may have formed of the causes of all these various phenomena which he investigated by means of extremely powerful electrical oscillations. The real value of Mr. Tesla's work is due to the fact that he was the first to succeed in constructing a magnificent array of physical apparatus, by means of which these exceedingly powerful Hertzian oscillations can be produced, and to point out a way of attacking old but still puzzling phenomena by weapons with which the refinements of modern electrotechnics can furnish us. It is a work which, for some time to come, will be consulted by the electrical engineer who wishes to become acquainted with the tendencies of recent advances in electricity, and it will be consulted by the physicist who wishes to make himself familiar with the practical bearing of these advances. In the performance of this work Mr. Tesla has, therefore, rendered invaluable service to all those who are interested in the science of electricity.

M. I. Pupin
Columbia College, New York, Oct. 15, 1893.

¹Vorträge und Reden, Vol. 2, p. 302, 1884. 
²The reviewer is responsible for the opinion enclosed in parentheses in this paragraph.