HEINRICH HERTZ

HEINRICH HERTZ1

N. Malov, Moscow

Fifty years ago, in 1888, one of the most accomplished experimental physicists of the nineteenth century—Heinrich Hertz—published a series of classical investigations that played an enormous role in the subsequent development of physics and found broad practical application in radio engineering.

This series of Hertz’s works was devoted to experimental proofs of the principal consequences of Maxwell’s theory—the production of electromagnetic waves and the discovery of their optical properties.

As is well known, Faraday’s ideas concerning the role of the medium surrounding charged conductors in the development of electromagnetic processes were given mathematical form by Maxwell, who wrote down his famous equations of the electromagnetic field. From these equations, among other things, it followed that a variable electromagnetic field must propagate through space with a finite velocity, coinciding with the velocity of light.

Maxwell held that a transverse electromagnetic wave is identical in its physical nature with a light wave (in the narrow sense of the word), and, proceeding from this proposition, constructed the electromagnetic theory of light.

The complexity of the mathematical side of Maxwell’s theory, the divergence of its basic propositions from the widely prevalent theory of “action at a distance” in those years, and, finally, the absence of experimental proofs of the most important consequences of the theory made it difficult for physicists of the seventies and eighties to accept it; only a few individuals properly appreciated Maxwell’s theory. Among them was, among others, Helmholtz, who played a major role in the development and direction of Hertz’s physical interests. In this connection it is of interest to note that, until his very death, Hertz recalled with gratitude his years of study under Helmholtz, and valued his approval more highly than all the numerous marks of distinction he received from various scientific institutions,

and, when publishing in 1891 a collection of his works on electromagnetic oscillations, he dedicated it to Helmholtz.

Hertz’s immortal achievement is the impeccable experimental proof of the existence of electromagnetic waves and the establishment of their optical properties.

Heinrich Rudolf Hertz was born on 22/II 1857 in Hamburg, in the family of a civil servant. Already in secondary school he showed great interest in experiment and made various instruments. Having completed secondary school, Hertz went to Munich to obtain a technical education, but soon saw that the applied sciences did not satisfy him, that natural science, in particular physics, attracted him much more. After long hesitation he decided to turn aside from the path originally intended and to engage in preparation for scientific work. In a touching letter to his parents that has come down to us, informing them of his intention and asking their consent, he wrote, among other things: “Earlier I often told myself that I should like more to be a great scholar than a prominent engineer; but, on the other hand, to be a mediocre engineer is, for me, preferable to being a mediocre scholar. But now I think that Schiller was right when he said: ‘Und setzt Ihr nicht das Leben ein, nie wird Euch das Leben gewonnen sein,’1 and that excessive caution on my part would be folly.”

Having received his parents’ consent, Hertz moved to Berlin and entered the university, where the teaching of physics was in the hands of Kirchhoff and Helmholtz. Already while working in the general physics practicum, Hertz was noticed by Helmholtz, at whose suggestion he soon began experimental work on the subject “The kinetic energy of moving charges,” proposed by the university on Helmholtz’s recommendation as a subject for a medal. The aim of the work was to determine the validity of Weber’s hypothesis concerning the existence, in the charges participating in the creation of an electric current in a conductor, of a certain inertia, i.e., to establish whether the inductance of a conductor should not be expressed by the formula

\[ L = L_{0} + m \]

where \(m\) is a constant independent of the geometry of the conductor, and \(L_{0}\) is the inductance in our understanding.

This work captivated Hertz, who devoted all his strength and all his time to it, even to the detriment of his studies. In letters to his parents he reports in detail on all these investigations, writes that “behind every difficulty overcome there rises a new one, still greater,” but that he does not despair and continues to work. Characteristic of Hertz is the following remark of his: “I doubt whether it is right to devote so much time to this matter (the experiment, N. M.), while my knowledge is still so imperfect.

But I cannot express how much greater satisfaction it gives me to extract from nature knowledge for myself and others than always to receive it from others and only for myself.”

The results of Hertz’s labors did not go to waste: he succeeded in showing that the increase in inductance assumed by Weber’s theory in fact did not exceed 0.4% of the total value, i.e., was very unlikely. For this work Hertz received a gold medal.

The following year Hertz carried out the brilliant work “On Induction in Rotating Bodies,” which he submitted as his doctoral dissertation and which received the highest mark from the University of Berlin.

After graduating from the university, Hertz became Helmholtz’s assistant and gained the opportunity to use the university’s rich equipment. During the next three years Hertz engaged in a wide variety of investigations—discharge in gases, hydrodynamics, the theory of elasticity, hydrometry, and so on.

In 1883 he moved as a Privatdozent to Kiel, and from there—in 1885—as professor of physics to the technical school in Karlsruhe.

In Hertz’s diary for 1884 there are two entries: “Thought about electromagnetic rays” and “Thought about the electromagnetic theory of light.” Evidently, by this time his interest in Maxwell’s theory had finally taken shape.

In one of his later articles, speaking of the state of Maxwell’s theory at the beginning of the 1880s, Hertz indicated that in order to prove the correctness of Maxwellian conceptions it was necessary to solve the following three problems:

1) to show that changes in the polarization of a dielectric are accompanied by the same electrodynamic forces as ordinary currents (i.e., to detect the magnetic field of the displacement current),

2) to show that electrodynamic forces are capable of creating dielectric polarization just as ordinary electrostatic forces do (i.e., to prove the existence of vortex electric fields),

3) to show that empty space (or air), in electrical respects, is similar to any other dielectric.

Reflecting on these fundamental questions, Hertz came to the conclusion that the third problem contains within itself the first two, and that to solve it it would suffice to show that an electromagnetic wave can exist in air.

Proceeding from the assumption that the speed of its propagation would be equal to the speed of light, Hertz realized that in order to detect a change in the phase of the wave in the comparatively small space represented by an experimental room, it was necessary that the period of oscillation of the propagating process be sufficiently small—smaller than the periods produced by the discharges of a Leyden jar through a coil, already studied at that time.

Thus, before Hertz’s mind’s eye there already arose at that time an extensive plan of investigations, which he brilliantly carried out and published in the period 1887–1889.

Gradually reducing the dimensions of the oscillatory circuit, Hertz passes from a closed vibrator to an increasingly open one; in the end, he arrives at a rectilinear wire—the well-known Hertz vibrator.

Investigating the mode of operation of the spark gap, Hertz noticed that illuminating it with some source of light sometimes worsens the mode of operation. Studying this phenomenon, Hertz found that the harmful influence was due to ultraviolet rays and thus gave the first indication of the existence of the photoelectric effect, subsequently studied in detail by Stoletov and Hallwachs.

To observe the propagation of the electromagnetic wave, Hertz at first constructs closed resonators, and then open ones. The negligible power of the oscillations greatly hampers the investigations; suffice it to point out that the spark gap of the resonator had to be adjusted with a micrometer screw and viewed through a magnifying optical system in a darkened room.

Already in 1887 Hertz obtains electromagnetic waves several meters long, creates standing waves in wires, studies their propagation in space with the aid of resonators, and establishes the complete agreement of the results of the greater part of his experiments with the conclusions of Maxwell’s electrodynamics.

Brilliant experimental investigations are accompanied by no less brilliant theoretical works. It is enough to indicate that in 1888 he considers the radiation field of a straight vibrator, and in 1890 publishes classical works on the electrodynamics of media at rest and in motion.

Already while working with meter waves, Hertz made attempts to discover their optical properties; these attempts proved unsuccessful.

Taking into account that this failure was due to a wavelength that was too great, requiring enormous mirrors, Hertz succeeds in obtaining shorter waves and, at the end of 1888, publishes his immortal work “On the Rays of Electric Force,” in which, using waves 60 cm long, he proves that their propagation obeys the usual optical laws. In this same work an irreproachable proof is given of the linear polarization of these waves.

This work is perhaps the most brilliant in execution, and in it the clarity of Hertz’s thought, the purity of his experiments, and their broad scope, enabling the phenomenon under study to be grasped from all sides, are revealed with exceptional distinctness. The authentic text of this work is published below.

Hertz’s works were duly appreciated by his contemporaries, gave rise to numerous repetitions, and contributed, as has already been indicated, to the establishment of Maxwell’s theory.

Hertz himself did not succeed in linking the electromagnetic spectrum with the optical one, which seemed desirable for the complete confirmation of Maxwell’s theory. The difficulties in obtaining still shorter waves lie in the reduction of their energy, associated with the reduction of the capacitance of the vibrator, and in the rapid triggering of the latter during spark formation in the spark gap.

Among Hertz’s successors in this direction, the best results were achieved by P. P. Lebedev, who in 1895 obtained waves 6 mm long; Glagoleva-Arkad’eva (1919–1923), who obtained “white” radiation, from which she was able to isolate waves of 180 and 300 μ, lying in the region of the long infrared rays investigated by Rubens and Baeyer; and M. A. Levitskaya (1920–1923), who also managed to obtain very short electromagnetic waves by producing an electric discharge between a chain of small spheres; unfortunately, the heating of Levitskaya’s vibrator, which was also accompanied by radiation, made it difficult to separate the two types of radiation.

In 1891 Hertz republished his works on electromagnetic oscillations under the general title Investigations on the Propagation of Electric Forces, supplying them with a preface in which he described in detail the path that led him to the solution of the extremely difficult problems that had confronted him.

Hertz spent the last years of his life (from 1889) in Bonn, where he was occupied chiefly with the foundations of mechanics, striving to present it in the form of a logically flawless system proceeding from a minimal number of premises. The results of Hertz’s work in this direction were set forth by him in the well-known book The Principles of Mechanics, which he was unable fully to complete because of a serious illness.

The experimental works of the last years of Hertz’s life concerned cathode rays, in particular the conditions of their propagation in thin metallic layers. In his last published work (1891), Hertz apparently observed, inside a discharge tube, the fluorescence of glass under the action of X-rays, but, giving this phenomenon an incorrect interpretation, he did not think to place a fluorescent screen outside the tube and did not make the discovery to which he had come so close.

Illness interrupted Hertz’s further experiments and soon prematurely cut short this remarkable life—Hertz died on January 1, 1894, at the age of 37.

The scientific significance of Hertz’s work was entirely clear to him; as for the possibility of its practical significance, it was underestimated by Hertz.

Not seeing the possibility of significantly increasing the energy of electromagnetic oscillations, Hertz believed that his discoveries had no practical value.

The work of A. S. Popov, Marconi, and others showed, after Hertz’s death, that in this assessment of his own work Hertz had been too modest.

Modern radio engineering, which plays such a colossal role in the life of humanity, admittedly makes use of more advanced methods for producing electromagnetic oscillations than the methods invented by Hertz. But the fundamental foundations of radio engineering, television, telemechanics, etc., laid down in Maxwell’s theory, were wholly realized in Hertz’s experiments; the significance of these works for radio engineering can scarcely be overestimated.

The field of application of Hertzian waves is continually expanding: in addition to their broad use in physics and technology, in recent years much attention has been devoted to their biological effect, which is of considerable interest for plant cultivation, medicine, and the food industry.

The test of time is the sternest and most impartial. The half-century that separates us from the appearance of Hertz’s works has not diminished the significance of his immortal labors, and we may with full right join in Helmholtz’s assessment of his beloved pupil and pay “a tribute of admiration to this great man, who died untimely, who combined deep and clear thought with an exceptional ability to notice unnoticed phenomena and to wrest from nature the secrets she jealously guards.”

1. Translation: “He who fears to risk his life will not know success in it.” ↩↩