New Methods for Generating Short Undamped Electromagnetic Waves.

1. W. White. The Plyotron Oscillator for extreme frequencies. General. El. Review—Sept. 1916.
2. B. Van der Pohl. The production of short continuous electromagnetic Waves. Phil. Mag. 38 (1919).
3. R. Whiddington. Oscillation in Three-Electrode thermionic Valves. Radio Rev. Nov. 1919.
4. H. Barkhausen und K. Kurz. Die kürzesten mit Vacuumröhren herstellbaren Wellen. Phys. ZS. I, p. 1 (1920).

A thermionic tube with three electrodes (triode) provides, as is well known, a very simple means of exciting undamped electrical oscillations in a circuit containing capacitance and self-inductance; this method is applied very widely in radio engineering and is beginning to enter into the practice of physical laboratories. By reducing the capacitance and self-inductance of the circuit, one can obtain very short waves; the limit to shortening \(\lambda\) is, of course, set by the dimensions of the triode itself.

In 1916 White (1) obtained by this method waves with \(\lambda = 600\) cm.
In 1919 Van der Pohl (2) obtained still shorter waves, \(\lambda = 365\) cm. The coupling of the circuit in the anode circuit with the grid, necessary for self-excitation of the oscillations, is effected in this arrangement by the electrostatic action of the oscillations of the anode potential on the grid; a condenser consisting of two plates 10 cm in diameter, which can be moved apart, serves both for regulating the wavelength and for the above-mentioned change in the coupling of the circuits; when it is varied, oscillations arise suddenly at a certain distance between the plates, which becomes noticeable by an increase in the deflection of the measuring instrument; the power of the oscillations is about 1 watt.

The detection of the waves and the measurement of \(\lambda\) were carried out by means of the Lecher system; a Duddell thermogalvanometer served as the indicator.

The production by this method of still shorter waves is evidently difficult to realize.

Whiddington (3) (November 1919) obtained electrical oscillations with a triode containing gas, without any oscillatory circuit; his arrangement is explained in Fig. 1. The grid is charged slightly positively with respect to the filament, the anode—with a somewhat higher positive potential; no circuits are introduced in the wires connecting the batteries, and nevertheless it is possible to detect by means of a wavemeter that such an arrangement gives oscillations with a definite wavelength. Whiddington explains this fact as follows: the filament \(F\) emits electrons at certain points—centers of emission; under the action of the accelerating field of the grid the electrons reach it with velocity

\[ u = \sqrt{2V\frac{e}{m}}, \]

where \(V\) is the potential of the grid, and \(\frac{e}{m}\) is the ratio of the electron charge to its mass; having passed through the grid, the electrons enter the stronger field of the anode, at once accelerate their motion and acquire sufficient velocity to ionize neutral atoms or molecules. The \(+\) ions formed fly back to the filament and strike it, thereby causing, according to the author’s supposition, a sudden increase in the emission of electrons, and in this way the process repeats again. The velocity of the \(+\) ions reaching the filament is determined from the relation

\[ u_1^2 = 2V\frac{e}{m_1}, \]

where \(m_1\) is the mass of the ion; since \(m_1\) is considerably greater than \(m\), the period of the oscillations is determined chiefly by the magnitude of \(m_1\); since the velocity of motion is inversely proportional to \(\sqrt{m_1}\), polyatomic ions will give oscillations with a frequency \(\sqrt{2}\), \(\sqrt{3}\), \(\sqrt{4}\), etc. times smaller than for mono-

atomic ion. The following table gives the results of the calculation for molecules of mercury vapor with 1, 2, 3, 4 . . . . . atoms and the corresponding results of frequency measurements in the experiment:

The frequency \(6.6 \cdot 10^5\) corresponds to the wavelength \(\lambda = 450\) mt.

Whiddington’s method evidently makes it possible to determine the ratio of the charge of ions to their mass, similarly to J. J. Thomson’s method.

The production by this method of very short waves is hardly possible; thus, the lightest H ions would give \(\lambda = 30\) mt. at \(V = 1\) volt; increasing \(V\) is possible only up to a less ionizing potential, i.e. no higher than 13 volts for hydrogen, and since \(\lambda\) decreases in proportion to \(\sqrt{V}\), a decrease of \(\lambda\) is possible by only a factor of 3.6, i.e. down to 8 mt.

Barkhausen and Kurz (4), quite independently, developed an analogous method of obtaining short electromagnetic waves, and they succeeded in obtaining much shorter waves than Whiddington’s, owing to the fact that electrons alone were brought into oscillation. The authors used the following arrangement: the grid (Fig. 2) was charged to a high positive potential (for example, \(+150\) volts), and the “anode” to a negative one (for example, \(-200\) volts) or to a small positive one, so that the field was directed from the grid to the anode. Under these conditions the electrons emitted from the filament move with acceleration toward the grid, pass through it, then move with retardation toward the anode, stop and fly back to the grid, pass through it and again are retarded, and reach the filament with the same velocity with which they were emitted; thereafter the same process is repeated. The authors consider it unclear how all the electrons, continuously emitted, can have one and the same phase of oscillation. This circumstance may be explained by assuming that, when the electrons strike the filament, enhanced emission of new electrons is produced.

Fig. 1.  Fig. 2.

From elementary considerations the authors calculate the wavelength of the oscillations:

\[ \lambda = \frac{1000}{\sqrt{V_g}} \cdot \frac{d_a V_g - d_g V_a}{V_g - V_a} \]

where \(V_g\) and \(d_g\) are the potential in volts and the diameter of the grid,

and \(V_a\) and \(d_a\) — ” ” ” ” ” anode.

The results of the calculation are compared with experiment in the following table:

If one takes into account the approximate nature of the calculation, which does not allow for space charge or for the dependence of mass on velocity, then the agreement of the calculations with experiment must be recognized as satisfactory.

Oscillations can be excited only in triodes of cylindrical form with a dense grid, having the most symmetrical construction. The shortest wave, \(\lambda = 43\) cm, was obtained with a Schott lamp having a cylindrical anode diameter \(d_a = 2.1\) cm (a similar lamp is intended for transmitters); the grid voltage was \(V_g = +500\) V. If a triode with a dense grid and \(d_a = 0.5\) cm is made, it should be possible easily to obtain \(\lambda = 10\) cm at \(V = 500\) V and less at higher grid voltage. To increase the radiation, two straight wires of length \(\dfrac{\lambda}{4}\) (an antenna) are connected to the grid and the anode; the same antenna also serves for reception, with a detector connected at its midpoint and, in parallel with it, a galvanometer, which gives a deflection when waves are emitted by the oscillator. The wavelength is also measured on a Lecher system by means of a detector with a galvanometer.

If a microphone was included in the anode circuit, a miniature radiotelephone station was obtained, with which the authors succeeded in establishing communication over 300 meters.

With such small antennas, Barkhausen and Kurz reproduced all the classical experiments of Hertz. The new method of obtaining short undamped waves will, of course, make it possible to investigate dispersion and absorption of them in various media more clearly and to elucidate contradictions in the experimental data of various investigators.

S. Rzhevkin.