FROM CURRENT LITERATURE

ELECTROGRAPHIC METHOD FOR RECORDING FAST ELECTRICAL PROCESSES¹

Zelenyi calls electrographic recording a method he has developed, the essence of which consists in fixing electrical impulses as an invisible electrical image on the surface of an insulator by means of charges carried by an electron beam in vacuum, or by a stream of ions in the ordinary atmosphere. This invisible image can subsequently easily be made visible. For this it is sufficient to place the electrical image in a stream of air containing a sprayed fine powder, such as talc, lycopodium, etc., as a result of which the invisible electrical image is “developed.”

Since the use of an electron beam for these purposes is associated with special vacuum apparatus and is therefore not very convenient, Zelenyi devotes particular attention to the second of the methods indicated above (ions in air), and here he has succeeded in creating a very simple “electrographic oscillograph.” The essence of the construction of this instrument is explained in Fig. 1, where the letter $K$ denotes the cathode of the “ion gun,” the letter $G$ its controlling electrode, and the letter $A$ a metal plate whose surface is covered with a thin layer of dielectric $S$.

Fig. 1

This plate constitutes the anode; the cathode of the ion gun—a platinum wire coated with barium oxide—being heated by battery $B_n$, emits electrons which form, with the gas molecules, negative ions; these ions, in accordance with the (usually grid) controlling potential of electrode $G$, rush toward the anode and, settling on the surface of the dielectric, impart to each point of it a charge whose magnitude corresponds to the intensity of the ion current at the given instant. This latter is determined both by the voltage with potential $A$ and by the potential of the controlling electrode. With a proper choice of the distances ($K—G \sim 0.5—1$ mm; $G—S \sim 0.5—1$ mm) and potentials ($K—A \sim 500—1000$ V), a 100% modulation at the electrodes can be obtained with a change of the potential $G$ by 5–10 V.

In order to obtain a record of the process, it is necessary to move the anode relative to the ion gun (arrow in Fig. 1). In practice the anode is made in the form of a drum rotating about an axis connected

with a worm screw that moves, parallel to the axis of the drum, a carriage with a corona point.

After the electrical image has been obtained and “developed,” it can be photographed (if it is necessary to preserve the record); the powder is removed from the surface of the insulator by a strong jet of air, and the surface is discharged, for example, by means of a Bunsen-burner flame. Thus one and the same surface can be used many times.

Selenyi successfully applied his instrument to the recording of a great variety of processes, such as: the recording of damped electromagnetic oscillations, the recording of vowel sounds and musical tones, the recording of relaxation oscillations, the investigation of the process of wire burnout at various current densities, the recording of discharges in a Geiger counter, etc. On the basis of the calculations and experiments carried out, Selenyi believes that the resolving power of such an oscillograph in air should reach \(10^{-5}\) sec.; when operated in vacuum it can be brought to \(10^{-6}\) sec.

N. Khlebnikov, Moscow

Literature

1. P. Selenyi, Journ. Appl. Phys., 9, 637, 1938.