From the American magazine "Radio News"  March 1925, published by Hugo Gernsback.

HE information given in this article will come as a shock to many well informed radio fans, amateurs of long standing, manufacturers and engineers in the electrical business. We have been led to believe that the origin of the three electrode idea lay with our own Doctor Lee DeForest.
All this is more or less ancient history for the average radio fan who keeps abreast of the times and reads the usual amount of radio literature. The date of Dr. DeForest's first radio tube patent is given as June 26, 1906, the patent being numbered by the Patent Office as 824,637; the next was 836,070, November 13, 1906.
Simultaneous with the work of Dr. DeForest, a group of foreign scientists were digging at the problem of the electronic relay.

 Wehnelt developed a series of metallic oxides which showed extreme sensitivity to heat and high cathode emission in a vacuum. Robert von Lieben, of Vienna, has been mentioned in many scientific works as an originator of novel ideas in cathode and other types of ray emitting tubes.

THE X-RAY TUBE.

Let us hark back to the discoveries of Röntgen. In the early '90's, the world of science was just beginning to make headway with the so-called cathode tube, produced and modified by Crookes, J. J. Thomson and others. Dr. Röntgen, in testing the strength of the field of cathode ray influence in 1895, found that the rays produced by an ordinary cathode tube caused phosphorescence in bodies outside the tube, and that a photographic plate was affected by the rays. He succeeded in photographing a door key through a leather pocketbook by means of this ray emission, which he named X-ray.
The ordinary X-rays as they were originally produced were developed by means of an evacuated tube containing two electrodes, a cathode having a semi-spherical surface and the anode being a flat plate, the center of which was also the center of curvature of the cathode. The terminals of a high frequency source, spark coil or electrostatic machine were connected to the two terminals of the tube, the negative lead being connected to the curved surface cathode.
The high degree of exhaustion within the tube caused a much greater difference of interior electrode potential, and this was found to accelerate the speed of
emission of the rays. In the simple cathode tube, having a comparatively gaseous content, or a vacuum of relatively low exhaustion, the speed of emission was slow, and the cathode particles being small, negative electrons moving slowly were easily deflected by magnetic or other means applied at right angles to the direction of flow.
Dr. Röntgen evacuated his tube to a high degree and the corresponding increased difference of interior electrode potential imparted a higher velocity to the cathode stream. In other words, the ordinary cathode stream gives off soft rays and the X-rays are hard. The harder these rays, the higher the degree of penetration beyond the tube. Determined by Dr. Marx as having the speed of light, they were not deflected by the application of a magnetic or electrostatic field.

Our own Dr. Coolidge found that he could heat the cathode by means of a separate external circuit and that the resulting stream of negative electrons emitted could be bombarded against the anode with a sufficiently high velocity to develop X-rays. His tube, being a great improvement upon other types, because of the economy of current, universality of use, and more sure control of the various functions of the tube, has found a ready use universally since its disclosure.

He showed conclusively that the penetrating power of the rays depended on the voltage of the secondary, and that the breadth or density of the emission stream was parallel to the degree of heat emission at the cathode. In the three electrode relay tube devised by Von Lieben, the influence of the Röntgen ideas are seen. Von Lieben, in working out a practical method to control emission, borrowed from Wehnelt the metallic oxide surface as a source of electron emission, took the curved surface of Röntgen as the proper shape for his emitter electrode, and projected the resulting electron beam through a tubular shaped grid member, the aperture of which was charged and discharged from an external circuit source electromagnetically or electrostatically.
In the original DeForest patent, both magnetic and electrostatic means for applying a third electrode current were included and in some of the DeForest cases a horseshoe-shaped magnet was wire wound and placed in such a position that its field would influence the electron flow or cathode stream in the tube.
It must be remembered that the "cathode ray" of 20 years ago, having a relatively slow electron flow, has become known as an electron stream today, and we have learned that this flow is composed of tiny electrons traveling from the heated filament to the positively charged plate, which because of the law of opposites exerts an attracting influence on the negative electrons.
At the time of the filing of the DeForest claims, and before, this truth about the character of flow was not known, and Dr. DeForest himself admitted that he could not explain the actual character of the phenomenon he had discovered and filed patent applications on.
The variations from the changing charges upon the surfaces of the control electrode produced corresponding changes in the external circuit of the collector electrode just as we know of it in modern radio tubes.

The patent was filed on March 4, 1906, in the German Imperial Patent Office three months, twenty-two days before DeForest's, and issued November 19, the same year, for a Cathode Ray Relay. The patent papers were received and stamped in the United States Patent Library, December 19, 1906, making them a matter of public record in the United States on that day. The direct translation clearly states that the invention relates to a device for releasing energy in a relay manner, and it definitety mentions telephone and telegraph relay purposes as its object, as well as the carrying of wireless telephone messages, the amplifying and building up of speech and music transmission, and telephotography. Von Lieben also clearly states that he desires to utilize a vacuum in which an electronic stream will be emitted in a circuit with " potential of 200 volts. This clearly outlines the degree of vacuum in use today in the modern radio receiving tube concerning which a controversy between Arnold and Langmuir is now raging, there being two conflicting patent claims before our Patent Office Board of Examiners, in which the use of a "hard" vacuum is claimed. These patents on the vacuum question have not been issued.

It will be noted in the circuit shown by Von Lieben that he provides for a "B" battery, and for a "C" battery in the external circuits connected to the tube electrodes. He also takes up the proposition of transmission and mentions the use of a microphone in the grid circuit, even showing its position. Another point Js that the external circuit of the collector electrode shows the positive terminal of the "B" battery connected to the plate, and that a means of feed-back is shown in "apparatus A," though not claimed in the patent specifications.

Altogether, this Von Lieben patent has a very strong bearing on the development of the vacuum tube today, for it shows a structure which will be very important in the coming 110-volt receiving tubes which are due to be placed upon the market within a short time. The Wehnelt oxide coating as a source of electron emission has withstood the test of science for more than 20 years. The Western Electric Co. has practically adopted the Wehnelt coating in every department of their tube work. In manufacturing tubes which utilize Wehnelt-coated sources of emission, the craftsman makes a compound of strontium and barium oxides with resin, wax or paraffin, in

a sort of soft, doughy solution. This compound is painted on strips of platinum-iridium alloy. As each painted coat dries, it is burned off, and successive coats applied and burned off until as many as 12 to 15 layers of oxide are applied, the final piece being used as a vacuum tube filament. This class of Wehnelt filament is known as the "dull emitter" type, since a very small amount of electrode heat is required to generate a heavy stream of electrons. In the recent attempts by manufacturers to create a design of 110-volt tube, the Wehnelt coat, heated from an electrically disconnected source, has been tried with fairly good results. In this type, glass tubes electrolytically coated with silver upon which the oxides are deposited are used as sources of emission. The heating element in such tubes is located adjacent to the emitter and operated from a 110-volt current supply.

Index

Next page