De Forest Radio Co v. General Electric Co 1931

• Decision Date: 25 May 1931
• Docket Number: No. 630,630
• Parties: DE FOREST RADIO CO. v. GENERAL ELECTRIC CO. Argued May 1-4, 1931
• Court: U.S. Supreme Court

283 U.S. 664

51 S.Ct. 563

75 L.Ed. 1339

DE FOREST RADIO CO.

v GENERAL ELECTRIC CO.

No. 630.

Argued May 1-4, 1931.

Decided May 25, 1931.

Rehearing Denied, Opinion Amended Oct. 19, 1931.¹

Messrs. Samuel E. Darby, Jr., of New York City, and Thomas G. Haight, of Jersey City, N. J., for petitioner.

[Argument of Counsel from pages 665-666 intentionally omitted] Mr. Ralph B. Evans, of Philadelphia, Pa., for respondent.

[Argument of Counsel from pages 667-669 intentionally omitted] Mr. Justice STONE delivered the opinion of the Court.

Certiorari was granted, 282 U. S. 836, 51 S. Ct. 333, 75 L. Ed. —, to review a judgment of the Court of Appeals for the Third Circuit, holding the Langmuir patent, No. 1,558,436, granted October 20, 1925, for 'electrical discharge apparatus and process of preparing and using the same,' valid, and infringed by petitioners. The District Court for Delaware, in which respondent, the assignee of the patent, brought suit for infringement, held the patent invalid for want of invention, and because of prior use and prior invention, and gave judgment dismissing the complaint, 23 F.(2d) 698, which the appellate court at first affirmed, and then, on reargument, reversed. 44 F.(2d) 931.

Infringement is conceded if the claims of the patent are valid. It is known as a high vacuum tube patent, and the alleged invention is exemplified in high vacuum tubes of familiar use as detectors or amplifiers in the art of radio communication and telephony. Correct appreciation of the contentions made requires, at the outset, an understanding and some exposition of the scientific principles which it is agreed are brought into play in the high vacuum tube or which at least are accepted as working hypotheses accounting for its operation.

A radio tube of the audion or three electrode type consists of a bulb, within which a vacuum has been created, enclosing a filament, which is a negative electrode, or cathode; a plate, which is a positive electrode, or anode; and a third electrode, known as a grid, located between the filament and the plate. The grid is connected with an input circuit, over which electrical radio activity, actuated at the sending station, is gathered from the ether and passes to the grid. When the tube is used as an amplifier, the plate is connected in circuit with a telephone receiver or loud speaker. In operation, the filament is heated to incandescence by passing an electric current through it. In its incandescent state, electrons, or negative charges of electricity, are developed at the filament and pass to the plate, attracted to it by its positive potential, and cause a flow of electricity through the plate loud speaker circuit. The sounds given out by the loud speaker are produced by variations in the current passing to it. Radio amplification depends on producing in the more powerful current of the loud speaker circuit, variations exactly corresponding to the variations in th wea ker input or voice current which are actuated by the sending station.

In the vacuum tube of the three electrode type, this is accomplished by passing the input or voice current over the grid. Variations in that current produce variations in potential of the grid which, by reason of its location between the filament and plate, effects like variations in the effective potential of the plate with corresponding variations in the loud speaker circuit. The number of electrons emitted by the filament is determined by its temperature. But the current passing through the plate loud speaker circuit depends on the number of electrons drawn from the filament to the plate, and this in turn depends on the voltage of the current passing to the filament. When it is high enough to force all the electrons emitted by the filament to pass from filament to plate, increase in the voltage at the filament will not produce an increase in current in the loud speaker circuit and the tube is then said to be 'saturated.' As successful operation of the tube depends on the response of the loud speaker current to changes in voltage effected by the voice or input current, the tube is most efficiently operated at a voltage of a range below saturation, and a current within this range is known as the 'space current.'

Of critical importance in the present controversy is the effect of the presence of gas within the tube. As in the practical art of bulb manufacture no scientifically perfect vacuum can be attained, air or other gas is always present within the vacuum tube. This consists of a small amount of residual gas, after the vacuum is created by pumping out the tube in the process of manufacture. There is also gas in the walls of the bulb and the electrodes, described as 'occluded,' which, if not expelled from them and removed in the course of manufacture, is later freed in varying amounts when the tube is in use, by the action of the heat of the filament and the electrons generated there.

The passage of electrons from filament to plate at certain voltages produces changes in the gas, known as 'ionization.' Ionization is the manifestation of a rearrangement of the constituent electrons of the gas atoms which occurs, in low vacuum tubes, if other factors of causation remain constant, at known voltages within a range of from 20 to 30, but varying somewhat with different gases. The atom, according to present day scientific theory, is composed of an electrically positive nucleus, around which revolve at high speeds electrically negative electrons. In its normal state, the atom, whose nucleus and electrons are in electrical balance, exhibits no electrical effects; but within a thermionic tube, the impact upon the gas atoms of the electrons, passing from cathode to anode at velocities induced by ionization voltages, forces off negative electrons from the atoms. The atoms from which the electrons have thus been detached are then electrically positive and are known as ions. Ionization, which begins at the ionization voltage, is increased with increasing voltages as the tube approaches saturation, when extreme ionization takes place; and, for reasons which need not be elaborated here, the tube then ceases to function as a radio tube, a condition visibly manifested by a blue glow within it.

Gas ionization in the vacuum tube is of great practical importance because of its effect on the current passing from filament to plate. Ionization, when it occurs, may operate within the range of the space current to increase 'conductivity' of the tube, that is, the discharge from filament to plate, above what it would be at the same voltage in the absence of ionization, through the development of the positive ions, which pass to the cathode, and of the negative electrons, which pass to the anode. The ions facilitate the flow of electrons from cathode to anode and increase their number by impact on the former, which raises its temperature. The result is that, in low vacuum tubes, saturation with the blue glow effect is reached, other fators remaining constant, at lower voltages than in high vacuum tubes. Hence, in the range of voltage above ionization and below saturation, within which the tube is commonly operated, a low vacuum tube, because of the increase of current due to ionization, is more responsive to slight changes in voltage produced by the operation of the grid or input current. In consequence, the low vacuum tube is more sensitive both as a detector and as an amplifier than a tube of high vacuum.

But this advantage is accompanied by a serious disadvantage, especially when the tube is used for amplification, in that ionization produces variations in the electronic discharge from filament to plate, which correspondingly affect the current passing through the loud speaker circuit. Ionization is affected by amount of gas in the tube, and hence by the degree of vacuum and the amount of occluded gas freed in operation by heat and bombardment. Since the discharge varies with the amount of ionization, the effective current in the loud speaker circuit varies with different tubes and with the same tube at different times; and critical adjustments of the current flowing to the filament are necessary to improve operation.

From what has been said, it is apparent that the problem of securing evenness or regularity of discharge from filament to plate and hence of current flowing through the loud speaker circuit is dependent upon the reduction of ionization in the tube, and this in ture is dependent, within certain ranges of limits, upon a number of variables, the more important of which are (1) the geometry of the tube, that is, its size and shape and the location of electrodes, (2) heat of the filament, (3) voltage of the filament, and (4) of vital importance here, amount of gas, that is, pressure within the tube. With the other variables controlled so as to remain approximately constant, as is practicable, reduction of pressure reduces ionization and increases steadiness of current and in turn raises the saturation point of the tube, permitting its use with higher than ionization voltages.

As in the low vacuum tube regularity or evenness of the loud speaker current was more or less imperfectly se- cured by varying the voltage at the filament with different tubes and at different times with the same tube, and desired result may also be attained, and far more effectively, by reducing the pressure in the tube and keeping other factors constant. When a vacuum is produced of as low a pressure as a few hundredths of a micron (a micron is equal to 1/1000 of a millimeter of mercury in terms of barometric pressure), the discharge is independent of the degree of vacuum, when the tube is used with appropriate space charge. The discharge then passing from cathode at constant temperature to anode, varies directly with the 3/2 power (square root of the cube) of the voltage imposed on the cathode. This is equivalent to saying that steadiness of current through the...