Stone had pointed out that the tuning of the antenna circuits shown in the prior art did not of itself afford sufficient selectivity. It was for that reason that he used the tuned closed circuit in association with the antenna circuit. But in the face of his emphasis on the desirability of tuning the transmitting and receiving apparatus, we cannot impute to him an intention to exclude from his apparatus the well known use of tuning in the antenna circuits as an aid to the selectivity which it was his purpose to achieve. The inference to be drawn is rather that he intended the tuned closed circuits which he proposed to add to the then known systems of radio communication, to be used in association with any existing type of vertical wire antenna circuit, including one so constructed as to be either resonant to a particular frequency, or adjustably resonant to any desired frequency, both of which involved tuning.

Stone's full appreciation of the value of making all of his circuits resonant to the same frequency is shown by his suggestion to insert, between the closed and antenna circuits at the transmitter and receiver, one or more additional closed circuits, so constructed as to be highly resonant to the particular frequency employed. He says that the purpose of such an intermediate circuit is "to weed out and thereby screen" the antenna circuit at the transmitter and the detecting device at the receiver from any harmonics or other impurities in the wave structure.

He states: "This screening action of an interposed resonant circuit is due to the well known property of such circuits by which a resonant circuit favors the development in it of simple harmonic currents of the period to which it is attuned and strongly opposes the development in it of simple harmonic currents of other periodicities." His original application thus disclosed the advantage, where vibrations created in one circuit are to be impressed on another, of making the latter circuit resonant to the same frequency as the former, in view of the "well

known property" of a resonant circuit to favor the "development" in it of forced vibrations of the same frequency as its natural periodicity.

Stone's application shows that these principles of resonant circuits were no less applicable to the antenna circuit, and suggests the use of "any suitable device" to "develop" in the antenna circuit the "simple harmonic force impressed" upon it. It was then well known in the art that every electrical circuit is to some degree resonant to a particular frequency to which it responds more readily and powerfully than to others. Although the degree of resonance attained by a vertical wire is small, its natural resonance is no different in kind from that of a closed circuit such as Stone's screening circuit. Stone recognized this in his application. In describing the complex natural vibrations set up by a sudden discharge in an antenna circuit, such as that commonly used at the time of his application, Stone said that "the vibrations consist of a simple harmonic vibration of lower period than all the others, known as the fundamental with a great variety of superimposed simple harmonics of higher periodicity superimposed thereon." And he says that the oscillations developed in the charging circuit of his system "induce corresponding oscillations in the vertical wire, which are "virtually" forced vibrations, and "practically independent, as regards their frequency, of the constants of the second circuit in which they are induced"-a plain recognition that the antenna circuit has electro-magnetic constants which affect its natural periodicity, and that that natural periodicity does have some effect on the frequency of the vibrations impressed upon the antenna circuit."

14 Stone's recognition of the similarity between his antenna circuit and his screening circuit is further shown by his direction that the coupling between the screening circuit and the charging circuit, like that between the antenna and charging circuits where no screening circuit is used, be loose. See note 12, supra.

Thus Stone did not, as the Marconi Company suggests, say that the antenna circuit had no natural periodicity. He recognized that its natural periodicity was less strongly marked than that of his closed circuit, and hence that the wave structure could be greatly improved by creating the oscillations in a closed circuit such as he described. But he also plainly recognized that the antenna circuit, like his screening circuit, was a circuit having a natural period of vibration which would therefore be more responsive to impressed oscillations of that same periodicity. Since he had previously said that "any suitable device may be employed to develop the simple harmonic force impressed upon the vertical wire," we think that Stone's specifications plainly suggested to those skilled in the art that they avail themselves of this means of developing in the antenna this simple harmonic force, and that they tune the antenna circuit in order to improve the strength and quality of the "forced" vibrations impressed upon it.

The Marconi Company argues that Stone's theory of "forced" oscillations presupposes that the open transmitter circuit be untuned. It is true that Stone said that such "forced" oscillations have a period of vibration which is "independent of the electrical constants of the circuit" on which they are impressed. But the fact that the "forced" vibration will retain its natural period whatever the frequency of the antenna circuit may be, does not preclude, as Stone showed, the tuning of that circuit so as to achieve maximum responsiveness to the vibrations impressed upon it. Stone's specifications indicate that he used the term "forced" merely as meaning that the vibrations are developed in another circuit and then transferred to the antenna circuit by inductive coupling, as distinguished from "natural" vibrations which originate in the antenna or radiating circuit-in short that "forced" is merely used as a synonym for "in

1

Opinion of the Court.

duced." Thus he states in describing the operation of his transmitter, "The high frequency current . . . passing through the primary I1 [of the antenna transformer] induces a corresponding high-frequency electromotive force and current in the secondary I, and forced electric vibrations result in the vertical conductor v... Hence there is ample support for the finding of the court below that

99 15

"By free oscillations is meant that their frequency was determined by the constants of the circuit in which they were generated. The Stone application as filed impressed these oscillations upon the open circuit, and therefore used 'forced' oscillations in the open circuit of the transmitter, that is, the frequency of the oscillations in the open circuit was determined by the frequency of the oscillations in the closed circuit.

"The effect of forcing vibrations upon a tuned and untuned circuit may be likened unto the effect of a tuning fork upon a stretched cord in a viscous medium. When the cord is vibrated by the tuning fork it has the same period as does the fork regardless of whether such period be that of the natural period of the cord, but when the fork vibrations are in tune with the natural period or

15 Stone's language here makes it plain that throughout his allusions to a frequency developed in one circuit as being "impressed" or "forced" on another circuit when the two circuits are coupled through a transformer, are used figuratively or metaphorically only as synonymous with "induced." Scientifically the oscillations in the charging circuit are not impressed or forced on the other. The stress in the magnetic field of the first circuit sets up or induces corresponding stresses in the magnetic field of the other circuit. The resulting frequency in the second circuit is affected both by the frequency of the oscillations in the charging circuit and the inductance and capacity in the second circuit. The result may be the superposition of two frequencies in the second circuit. This may be avoided and a single frequency developed, as Stone showed, by tuning the second circuit so as to be resonant to the frequencies created in the first.

Opinion of the Court.

320 U.S.

fundamental of the cord, then the amplitude of vibrations in the cord is a maximum."

Thus Stone's application, prior to Marconi, showed a four-circuit system, in which the oscillations were produced in a closed charging circuit and impressed on an open antenna circuit in the transmitter, and were similarly received in an open antenna circuit and by it induced in a closed circuit containing a detector. He showed the effect of resonance on the circuits resulting from their tuning to a desired frequency, and emphasized the importance of making the transmitting and receiving apparatus resonant to that frequency.

Stone's patent,16 granted a year and a half before Marconi-although after Marconi's application was filed— makes explicit, as the patent law permits, what was implicit in Stone's application. By amendments to his specifications made April 8, 1902, he recommended that the frequency impressed upon the vertical conductor at the transmitter "may or may not be the same as the natural period or fundamental of such conductor" and that the antenna circuit at the transmitter "may with advantage be so constructed as to be highly resonant to a particular frequency and the harmonic vibrations impressed thereon may with advantage be of that frequency." Since Stone used a variable inductance to alter at will the frequency of the charging circuit, this direction plainly indicated that the frequency of the antenna circuit might also be variable, and suggested the inclusion of the wellknown Lodge variable inductance in the construction of the antenna circuit to achieve that result. And since Stone had specified that "by my invention" the operator at the receiving station is able to "adjust" the receiving

16 At the insistence of the Patent Office Stone divided his original application, and was granted two patents, No. 714,756 for a method and No. 714,831 for apparatus. The former is the one particularly relied on here.