Special Findings of Fact 123 C. Cls. In all cases the external radio frequency field was of such a small value as not to cause any serious interference with a properly installed radio receiver adjacent the ignition system. The ground return path in the Government structure formed by the engine block functions to provide a return path for a portion of the unidirectional component and also a portion of the radio frequency component. PRIOR ART 37. The following patents and publications were available to those skilled in the art prior to October 21, 1926, the filing date of the patent application which matured into the patent in suit: Article by R. L. Smith-Rose entitled "Screening of an Oscillator," published June 15, 1922, in Proceedings of The Physical Society of London (defendant's Exhibit 6). Article entitled "Precautions Against Magneto Interference" appearing in the book Direction and Position Finding, by R. Keen, published 1922 by Wireless Press, Ltd. (defendant's Exhibits 1 and 1a). British patent to Kenelm Edward Lee Guinness, No. 166,680 accepted July 18, 1921 (defendant's Exhibit 2). British patent to George Edward Bairsto and Fritz Gordon Haydon, No. 230,902, accepted March 17, 1925 (defendant's Exhibit 3). Article entitled "Wireless Navigation for Aircraft" by Capt. J. Robinson appearing in The Electrician, for October 10, 1919 (defendant's Exhibit 4). Article entitled "The Elimination of Magneto Disturbance in the Reception of Wireless Signals on Aircraft" by Capt. James Robinson, published December 1919 in The Radio Review (defendant's Exhibit 4a). The Year-Book of Wireless Telegraphy & Telephony, published 1922 by The Wireless Press, Ltd., London, Article by H. C. Van de Velde entitled "The Progress of Wireless Telephony in Aircraft" (defendant's Exhibit 5). U.S. Patent to E. R. Gill, No. 761,372, granted May 31, 1904 (defendant's Exhibit 7). U. S. Patent to Henry D. Gardy, No. 873,123, granted December 10, 1907 (defendant's Exhibit 8). Handbook entitled "Marconi Aircraft Telegraph and Telephone Apparatus: Its Installation, Testing and Maintenance" prepared and printed in 1921 by Mar 591 Special Findings of Fact coni's Wireless Telegraph Co. Ltd. (defendant's Exhibits 15 and 22). British patent to Lefroy No. 235,351 sealed September 9, 1925 (defendant's Exhibit 38). 38. The Smith-Rose article (defendant's Exhibit 6) discusses in considerable detail the "screening" 5 of a radio oscillator. A radio oscillator is a device for generating radio frequency currents, and the screening of this device discussed in this article was for the purpose of preventing, as far as possible, radio frequency emanations from affecting adjacent radio receivers. The experiments described in this article relate to the shielding of the oscillator by placing the same in a complete metallic box-type enclosure. The article discusses the relative shielding effects when the enclosure is made of copper, galvanized iron, or thin brass wire gauze. For complete interception of the radio frequency emanations the article recommends that the sheet copper for the box and its lid be of a thickness of at least 1" and that the lid should be soldered on the box so that no crack or joint is left for leakage. Leakage effects varied at different frequencies. In one experiment that is described, leakage of radio frequency energy from the box was easily detected through a slit 11⁄2" long. This article teaches those skilled in the art that the shielding or screening of radio frequency energy by a metallic enclosure is a matter of degree and varies with the frequency involved, on the particular metal used for the shielding, its thickness, and upon the completeness of closure of all openings, cracks, or joints. 39. The Keen article (defendant's Exhibit 1) discusses problems relating to radio interference caused by radiations from the ignition systems of aircraft engines. The article states The final solution was found to be the complete screening of the whole of the ignition system, including the magneto, the leads to the sparking plugs, the plugs themselves, and the controls from the magneto to the cockpit. Analogous to the American technical term "shielding." Special Findings of Fact 123 C. Cls. A Rolls-Royce Eagle engine, with screened ignition, is shown in Fig. 242, installed in a Short flying boat. A metal screen completely encloses the magneto, and the wiring is covered with copper braid. If the screening is stopped at the point where the high-tension lead is connected to the plug, the radiation from the plug and short length of exposed lead is enough to prevent wireless reception. Fig. 243 shows a specially screened plug developed by the Marconi Company and the Robinhood Engineering Works, which has proved an efficient remedy for the trouble. The engine in Fig. 242 is fitted with these plugs. The cylinder wall forms the return path from the plug to the engine casing, and the interference due to this varies considerably with the type of cylinder construction. Steel and iron cylinders offer the greatest impedance to the high-frequency oscillatory currents, and are the worst offenders; but an improvement is effected by fitting a copper bonding between the body of the plug and the engine framing. Less trouble is noticed in the case of aluminum cylinders having a steel liner; a copper water jacket also acts as an efficient low-impedance path. When it is necessary to screen an ignition system which has not been designed with a view to the possible requirements of the wireless equipment, the best method is to enclose the wiring in copper gauze, put on in strips, the seams soldered in as many places as possible and well bonded to the earth trunk. If the ignition wiring is taken through aluminum tubes at any point, this will provide efficient screening, but the tubing must be bonded by copper gauze at each end to the main earth. In the case of large or multi-engined machines in which the wiring is carried to the engines through stream-lined control spars, the spars should be covered with gauze and the joints soldered, the whole covering being well secured with binding wire. The spark plug illustrated in this article has a metallic screen extending from the braided shielding on the ignition wire to the external shell of the plug, this plug being similar to that referred to in the subsequent finding. The Keen article teaches those skilled in the art that the whole of the ignition system should be completely screened 591 Special Findings of Fact or shielded and that any discontinuity in the screening of the ignition system results in the leakage of radio frequency emanations. It also teaches that the shielding be grounded to the engine at the spark plugs and that it be bonded or grounded at other points. The Keen article indicates that the return path for the ignition current is through the cylinders and it does not indicate any connection between the return terminal of the magneto and its enclosure which would permit current to return to the magneto by means of the ignition cable shielding. 40. British patent to Guinness #166,680 (defendant's Exhibit 2) relates to shielding of spark plugs and leads in order to prevent disturbance of wireless-receiving apparatus on aircraft. The specification states as follows: The present invention therefore provides for a sparking plug, a metallic screen constructed to extend between the body thereof and the source of supply therefor in such a manner as to afford an electrically continuous path of substantially tubular form surrounding the plug terminals and the supply conductor thereto between the said limits. It is desirable to obtain screening that shall be "electrically complete," that is to say, that shall effectually prevent disturbance by the ignition system for the engines, of wireless apparatus used for signalling purposes. This invention therefore further provides for a sparking plug, a metallic screen constructed to be of sufficiently complete physical continuity in all directions to afford "electrically complete screening" for the portions which it encloses. It is furthermore within this invention to provide at the generator end of the supply conductor a screening sheath similar in function and construction to that provided at the plug end of the conductor. Fig. 1 of the patent, which illustrates in detail the connection of the ignition cable and the concentric shielding therefor with a spark plug, is reproduced herewith. This patent, which relates to details for connecting the shielded cable to the spark plug, suggests to those skilled in the art that a similar method of connection should be used at the generator end of the spark plug lead, that is to say, the connection between the output terminals of the ignition generator should be similar to those shown in connection with the spark plug and suggests that the shielding should be continuous. There is no disclosure of the ignition generator or the character of its shielding and no indication of any connection of the shielding to the return terminal of the ignition generator so that it could be utilized for the return circuit from the plug. 41. British patent to Bairsto, et al., No. 230,902 (defendant's Exhibit 3) relates to improvements in the shielding of electrical ignition systems on aircraft. More specifically, this patent relates to the details of the metallic connection between the braided metal shield of the spark plug cables, the metal shield of the magneto distributor and the outer shell of the spark plug. The details of this connection disclosed in the patent include securing the braid between an inner and outer metallic sleeve having rounded flanges or edges. It is stated that this avoids source of damage to the insulation. The specification refers to the "distributor screen" being made an integral part of the distributor by means of stops or insets molded into the distributor insulation. Such construction is shown in Fig. 6 of the patent, reproduced herewith. |