Not only was it necessary to exclude the roar of the engine and the rattle of the machine gun from the ears of the men receiving the radio communication, but it was also necessary to filter out these sounds from the telephone transmitter. Every person who has ever shouted into a telephone knows how sensitive the ordinary telephone transmitter is to extraneous noises. It requires no wide stretch of the imagination to hear in fancy how an ordinary transmitter would behave when beside the exhaust of a 400-horsepower Liberty engine. A brilliant line of experimentation conducted by one of the scientists at the laboratory resulted in a telephone transmitter or microphone which possessed the extraordinary quality of being insensitive to engine and wind noises and at the same time highly responsive to the tones of the human voice.
With the receiver and the transmitter perfected the scientists thought that the problem of airplane telephoning was solved; but nevertheless three months of hard work were required before the entire system could be adjusted and put in such shape that it might be considered a practical device for everyday use.
The question of weight was of utmost importance, and a structure that would adequately house and protect the delicate parts of the mechanism from the vibration and jars of flying and landing and at the same time not be too heavy for practical use on the plane was a difficult problem in mechanical design. Day after day the inventors took the mechanism up in flying machines and brought it back night after night for more work in the laboratory.
INTERIOR VIEW. AIRPLANE RADIO TELEPHONE SET BOX.
EXTERIOR VIEW OF SAME.
This was a period, however, of rapid progress. Officials appearing on Langley Field from time to time witnessed informal demonstrations of this development. In August Mr. Baker, Secretary of War, and Gen. Scott, Chief of Staff, listened to a conversation being carried on in the air, and some six weeks later Brig. Gen. Foulois witnessed a similar demonstration and from the ground directed the movements of the airplane in flight. The experimental apparatus had reached such a state of efficiency that on October 16, at Langley Field, communication by voice was carried on between airplanes in flight 25 miles apart and from airplane to ground over a distance of 45 miles. By September cables had been sent abroad telling of the progress made in this country on the development of this apparatus. Our officers abroad were skeptical and could not believe that this country could outdistance the scientists of the allies who had had three years of war experience to draw upon. By October the designers had brought the system to a perfection where they were willing to risk its use in actual war flying, and Col. Culver took to the American Expeditionary Forces in France several trunk-loads of the apparatus to acquaint those abroad with what had been done and to test the apparatus under service conditions. Meanwhile the development work continued in this country. Early in December the operation of the apparatus was exhibited in an official test at the Morraine Flying Field at Dayton, Ohio.
A large number of military and civilian officials not only of our own country but of the allies had been invited to witness this test. It must be remembered that at this time even those who had heard about the progress being made were skeptical of the possibilities of the successful adaptation of the radio telephone to airplane work. The designers of aircraft never look with favor upon additional equipment which may clutter up the machine with trailing wires and the like and possibly compel alterations in standard lines. The pilots, also, do not usually give a friendly reception to new equipment for their planes.