Captain Bougier was working on a device for determining the direction from which hostile aircraft were approaching by causing the sound vibrations falling on two widely separated horns to vibrate two light mirrors mounted at right angles to each other; a beam of light reflected from one mirror to the other and then to a screen traced a more or less complicated curve known to physicists for the past half century as a Lissajous figure, from the shape of which the direction of the source of the sound could be determined. I made a slight improvement in this apparatus by placing the mirrors closer together and viewing a minute source of light directly in the second mirror. The French now needed an instrument imitating the sound of an airplane for testing these and other direction finders. I said, “Why not use an old airplane?” (This same problem came up years later in the broadcasting studios. The sound-effect experts had spent fruitless days in searching for something that would imitate the sound of an opening or closing door, and finally agreed that the only thing that would imitate the sound perfectly was a door; in every studio you now see a little door about three feet square, with handle and latch complete, on a frame which rolls on rubber- tired wheels. Opening and shutting this does the trick.) Some objection was raised against this obvious solution, and I constructed in a half hour or so a horn made by separating the trumpet and sound box of a “Strombos” auto horn, operated by compressed carbon dioxide, and inserting a brass tube about three feet long between the parts. This, when operated by the compressed gas, produced a low note of about 120 vibrations a second, and imitated the low hum of an airplane quite perfectly. They liked this very much, as it could be carried anywhere under the arm. When operated in the laboratory the effect was very peculiar. Stationary waves were produced. At some places its roar was very loud, and at other places only a few feet away there was almost complete silence. We used to poke it out of the window at noon and turn it loose, and the crowds going to lunch down the “Boul. Mich”. would stop and gaze skyward in alarm. (This was the progenitor of the subaudible horn I made later for John Balderston for stage effect.)
Professor Jean Perrin, Nobel laureate, now disguised as a Commandant in horizon-blue uniform with red and gold tabs, but, with his white hair and beard and perpetual good humor, looking more like Santa Claus than an officer, dashed back and forth between his laboratory and the proving ground at St.-Cyr in a military car driven at a furious pace and squeaking “toot-toot” every five seconds like a Paris taxi. He was testing his gigantic “loud-speaker” or honeycomb horn, as we called it. Hundreds and hundreds of little hexagonal horns were gathered together on a plane like the cells of honeycomb, with tubes of equal length leading to a single mouthpiece, the idea being that the sound would emerge from each trumpet at the same instant, and consequently would go off into space as a parallel beam like the rays of a searchlight. It was a terrific contraption, with its tangled network of twisted brass tubes, and did not work much better, it seemed to me, than the big ten-foot megaphone with which we used to sass the policemen two or three blocks away when I was a student at Johns Hopkins. After the armistice I tried to induce the French to present one of these to the War Museum of the Smithsonian Institution, but they wanted three thousand dollars for it!
The great flat collection of small hexagonal trumpet mouths must have been eight or ten feet in diameter. It was pointed down the field, and a narrow-gauge railway led away from it, on which operated a hand car, with two officers, armed with pens and notebooks, who recorded the distance at which they could hear speech correctly. The device was designed to enable a commander to give orders during the din of battle. How this gigantic acoustic engine on its great truck would have fared in battle seems open to question. “Gutenberg soixante-quatorze deux zéros” bellowed Perrin through the cells of the honeycomb. The observers, three hundred yards away, entered this Paris telephone number in their ledger, and drew away, pumping their hand car vigorously. “Louvre quatre-vingts soixante et un” thundered Jove again. This went on for some time, when the hand car dashed back to report observations, and I, who had been standing directly in front of the horn, told Perrin I had been learning French by a surgical operation.
Then there was Chilofski, who was experimenting with a seventy-five millimeter shell fitted with a slender rod in front, at the tip of which a flame of burning phosphorus streamed back over the shell during its flight. This was supposed to decrease the air resistance and increase the range. Since he could not fire the shells in his little laboratory from a “seventy-five”, he mounted them on the arm of a “dynagraph” and secured records of the pressure exerted by a blast of air having a velocity of 1,200 feet per second, with and without the flame. These tests showed a marked decrease in the pressure, but ballistic experts have since told me that an equal decrease could be obtained by giving the shell a long, tapering point.
The work of Professor Paul Langevin was much more promising, however. He was developing a method of locating submarines by sweeping the sea, under water, with a narrow beam of high-frequency sound waves, and picking up the “echo” reflected from the submarine by suitable electrical apparatus. As I had asked permission to devote particular attention to this work, I spent more time with Langevin than with the others. We went together to the Naval Arsenal at Toulon where the apparatus was in operation. The source of the supersonic vibrations was a system of square quartz plates properly oriented and cemented side by side to a steel disk. The quartz plates have the remarkable property of expanding and contracting when the opposite sides are put in electrical contact with the terminals of a high potential electrical generator, at the same frequency as that of the electrical oscillator. In this way sound waves of such high frequency can be caused to radiate from the steel disk that, instead of spreading out in all directions, as do audible sound waves, they are projected in a narrow beam. We saw fish die and turn belly up when they swam across the beam, and if a hand was held in the water in front of the plate, there was a painful burning sensation in the bones.
Throughout all this time, Wood’s chief, General Russell, who had the military martinet’s horror of anything that savored of free-lancing, had been trying to hold Wood in one groove. He had sensed, however, the great importance of Langevin’s work, and had willingly let Wood give all the time he wanted to that. The Creusot gun works had asked the Bureau of Inventions for suggestions on a method to measure the pressure in high caliber guns, from point to point, as the shell traveled along the barrel. Wood suggested the insertion of piezoelectric cylinders of quartz, each of which would give out an electrical impulse of magnitude proportional to the applied pressure. This method is standard procedure today, and is generally ascribed to Sir J. J. Thomson, who developed it independently in England a year or two later. It was fortunate for pure science that General Russell gave Wood free rein with Langevin, for it led later to the important researches in supersonic vibrations which were carried out by Wood and Alfred Loomis in the latter’s laboratory at Tuxedo Park in 1927.
There was a good deal of shuttling of scientific and technical officers back and forth across the pond, and toward the end of the year Wood began to feel that he could obtain better laboratory facilities and consequently be more useful for a while back in America. So he applied for transfer, and arrived in New York in January, 1918.
He stopped in on Professor Michael Pupin of Columbia University, the great electromechanical wizard, who was working for the Navy on submarine detection. Pupin was interested in hearing of Wood’s work with Langevin and spent some time, with his staff, getting the details of the piezoelectric quartz vibrations.