Bohr, the great Danish physicist, whose explanation of the Balmer series of hydrogen had created a sensation at the Birmingham meeting of the British Association, had told me that he believed that the absence of the higher members of the series in vacuum tubes might result from the circumstance that the atoms were too close together to allow room for the larger electronic orbits, which, on his new theory of radiation, were responsible for the shorter ultraviolet radiations, whereas in hydrogen stars there might be room for these orbits owing to the lower pressure of the gas. This didn’t seem a promising line of attacking the problem in the laboratory, since the luminosity in a vacuum tube decreases enormously as the pressure is lowered, but I decided to give it a trial.
To make up for the loss of light, which was sure to result from the lowering of pressure, I made a tube over three feet in length. The two ends, terminated by large bulbs for the electrodes, were bent at a right angle, so that light from the entire tube could escape from a small, thin-walled bulb blown at the bend. The tube was excited by a powerful high-potential transformer, but when pumped to very low pressure showed only two or three of the Balmer lines and a host of the hundreds of lines that we now know are due to molecular hydrogen. This was clearly the wrong idea, but at higher pressures, the lines that I wanted came out much stronger, and the other lines grew fainter, and conditions seemed to be improving from day to day. Moist hydrogen was flowing into the tube all the time through a long tube the size of a horsehair, and the pump was working at the other end continuously. On the third day the central part of the tube was shining with a fiery purple color of almost unsupportable brilliancy, the spectroscope showed that only the Balmer series of lines were being emitted, and I eventually succeeded in photographing twenty- two members of the series, more than doubling the number previously found in the laboratory. Further study showed that the improvement resulted from the circumstance that only hydrogen atoms were present in the tube. These emit the Balmer lines, while the molecules, which consist of two atoms bound together, give a very complicated spectrum made up of thousands of lines. These, and the continuous background which accompanies them, were the cause of the obliteration of the Balmer lines of shorter wave length in all previous work. As the work proceeded I found that the success of the operation resulted from the fact that the atomic hydrogen formed by the powerful discharge could combine into the molecular gas only by coming in contact with the walls of the tube or the aluminum electrodes. The central part of the long tube was so far removed from the electrodes that they were inoperative in this region, and the water vapor, which entered along with the hydrogen, formed a film on the walls, which “poisoned” them, as Langmuir pointed out, so that they were no longer operative in recombining the atoms into molecules.
The most remarkable observation of all was made when a short loop of fine tungsten wire had’been mounted in a short side tube, for another experiment. It was to be heated white hot by a storage battery to see whether shooting free electrons into the discharge would have any effect. To my amazement the wire remained white hot after I had opened the switch to the storage battery, though it was not in the line of the discharge, but in a little side tube. Aston, the English physicist, happened to come into my room at the moment, and opened his eyes when he saw what was happening. He suggested that a parasitic discharge might be flowing from the main current to the battery, which was still connected by one wire with the tungsten filament, so I disconnected both wires where they were attached to the tungsten; but the filament continued to shine like an automobile lamp. It turned out that the tungsten was causing the recombination of the hydrogen atoms into molecules, and the heat of recombination was sufficient to maintain the wire at a white heat. The results of these experiments were published in two papers in the Proceedings of the Royal Society.
Shortly afterwards I demonstrated the effect before the research staff of the General Electric Company at Schenectady, making the vacuum tube on the spot. As I showed it here the tungsten filament was mounted in the side tube leading to the pump. The pressure was only about 1/700 of that of the atmospheric, and the atomic gas was practically at room temperature, yet the wire was kept at incandescence by a cold stream of atomic hydrogen. Dr. Langmuir was much intrigued and began to speculate on what could be accomplished with a stream of the gas at atmospheric pressure. His speculation led to an important invention, for in less than six months he took out a patent for an atomic hydrogen welding torch, which proved of immense value, since all sorts of metals could be welded in a hydrogen atmosphere without showing flaws or blowholes.
It was as a consequence of Wood’s scientific zest and social strenuousness that fate brought him, about this time, the facilities of a great private laboratory backed by a great private fortune. He had met Alfred Loomis during the war at the Aberdeen Proving Grounds, and later they became neighbors on Long Island. Loomis was a multimillionaire New York banker whose lifelong hobby had been physics and chemistry. Loomis was an amateur in the original French sense of the word, for which there is no English equivalent. During the war, he had invented the “Loomis Chronograph” for measuring the velocity of shells. Their friendship, resulting in the equipment of a princely private laboratory at Tuxedo Park, was a grand thing for them both. To say that the conjunction was like that of Leonardo da Vinci and Lorenzo the Magnificent would be a wrong comparison, since Wood’s nature is such that not even God Almighty could ever be a patron to him.
A happy collaboration began, which came to its full flower in 1924. Here is Wood’s story of what happened.
Loomis was visiting his aunts at East Hampton and called on me one afternoon, while I was at work with something or other in the barn laboratory. We had a long talk and swapped stories of what we had seen or heard of “science in warfare”. Then we got onto the subject of postwar research, and after that he was in the habit of dropping in for a talk almost every afternoon, evidently finding the atmosphere of the old barn more interesting if less refreshing than that of the beach and the country club.
One day he suggested that if I contemplated any research we might do together which required more money than the budget of the Physics Department could supply, he would like to underwrite it. I told him about Langevin’s experiments with supersonics during the war and the killing of fish at the Toulon Arsenal. It offered a wide field for research in physics, chemistry, and biology, as Langevin had studied only the high- frequency waves as a means of submarine detection. Loomis was enthusiastic, and we made a trip to the research laboratory of General Electric to discuss it with Whitney and Hull.
The resulting apparatus was built at Schenectady and installed at first in a large room in Loomis’s garage at Tuxedo Park, New York, where we worked together, killing fish and mice, and trying to find out why and how they were killed, that is whether the waves destroyed tissue or acted on the nerves or what.
The generator was an imposing affair. There were two huge Pliotron tubes of two kilowatts output, a huge bank of oil condensers, and a variable condenser with intersecting wings of the type familiar to every amateur radio operator, but about six feet high and two feet in diameter. Then there were the induction coil for stepping up the voltage and the circular quartz plate with its electrodes in an oil bath in a shallow glass dish. With this we generated an oscillatory electric potential of 50,000 volts at a frequency of from 200,000 to 500,000 alternations per second. This oscillating voltage applied to the electrodes on the quartz plate caused it to expand and contract at the same frequency, and generate supersonic waves in the oil, the pressure of which against the surface of the oil raised the thick liquid in a mound nearly two inches in height, surmounted by a fountain of oil drops some of which were projected to a height of a foot or more. We could conduct the sonic vibrations out of the oil into glass vessels and rods of various shapes by dipping them in the oil over the vibrating plate, and found they could be transmitted along a glass thread the size of a thick horsehair to a distance of a yard or more. If the end of the thread was held lightly between the thumb and finger, no sensation was produced, but if it was pinched it felt almost red hot, and in a second the skin was burnt white in the form of a groove. A thin glass rod when carrying the waves and pressed firmly against a pine stick caused it to emit smoke and sparks, the rod burning its way through the wood, leaving a hole with blackened edges. If a glass plate was substituted for the pine stick, the vibrating rod drilled its way through the plate, throwing out the displaced material in the form of a fine powder or minute fused globules of glass. If the waves were passed across the boundary separating two such liquids as oil and water or mercury and water, more or less stable emulsions were formed. Blood corpuscles were exploded, the red coloring matter escaping and staining the saline solution with which it had been mixed, making a clear transparent red like an aniline dye. These and a host of other new and interesting effects were discovered in the first two years of our experiments.