Doctor Wood. Modern Wizard of the Laboratory: The Story of an American Small Boy Who Became the Most Daring and Original Experimental Physicist of Our Day-but Never Grew Up

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.