Science: Cathode Rays

The members of the Franklin Institute of Philadelphia took their seats for their annual meeting last week in an atmosphere of ill-concealed excitement. Weeks before, the committee on awards had notified Dr. William David Coolidge, assistant research director of the General Electric Co., that he was this year’s recipient of the Howard N. Potts gold medal, in recognition of his now universally used invention, the Coolidge X-ray tube. And Dr. Coolidge had replied, saying that he would present himself for the reward, and at the same time submit a demonstration called: “A Method of Producing High Voltage Cathode Rays Outside the Generating Tube.”

To electro-physicists this title was a great deal more significant than it would have sounded to laymen. It meant that Dr. Coolidge—the man who, besides his X-ray work, first learned to make brittle tungsten ductile and so suitable for electric light bulbs (“Mazda”) of low price and long life—that this man of results had been exploring a field discovered 50 years ago by Sir William Crookes of England and only faintly understood ever since.

The Rays. Sir William Crookes found, when he passed an electric current through an air-exhausted glass vessel, that not only X-rays were formed (by the impact of the electrons upon the electrodes) but also other rays, composed of streams of electricity from the negative electrode, or cathode. His voltages were weak, however, and his tube clumsy. He was never able to ascertain much about these “cathode rays” because he could not get them outside their glass container to experiment with them.

The Coolidge Tube. After receiving his gold medal last week, Dr. Coolidge brought out upon the platform an astonishing tangle of insulators, wires, supports, switches, a huge induction coil—equipment for supplying 350,000 volts. In his hand he held, a glass vessel, five feet long, bulging in the middle. This was a vacuum tube, made portable so that it could be attached to an exhaustion pump in any laboratory. Into one neck ran the usual filaments to conduct electric current. These filaments ended in electrodes, of which the negative one or cathode could be heated white hot electrically before introducing the main current. About this cathode was built another innovation in vacuum tubes, a metal cup designed to repel electrons backfiring against it and converge them forward in a narrow stream at greatly accelerated speed. This stream was pointed down the tube’s other neck, a foot long, the sides of which were likewise sheathed in metal to guide the electrons on their way. At the tube’s end was the main feature of the invention, the “window.” This constituted a vast improvement upon the aluminum disc of earlier experiments. It was a sheet of nickel 1/2000 of an inch thin and three inches in diameter, supported against the 100-pound suction of the vacuum tube by skeleton struts of molybdenum. The molecular structure of nickel is such that molecules of air (oxygen, nitrogen) cannot pass through it, though it offers a minimum of resistance to those billionth parts of molecules, electrons.

When Dr. Coolidge ordered his 350,000-volt current turned on, a prodigious stream of electrons leapt from the hot cathode, moving perhaps two miles per second. Rebounding from the metal cup about the cathode, they raced off down the 12-inch exit passage of the tube until, when they reached the “window,” they were going some 150,000 m.p.s. (four-fifths the speed of light). Their volume was virtually undiminished as they shot through the thin nickel foil and out into heavy, molecular air, where their effects were at once visible and startling.

Effects. Hurtling into a crowded world of molecules, the stream of electrons (cathode rays) bombarded whatever they met so violently that the electrons of that world were knocked abruptly out of their orderly orbits around protons (positive atomic nuclei). Instantly, X-rays were set up in the local ether. As the dislocated electrons struggled back to their original positions they made another kind of vibration, weaker than X-rays and visible as bright luminosities. Thus, as soon as the 350,000 volts were switched on, a purple ball appeared at and enveloped the “window” end of the tube, caused by the vibrations of electrons in molecules of the air hustling back into position.

There had been a premature announcement of Dr. Coolidge’s ray and tube, saying that they were so powerful they would completely disintegrate the body of a mouse in a fraction of a second’s exposure. Dr. Coolidge did not verify this report, but the Franklin Institute members heard of or witnessed the following:

Acetylene gas in a sealed tube was reduced to a surprisingly large quantity of yellow powder, resembling varnish, which resisted all chemical reagents and a heat of 4000°. The powder was a substance utterly unknown to chemists. Precipitated by the ray upon an aluminum disc, the powder became an enamel which could not be removed.

Castor oil was changed quickly from a liquid to a solid; cane sugar was turned white; cane sugar in solution was turned acid; common salt was turned brown and rock salt black—all by momentary exposure to the rays.

A lump of fused quartz, clear as water, turned purple; a lump of feldspar glowed blue, amber, ruby, amethyst, with patches of brilliant green, successively; a lump of limestone burned angry orange. After exposure to the rays, these minerals looked searing hot but were not. Their fluorescence was without rise in temperature and in some cases persisted for hours after the exposure (as displaced electrons worked slowly back to their places in the atoms). The application of heat and cold (liquid air) altered the speed and intensity of these effects. Diamonds were only temporarily affected by exposure to the ray.

Milk submitted to the rays was pasteurized (rid of bacteria, including spores, which it ordinarily takes three heatings to kill) al-most instantaneously—but contracted an unpleasant flavor.

Fruit flies and other insects withered under a fraction of a second’s exposure, soon died. A rubber-plant leaf oozed white latex from millions of tiny punctures at one short dose of the ray.

Rabbits had been used for the tests on animal tissue. A tenth of a second’s exposure for a patch of ear-skin had made the patch lose its hair and turn dark. Not for seven weeks did hair reap, pear. Another patch was exposed for a second. A scab formed in a few days, fell off taking the hair with it—and in two weeks a growth of new hair, white instead of grey and thrice as profuse as previously, sprouted.

Problems. Dr. Coolidge knew that the changes wrought by his tube and ray were accomplished, basically, by electronic dislocations and rearrangements. He had experimented with as much as 350,000 volts. What effects might be obtained with, say, two million volts, remained to be seen. How and why these effects came about were a whole volume of problems.

Of one thing he could be certain: the Cathode Ray would never be a war weapon unless whole armies were “marched right into it,” for once outside their vacuum birthplace, the hurtling electrons all hit something soon and got slowed up; within a space of three feet when 350,000 volts were used; within five feet (calculated) if two million volts were used.

Since cathode rays are identical in nature with the beta rays of radium, computations were made showing that Dr. Coolidge had invented the equivalent of a ton of radium (2,000 times the world’s present supply), worth a hundred billion dollars. But these computations were misleading. It is the gamma rays (ether vibrations) of radium that are most potent and healing. Radium’s beta rays (streams of electrons) are screened off during most applications of radium to diseased tissue; their effect is superficial, while gamma rays penetrate deeply, cause more violent atomic changes.

What therapeutic significance attaches to cathode rays was to be suggested shortly by a preliminary report of doctors at the Albany Medical College, now experimenting. Experts at Columbia University reported having tried cathode rays in cancer treatment and found them practically useless.

Nevertheless, the Franklin Institute adjourned well aware that it had celebrated a moment in scientific history that would mark an epoch of no small proportions. The Massachusetts Institute of Technology, the University of Leipzig and the General Electric Co., where Dr. Coolidge had studied and worked, rejoiced generally and in unison.