Radio broadcast .. (1922-30)

AUGUST, 1926 ONE BILLIONTH OF A METER 291 the atomic number refers to the average, so chlorine is number 17 and oxygen number 8. Among the early workers in the subject of X-ray spectra was a young Englishman named Henry G. J. Moseley, who worked with Sir Ernest Rutherford, then at Manchester, and at Oxford University, his Alma Mater. One of his experiments was to test with the X-ray spectrometer the X-rays with different substances as targets. This led him to the important discovery, at the age of 25, that the spectra of the X-rays from different elements were remarkably similar in regard to the position of their dark lines, but with the difference that the wavelength of the radiation which corresponded to the lines becomes shorter as the atomic number of the element increases. For example, nickel being number 28 and copper number 29, the position of the lines in the copper spectrum shows V It is ment no volt a.c. COOLIDGE X-RAY TUBE WIRING As is the case with the ordinary radio vacuumtube circuit, an electron flow is emitted from the filament. To the cathode of the Coolidge tube, which corresponds to the filament in the radio tube, is applied an i lo-volt current. A highvoltage current from a transformer also flows through the tube and, as the electrons bombard the tungsten target, X-rays are produced X-RAY DIFFRACTION APPARATUS used for obtaining X-ray spectra from which the arrangeof the atoms in metals and other solids may be determined them to be of slightly shorter wavelength than in the spectrum of nickel. In the summer of 1914, after he had made this discovery, Moseley went to Australia to attend the meeting of the British Association for the Advancement of Science, and to tell them of his work. August came, and with it the outbreak of the World War, and Moseley hurried back to England and joined the Royal Engineers. He was soon in the thick of fighting and on August 10, 1915, was in the Gallipoli campaign, when a Turkish bullet ended the life that had already made such great contributions to science and which had given every prospect of many more. The work he began has already led to brilliant discoveries in the hands of his successors. Since each element has its characteristic X-ray spectrum, and since the wavelength of the lines varies according to the atomic number, a single X-ray spectrum photograph gives an infallible means of identifying the element. Not only is the method applicable to already known elements, but as blank numbers are left for ones which have still been undiscovered, the presence of a hitherto unknown element may be demonstrated as easily as one that is already common. Until about a year ago, there were five empty ranks in the parade of elements. Since then, three have been filled. Numbers 43 and 75 were filled by Dr. Walter Noddack and his associates, working at the University of Berlin. They concentrated solution of ores containing platinum, and when their X-ray spectra showed the lines in the positions corresponding to their numbers, according to Moseley's law, they knew they had found the new elements, and named them Masurium and Rhenium. Still later, Dr. S. B. Hopkins, at the University of Illinois, found number 61, which he named Illinium after his college, and with it, too, the X-ray spectrum corresponded precisely to Moseley's law. Elements number 85 and 87 are still absent and unaccounted for, though we know what their X-ray spectrum will be when they are discovered. CaJhodf {Support AN EARLY X-RAY TUBE This was the type employed by Rontgen when he discovered X-rays. The cathode rays impinged on the broad end of the tube THIS SCALE HAS BEEN SPECIALLY DEVELOPED FOR MEASURING X-RAY SPECTRA As seen here, it is being used with two spectrum photographs. The upper photograph is of the tungsten spectrum while the lower one is of ordinary table salt. The numbers on the scale indicate the wavelengths of the rays in 250/1, ooo.oooths of an inch.