On X-Ray Wave-Lengths

Abstract

A T its Philadelphia meeting (December, 1914) one of us presented a paper to this Society describing experiments on the relation between the wavelengths of X-Rays and the voltages required to produce them. The X-Rays came from the tungsten target in one of the tubes designed by W. D. Coolidge, 2 and belonged to the general or "white light" X-radiation and not to the characteristic X-radiation. The generator supplying current to the tube produced a roughly constant difference of potential, which was measured by means of the voltmeter attached to the generator and also by means of the length of sparks between two metal balls each 2 cm. in diameter. This gave, of course, only an approximate estimate of the voltage. The wavelength X e was calculated from measurements of the mass-coefficient of absorption fjt/p of the X-rays in aluminium, the formula being-= 14.9 X e 3 (h e expressed in angstroms). P This method gives what may be called the effective wavelength of the X-rays. With V = 43,000 volts difference of potential between the electrodes of the tube the effective wavelength of the X-rays after they had passed through 2 mm. of aluminium was found to be \ e = .45 angstrom. Calling the ratio of the energy of a cathode-ray particle to the effective frequency of the X-rays h e , this gives h e-10 X io-27 , a quantity of the order of magnitude, but somewhat greater than Planck's radiation constant h = 6.41 X io~ 27. The authors of this paper have repeated the experiments using a high tension storage battery to supply current through the tube. The difference of potential between the electrodes of the tube was measured by means of an excellent electrostatic voltmeter designed by Dr. E. L. Chaffee. We calibrated this instrument by measuring the current flowing from the storage battery through a number of manganin wire coils of known resistance. The total resistance of the coils joined in series amounted to about one million ohms. The measurements appear to be thoroughly reliable. 1 Abstract of a paper presented at the Washington meeting of the Physical Society, April 23-24, 1915. 2 PHYSICAL REVIEW, December, 1913.