Electron Speeds

By a clever device Chairman Robert Andrews Millikan of the California Institute of Technology was able to measure the electrical charge of the electron, the indivisible unit of all electricity. For that Dr. Millikan won a 1923 Nobel Prize. Last week two other Caltech men--Jesse W. M. du Mond and Harry Kirkpatrick-- reported the perfection of another device, to measure the speed of electrons moving within atoms. A serviceable description of the structure of an atom is this: At its core are, according to the particular kind of atom, 1 to 238 protons (positive charges of electricity). The hydrogen atom (simplest) has one proton at its nucleus. Helium (next simplest) has four nuclear protons. But two are herded into inaction by two nuclear electrons. This leaves two positive sports on the helium nucleus. They in turn are kept from rampage by two more orbital electrons which whirl about the nucleus at a comparatively vast distance. The atomic structures of elements heavier than helium are like helium's--a nuclear core of protons held together and neutralized by fewer electrons and the difference between protons and nuclear electrons made up by an equal number of electrons in one or more enveloping orbits. To understand how the speed of an atom's orbital electrons might be measured, take the illustration of a ball bounced against a figure on a moving merry-go-round. The speed of the ball as it strikes the moving figure will differ from the speed of the ball as it rebounds. By calculating with the two speeds it is possible to compute the merry-go-round's rate of revolution. What Caltech's du Mond and Kirkpatrick did was to shoot X-rays of known wavelength at the atoms of various elements. When an X-ray strikes an atom, the X-ray presumably hits and bounces off the orbital electrons, which are moving so fast that they give the effect of an impervious surface. The impact of the electrons alters the length of the X-ray wave. The difference between the original and altered wave is the measure of an orbital electron's speed. The nicety of the du Mond-Kirkpatrick experiment lay in their photographing thq rebounding X-rays in such fashion that the wavelength change could be exactly measured. The value of their work lay in its measuring electrons bound in atoms. An older method measures velocities of free electrons by use of an electric field, a magnetic field and a fluorescent screen.* Such methods disclose that some electrons move as slowly as 1% (within beryllium) the speed of light and others as swiftly as 90% (radium's Beta particles) light's speed. Light is known to travel (until Professor Albert Abraham Michelson, who last week was in a serious nervous collapse at Pasadena, figures it more accurately) 186,285 mi. per sec.

*Fluorescence is the visible effect of short waves being absorbed by an object which then emits longer, visible rays (in the direction of violet towards red).

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