Davisson and Germer at Bell Telephone Laboratories tested this idea, in 1927, by passing a thin beam of accelerated electrons through a metal foil. The pattern that they obtained, like that shown on page 21, obviously is a pattern produced by diffraction of some kind of waves by the metal atoms.

Knowing the wavelength of the x rays used, a physicist can measure the radii of the diffraction rings in an x-ray pattern, such as that shown on the last page, and calculate the spacings between atoms in the metal.

With this information, he then can work backward from the radii of the corresponding rings in the electron-diffraction pattern to calculate the effective wavelength of the beam of electrons. It was found that the wavelength of a beam of electrons depends on the speed of the electrons. The faster the electrons the shorter their wavelength. Electrons accelerated through a potential of 40,000 volts, or given a kinetic energy of 40,000 electron volts, have a wavelength of 0.06 .