The intensity of scattering of a homogeneous beam of electrons of adjustable speed incident upon a single crystal of nickel has been measured as a function of direction. The crystal is cut parallel to a set of its I 111j-planes and bombardment is at normal incidence. The distribution in latitude and azimuth has been determined for such scattered electrons as have lost little or none of their incident energy.Electron beams resulting from diffraction by a nickel crystal. -Electrons of the above class are scattered in all directions at all speeds of bombardment, but at and near critical speeds sets of three or of six sharply defined beams of electrons issue from the crystal in its principal azimuths. Thirty such sets of beams have been observed for bombarding potentials below 370 volts. Six of these sets are due to scattering by adsorbed gas; they are not found when the crystal is thoroughly degassed. Of

No abstract

Continuing our investigation of the interaction between a beam of electrons and a crystal of nickel (Phys. Rev., 30, 705 (1927)) we are now directing the electron beam against a 1 1-face of the crystal at various angles of incidence, and are measuring the intensity of scattering in the incidence plane as a function of bombarding potential and direction.We find that under certain conditions a sharply defined beam of scattered electrons issues from the crystal in the direction of regular reflection. This occurs whenever the speed of the incident electrons is comprised within any of certain ranges which change in location as the angle of incidence is varied. Within each of these ranges there is an optimum speed at which the intensity of the reflected beam attains a maximum.That regular selective reflection of electrons from a crystal face would be observed under appropriate conditions was anticipated from our earlier observations on electron diffraction. The phenomenon is cdearly the analogue of the regular selective reflection of x-rays on which the Bragg method of x-ray spectroscopy is based, and is, of course, to be interpreted in terms of the undulatory theory of mechanics. The incident beam of electrons of speed v is equivalent to a beam of waves of wave-length h/mv; a portion of the incident beam is regularly reflected, through the process of coherent scattering, from each of the layers of atoms lying parallel to the crystal face, and the intensity of the resultant beam is a maximum when the elementary beams proceeding from the individual layers emerge from the crystal in phase. The condition for such a maximum in the case of x-ray reflection is that the wave-length and angle of the incident beam be related to the separation between successive atom layers of the crystal through the Bragg formula nX = 2d cos 0. The condition in the case of electron reflection is somewhat different. The wave-length )X(=h/mv) of the reflected beam at maximum intensity is not given by the Bragg formula. * These results, including the failure of the data to satisfy the Bragg formula, are in accord with those previously obtained in our experiments on electron diffraction. The reflection data fail to satisfy the Bragg relation for the same reason that the electron diffraction beams fail to coincide with their Laue beam analogues. These differences between the electron and x-ray phenomena can perhaps be accounted for by assuming, as first suggested by Eckart,' that the crystal is characterized by an index of refraction for electrons as it is for x-rays, and that for elec-VOL,. 14, 1928 317

Prebreakdown electron current between electrodes closing at voltages below the minimum which can give breakdown by successive ionization of air molecules has been measured by two different methods. This field emission current varies widely in successive experiments, increasing in general with decreasing voltage, with maximum values of the order of 10−7 amp. At the small electrode separations characteristic of breakdown at voltages below 300, it is shown that the ions necessary for breakdown come from the anode surface. The number of ions in the space at one time is so small that they cannot cooperate to enhance the gross field at the cathode, which is a conclusion having important consequences for the theory of breakdown.

Currents preceding breakdown have been measured between closely spaced tungsten electrodes in high vacuum. It is found that field emission currents sufficient to evaporate anode metal flow before breakdown. These currents follow the Fowler-Nordheim equation when field magnification due to surface irregularities on the cathode is taken into account. The field magnification is a function of distance at electrode separations less than 4×10−4 cm. Explanation of the observed breakdown at low voltage and small spacing requires an unusually high yield of electrons at the cathode per ion formed in the gap. Furthermore there is no measurable direct enhancement of the current by ionization even at higher voltages. The high electron yield must therefore exist over the entire observed range of breakdown voltages. This high yield is satisfactorily accounted for by the increase in field emission due to the positive ion space charge, which in turn increases the positive ion current density until there is breakdown. It is shown that breakdown occurs when the field emission current is increased by only 65 percent. This condition is reached with the ion current density much smaller than the electron current density.