Measurements on the contact potential difference between the (111) and (100) faces of copper single crystals, at different stages of outgassing, were made by the Kelvin null method. Two sets of observations with different experimental tubes show that the (111) face assumes a positive potential with respect to the (100) face. This value increased rapidly from near zero to about 0.4 volt during the first few hours of outgassing at a dull red heat and then more slowly to a limiting value of 0.446±0.002 volt at 20 hours in the first series and 0.463±0.002 volt at 70 hours in the second series of measurements. Further heating at temperatures which exposed other faces by evaporation caused the value, in the first series, to decrease to 0.384 volt after 100 hours of outgassing, and in the second series, after 275 hours, it decreased to 0.378 volt. This value, 0.378 volt, remained constant for the next 770 hours of outgassing. This difference between the results of the two series of measurements is due to different rates of outgassing. Observations were taken at pressures of about 5×${10}^{-8\mathrm{}}$ mm Hg as measured on an ionization gauge. The outgassing was more than that required to reduce the surface gas layer to such thickness that it possesses a definite crystal structure related to that of the underlying copper, as shown by Farnsworth. Thus the above results should be characteristic of the single crystals contaminated with undesired crystal facets and probably covered with small traces of gas of crystalline structure, both of which reduce the effective value. These results definitely indicate that the contact potential difference between the (111) and the (100) faces of uncontaminated copper single crystals must be greater than the final value obtained here and most probably greater than the maximum value of 0.463 volt.

• Received 24 June 1933

DOI:https://doi.org/10.1103/PhysRev.44.585