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The thermionic and hotoelectric work functions and the temperature variation of coating conductivity have been measured for an oxide cathode. Thermionic and photoelectric currents were measured in both retarding and accelerating fields. The two work functions are found to be different, but not by the amount predicted by applying simple semi-conductor theory to the data. Photoelectric currents fit a Fowler plot rather well over a limited range of frequencies.The large decrease in work function with applied field, the poor "saturation" of thermionic currents typical of oxide cathodes, and the behavior of the photoelectric currents in accelerating fields all suggest that one is dealing with a "patchy" surface. Applying "checkerboard" patch theory to the experimental data, one finds that most of the data is accounted for by assuming patches about 3xlO -4 cm on a side differing in work function by about .2 volt.*

This communication comments on some of the work of Fleming and Alberg, and presents some data tending to substantiate a ``numbers'' theory for photoconduction in certain types of chemically prepared photoconducting films of lead sulfide. The data were obtained some years ago but were not published, and were taken during an extensive program of measurements of the basic properties of PbS films, some of the results of which were published [G. W. Mahlman, Phys. Rev. 103, 1619 (1956); J. C. Slater, ibid. 103, 1631 (1956)].

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