Photoelectric Effect and Planck's Constant in th Introductory Laboratory

This paper describes another approach to the photoelectric measurement of $h/e$ which is practicable for the introductory laboratory. The method owes its success to a new way of minimizing photoemission by the anode. A common vacuum photocell (1P39) and slit replace the eyepiece and cross hairs of a student grating spectrometer, illuminated by an ac mercury arc. If the cell is favorably oriented to the incident light, photoemission by the anode at high cathode retarding potential (6 V) can be reduced to less than 0.4% of the maximum photocurrent with zero retarding potential at the 3650-Å line. The 120-Hz component is amplified and rectified for presentation on a 100 μA dc meter. The maximum gain is sufficient to indicate full scale for a photocurrent of $10−11$ A. Stopping potentials for six lines between 3650 and 5770 Å are determined by plotting the square root of the photocurrent against retarding potential and by extending the straight portions of these plots to the axis. The slope of a straight line drawn through the stopping potentials, plotted against photon frequency, comes close to the accepted value of $h/e$⁠. Perhaps the most instructive aspect of this experiment is the experience with electron and photon energies. The student observes directly that electrons ejected by uv photons are much harder to stop than those ejected by photons of yellow light.