Photoelectric thresholds for sandpapered commercial samples of fifteen metals and graphite. Light from a carbon arc, sometimes with cored carbons, after dispersion by a mirror monochromator with a fluorite prism, was focussed through a fluorite window on the slit of the evacuated photoelectric cell containing a strip of the element being studied connected to a sensitive string electrometer, and the saturation photocurrent as a function of wave-length was determined. Scattered light was reduced to a minimum by the use of screens, and the effectiveness of these was tested by introducing filters. This method eliminates many of the usual difficulties. Commercially pure samples were tested with surfaces cleaned by sandpapering only. The fairly great intensity of the arc even to 2600 A gave reasonably large photocurrents as soon as the threshold wave-length was passed and enabled it to be determined from within ±30 A at 2600 A to ±100 A at 3600 A, corresponding to from ±.06 to ±.10 equivalent volts, as determined by the formula V = hv0/e. The results in Angströms for Ag, Al, Bi, C (graphite), Ca, Cd, Cu, Fe, Ni, Pb, Pt, Se, Sn, Th, W and Zn are respectively: 3390, 3590, 2980, 2615, 4000, 3130, 2665, 2870, 3050, 2980, 2780, 2670, 3180, 3460, 2615, and 3420. These results are compared with those of others.
This research was undertaken with a view to determining directly the limiting frequencies for the various elementary metals under ordinary conditions. The principle of the method adopted was to use a source giving an approximately continuous spectrum extending over the experimental range of the limiting frequency for an element and to determine the threshold value by plotting the last values of the diminishing saturation photocurrents against the increasing wave-length of the incident light, or simply to take the average of the various wave-length limits observed in different determinations.
No special preparation of the metallic surfaces was made save that of rubbing with sandpaper, blowing with dry air, and placing them in the photoelectric cell which was immediately evacuated to a pressure of about 0.006 mm of mercury.
© 1924 Optical Society of AmericaFull Article | PDF Article
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