Abstract: We develop a method for measuring the cut-off voltage of a photoelectric tube by charging capacitance from the photocurrent produced by the photoelectric tube.In our experiment,LED with different color is used as light sources,and a data acquisition system of computer is used for measuring the cut-off voltage of the photoelectric tube.The difference between the measured and the standard Plank＇s constant is found to be within 3%.

Key words: LED, photoelectric tube, Plank＇s constant, cut-off voltage, data acquisition