Stopping Voltage

# 7A10.30


The apparatus can be used for studying the photoelectric effect and measuring Planck's constant. It shows that the kenetic energy of photoelectrons knocked out of a photosensitive surface depends on the color (and hence the frequency) but not on the intensity of the incident light. The aqqaratus consists of a photocell in which photoelectric emission occurs. The voltage applied to the tube can be adjusted by rheostat. If the voltage decreased and made negative the electrons ejected from the cathode will not have enough energy to reach the anode. At certain voltage called the "stopping voltage" or "cut-off voltage" the electron current from the cathode to the anode will become to zero.

Location and Notes

Setup1: THORNTON Phototube unit PVB-100(16H) w/a high intensity incandescent lamp; THORNTON Amplifier-Power supply unit APS-101(39 B)
When continue to turn the voltage control knob past the cut-off voltage, the output signal remains unstable. This is because of the emmision of the anode. The stopping voltage is set for the minimal stable output signal.

Setup2: WELCH Phototube unit 2120(16H) w/SPECTOLINE Mercury lamp(15E)
Because of the anode emission, at the stopping voltage the microameter appears negative.