Figure 6-13 is a schematic of vacuum phototube circuit similar to the one used by Millikan to study the photoelectric effect. The surface of the large photocathode on the left usually is coated with an alkali metal or one of its compounds, but other metals may be used. When monochromatic radiation impinges on the photocathode, electrons are emitted from its surface with a range of kinetic energies. As long as the voltage V applied between the anode and the cathode is positive, the electrons are swept from left to right through the phototube to produce a current I in the circuit. When the voltage across the phototube is adjusted so that the anode is slightly negative with respect to the cathode, the photoelectrons are repelled by the anode, and the photocurrent decreases as expected. At this point in the experiment, however, some of the electrons have sufficient kinetic energy to overcome the negative potential applied to the anode, and a current is still observed.