An increasing negative voltage prevents all but the highest-energy electrons from reaching the collector. When no current is observed through the tube, the negative voltage has reached the value that is high enough to slow down and stop the most energetic photoelectrons of kinetic energy K max. This value of the retarding voltage is called the stopping potential or cut off potential V o. Since the work done by the retarding potential in stopping the electron of charge e is eV o, the following must hold eV o = K max.