An experimental setup of verification of photoelectric effect is shown in the diagram. The voltage across the electrodes is measured with the help of an ideal voltmeter and can be varied by moving jockey J on the potentiometer wire. The battery used in potentiometer circuit is of 20V and its internal resistance is 2$\mathrm{\Omega }$. The resistance of 100 cm long potentiometer wire is 8$\mathrm{\Omega }$. The photocurrent is measured with the help of an ideal ammeter. Two plates of potassium oxide of area 50cm² at separation 0.5 mm are used in the vacuum tube. Photo current in the circuit is very small so we can treat potentiometer circuit an independent circuit. The wavelength of various colours are as follows :