Millikans oil Drop Apparatus is used to measure the Elementary Electric Charge. To achieve this, tiny charged droplets of oil are finely sprayed into two metal electrodes placed horizontally. Heavier drops tend to fall downwards fastly due to their thermal velocity but the lighter ones remain almost steady inside the metal electrode condenser chamber. An adjustable electric voltage is applied between the metal electrode plates which creates an electric field inside the condenser chamber. By adjusting the electric voltage, drops can be moved upwards or downwards according to their charge.In the original design of Millikans oil drop apparatus, it was not easy to locate the oil drops in the condenser because the setting was critical.