In this method, oil droplets in the form of mist, produced by the atomiser, were allowed

to enter through a tiny hole in the upper plate of electrical condenser. The downward motion

of these droplets was viewed through the telescope, equipped with a micrometer eye piece. By measuring the rate of fall of these droplets, Millikan was able to measure the mass of oil droplets. The air inside the chamber was ionized by passing a beam of X-rays through it. The electrical charge on these oil droplets was acquired by collisions with gaseous ions. The fall of these charged

oil droplets can be retarded, accelerated or made stationary depending upon the charge on the droplets and the polarity and strength of the voltage applied to the plate. By carefully

measuring the effects of electrical field strength on the motion of oil droplets,

Millikan concluded that the magnitude of electrical charge, q, on the droplets is always an integral multiple of the electrical charge,

e, that is, q = n e, where n = 1, 2, 3... .

The Millikan oil drop apparatus for measuring charge ‘e’. In chamber, the forces acting on oil drop are:

gravitational, electrostatic due to electrical field and a viscous drag force

when the oil drop is moving.