Question

Why telescope is used in milikans oil drop experiment?

Answer 1

The oil drop experiment was performed by Robert. Millikan and Harvey Fletcher  in 1909 to measure the elementary Electric charge.

The experiment entailed observing tiny electrically charged droplets  of oil located between two parallel metal surfaces, forming the plates of a Capacitor. The plates were oriented horizontally, with one plate above the other. A mist of optimised  oil drops was introduced through a small hole in the top plate. First, with zero applied electric field, the velocity of a falling droplet was measured. At terminal velocity
, the drag force equals the gravitational  force. As both forces depend on the radius in different ways, the radius of the droplet, and therefore the mass and gravitational force, could be determined (using the known density  of the oil). Next, a voltage inducing an electric field was applied between the plates and adjusted until the drops were suspended in mechanical equilibrium , indicating that the electrical force and the gravitational force were in balance. Using the known electric field, Millican and Fletcher could determine the charge on the oil droplet. By repeating the expermemt for many droplets, they confirmed that the charges were all small integer multiples of a certain base value, which was found to be1.5924(17)×10−19, about 0.6% difference from the currently accepted value of1.602176487(40)×10−19 C. They proposed that this was the magnitude of the negative charge of a single electron.