1. Robert A. Millikan

2. What led to a Change in the Previous Model
Before Millikan developed his oil drop method, finding the charge of an electron was difficult and erratic due to rapid evaporation. A scientist named H. A. Wilson was responsible for creating the first method but was later proved the be ineffective by millikan. Since electrons were playing an increasingly important role in the atomic theories, the precise measurement of its charge had become a pressing problem which eventually led to the change in the model made by Robert A. Millikan.

3. Summary of Millikan’s Experimental Design
Robert Millikan worked with Harvey Fletcher to determine the charge of a single electron and to carry out this experiment, he used a atomizer (a device that is used for emitting a fine spray) to reduce the oil to a mist and sprayed it into a chamber where he measured their terminal velocity and calculated their mass. Millikan exposed the droplets to x-rays to ionize (to remove electrons) the molecules in the air and cause electrons to stick to the oil droplets making them charged. He placed batteries at the top and bottom of the chamber and the difference between the two batteries created and electric field that acted on the charge of the oil droplets. Eventually, he adjusted the voltage perfectly and was able to balance the force of gravity with the force of the electric field on the charged particles, causing the oil droplets to be suspended in mid-air.

4. How Millikan Changed the Atomic Model
Millikan’s oil-drop experiment proved that the electron was a fundamental particle with a fundamental charge.
This experiment changed the atomic model by determining the charge of an electron. He discovered that the charge was negative.
Millikan knew this because of the charge of the plates and how the oil drops were repelled from the negative charge.

5. Atomic model image