Answer:

Learning Objective

Describe the major findings of Millikan’s oil drop experiment

Key Points

Millikan’s oil drop experiment measured the charge of an electron. Before this experiment, existence of subatomic particles was not universally accepted.

Millikan’s apparatus contained an electric field created between a parallel pair of metal plates, which were held apart by insulating material. Electrically charged oil droplets entered the electric field and were balanced between two plates by altering the field.

When the charged drops fell at a constant rate, the gravitational and electric forces on it were equal. Therefore, the charge on the oil drop was calculated using formula Q = [latex]\frac {m\cdot g}{E}[/latex] Millikan found that the charge of a single electron was 1.6 x 10-19 C.

Terms

oil drop experimentExperiment performed by Robert Millikan and Harvey Fletcher in 1909 to measure the charge of the electron.

electronThe subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.

The Oil Drop Experiment

In 1909, Robert Millikan and Harvey Fletcher conducted the oil drop experiment to determine the charge of an electron. They suspended tiny charged droplets of oil between two metal electrodes by balancing downward gravitational force with upward drag and electric forces. The density of the oil was known, so Millikan and Fletcher could determine the droplets’ masses from their observed radii (since from the radii they could calculate the volume and thus, the mass). Using the known electric field and the values of gravity and mass, Millikan and Fletcher determined the charge on oil droplets in mechanical equilibrium. By repeating the experiment, they confirmed that the charges were all multiples of some fundamental value. They calculated this value to be 1.5924 × 10−19Coulombs (C), which is within 1% of the currently accepted value of 1.602176487 × 10−19 C. They proposed that this was the charge of a single electron.

Charge of an Electron: MillikanHow did scientists discover how much negative charge an electron had? Robert Millikan and Harvey Fletcher used the oil drop experiment.

How did the process work?

The figure below shows a simplified scheme of Millikan’s oil drop experiment. The apparatus incorporated a pair of metal plates and a specific type of oil. Millikan and Fletcher discovered it was best to use an oil with an extremely low vapor pressure, such as one designed for use in a vacuum apparatus. Ordinary oil would evaporate under the heat of the light source, causing the mass of the oil drop to change over the course of the experiment.

By applying a potential difference across a parallel pair of horizontal metal plates, a uniform electric field was created in the space between them. A ring of insulating material was used to hold the plates apart. Four holes were cut into the ring—three for illumination by a bright light and another to allow viewing through a microscope. A fine mist of oil droplets was sprayed into a chamber above the plates. The oil drops became electrically charged through friction with the nozzle as they were sprayed. Alternatively, charge could be induced by including an ionizing radiation source (such as an X-ray tube).