In 1897, the experiment of J. J. Thomson precisely determined the charge to mass (e/m) ratio for an electron. In 1909, Robert A Millikan conducted an experiment known as “oil-drop experiment” to determine the charge (e) on an electron. He accurately measured the electric charge of an electron through his famous oil-drop experiment.
Millikan’s work not only determined the electron’s charge but also confirmed that the electric charge is quantized, meaning it comes in discrete amounts rather than being continuous.
Millikan’s Oil-Drop Experiment Apparatus
In 1909, American physicist Robert A. Millikan and Harvey Fletcher performed the oil drop experiment at the University of Chicago to measure the fundamental charge of an electron. The apparatus used in the oil-drop experiment is shown in the below figure.
In this experiment, Robert Millikan used a special apparatus consisting of the following components:
(1) Chamber with two metal plates: Millikan used a chamber (i.e., container) that contained two horizontal metal plates placed parallel to each other, about 1.5 cm apart. The plates were positioned one above the other.
- The upper plate was positively charged.
- The lower plate was negatively charged.
These plates were used to create an electric field within the chamber. The upper plate had a small hole in it to allow oil droplets to pass through.
(2) Atomizer: An atomizer is used to produce a spray of tiny oil droplets. These droplets enter the chamber through the small hole in the upper metal plate and fall between the two parallel metal plates, which are charged.
(3) Microscope: Millikan used a microscope to observe the motion of oil droplets. This microscope allowed him to see the tiny droplets clearly and measure their movement within the chamber.
(4) Light source: A light source was used to illuminate the droplets so that they could be visible through the microscope. This helped in tracking their motion accurately.
(5) X-rays: X-rays were used to irradiate the space between the charged plates. X-rays ionize the molecules of air (or gas) inside the chamber. When the oil droplets entered the chamber, they picked up charges (i.e. electrons) from the ionized air, causing the droplets to become negatively charged.
(6) Electrical Voltage Supply: To create an electric field between the two metal plates, Millikan used an electrical voltage supply.
The electrical voltage supply was connected to the two metal plates in the chamber. The positive terminal was connected with the upper plate to make it positively charges. While, the negative terminal was connected with the lower plate to make it negatively charged.
By applying a voltage (electrical potential difference) between these plates, an electric field was created in the space between them. This electric field exerts a force on any charged particles present between the plates, such as the negatively charged oil droplets.
Initial Observation of Droplets Motion Without Electric Field
Initially, both metal plates are earthed (connected to the ground), which means there is no electric field between them. In this state, the oil droplets entering the space between the plates fall freely under the force of gravity.
As the droplets fall, they eventually reach a constant velocity known as the terminal velocity. This happens when the downward gravitational force is exactly balanced by the upward forces of air resistance (drag) and buoyancy (upthrust) from the surrounding air.
This constant terminal velocity is measured by observing the droplet’s motion through the microscope. Let this initial velocity be denoted as v₁.
Observation of Charged Oil Droplets Motion with Electric Field
Then, an electric field is applied between the plates and it is adjusted so that the droplets start moving upwards against the gravity. The upward velocity would be uniform if the charge on the droplet remained the same. However, the velocity does not remain constant. It increases suddenly as the droplet picks up more charge from the ions present in its path.
In the course of a prolonged series of observations on the oil drop, let us suppose two different velocities, v2 and v3 are observed for the oil drop. The difference in velocity is due to different amounts of charge on the oil drops. Let us suppose v2 is the velocity when the charge on the drop is e2 and v3 is the velocity when the charge is e3.
When observing the motion of the oil droplets, the difference in their velocities (v2 – v3) is directly proportional to the difference in their charges (e2 – e3).
When many observations had made on a single droplet, the differences in the value of velocities (v2 – v3) are found to be integral multiples of the minimum value among them, though some are positive and others are negative.
This implies that the differences in charge (e2 – e3) are also an integral multiple of the minimum value, regardless of the sign. This minimum value represents the charge on a single electron.
Robert A. Millikan conducted thousands of observations on droplets of different liquids and various sizes. From his experiments, he determined the value of the electric charge to be
1.59 × 10−19 coulombs (C). The currently accepted value of the charge on an electron is 1.602192×10 −19 C. This is taken as one unit negative charge.
Mass of an Electron
The mass of an electron can be determined using the values of charge-to-mass ratio (e/m) and the electric charge (e).
Thomson experiment: e/m = 1.76 * 108 coulombs/g
Millikan experiment: e = 1.60 * 10-19 coulombs/electron
Mass of an electron, m = e / e /m = 1.60 * 10-19 coulombs/electron / 1.76 * 108 coulombs/g
Mass of an electron m = 9.1 * 10-28 g/electron = 9.1 * 10-31 kg/electron.
The mass of an electron is very small and is approximately 1/1837 times the mass of an atom of hydrogen.
From the above discussion, it is clear that an electron is a fundamental particle of an atom that carries a single negative charge and has mass nearly equal to 1/1837th of mass of an atom of hydrogen.
