Robert Millikan’s work in the early 20th century provided foundational evidence for the modern understanding of the atom. His most significant contribution to atomic theory was the determination of the precise value of the electric charge carried by a single electron. This measurement established a fundamental constant of nature, proving that electric charge exists not as a continuous fluid but as discrete, quantifiable packets. The accurate calculation of this elementary charge helped transition physics from classical concepts into the modern era of subatomic particles.
The Context of the Electron
Before Millikan’s experiments, the electron was already a recognized particle thanks to the work of J.J. Thomson. In 1897, Thomson’s cathode ray experiments demonstrated the existence of a negatively charged particle approximately 1,800 times lighter than the lightest atom, hydrogen. Through this work, Thomson was able to calculate the electron’s charge-to-mass ratio (\(e/m\)).
This \(e/m\) ratio confirmed the electron was a universal component of matter, regardless of the source material. However, knowing the ratio alone was insufficient because neither the individual charge (\(e\)) nor the mass (\(m\)) could be determined separately. Scientists lacked the specific numerical value for its charge, which was necessary to fully incorporate the electron into a physical model of the atom. Millikan sought to isolate and measure the magnitude of the electron’s charge directly.
The Oil Drop Experiment
To resolve the unknown value of the electron’s charge, Millikan developed the oil drop experiment around 1909. The setup consisted of a pair of horizontal, parallel metal plates separated by a small distance, creating a uniform electric field when a voltage was applied. A fine mist of non-evaporating oil droplets fell through a small hole in the top plate into the chamber.
The oil droplets acquired an electric charge, typically through friction or by passing X-rays through the air to ionize gas molecules. Millikan used a viewing telescope to observe the motion of a single, illuminated oil droplet. The experiment involved a two-step process to calculate the charge on the droplet.
The first step was conducted with the electric field turned off, allowing the droplet to fall solely under the influence of gravity and air resistance. The droplet quickly reached a constant terminal velocity, measured by timing its fall over a fixed distance. This measurement, combined with the known viscosity of air and the density of the oil, allowed Millikan to calculate the mass and radius of the droplet.
In the second step, the electric field was turned on and adjusted until the upward electric force exactly balanced the downward force of gravity. In this suspended state, the electric force was equated to the gravitational force, allowing Millikan to calculate the total electric charge (\(q\)) on the oil droplet. He repeated this measurement for thousands of individual droplets.
Quantifying the Elementary Charge
The analysis of the collected data revealed that the calculated charge on every oil droplet was always an integer multiple of a smallest, common value. No droplet carried a fractional charge or a charge smaller than this minimum unit. This smallest, indivisible unit of charge was defined as the elementary charge, \(e\), representing the magnitude of the charge on a single electron.
Millikan’s value for the elementary charge was determined to be approximately \(1.602 \times 10^{-19}\) Coulombs, a figure close to the currently accepted constant. This finding provided the first experimental proof that electric charge is quantized, meaning it exists in discrete packets rather than as a continuous quantity.
With the value of \(e\) known, scientists combined it with Thomson’s charge-to-mass ratio (\(e/m\)) to calculate the electron’s mass (\(m\)) for the first time. The mass was confirmed to be about \(9.10 \times 10^{-31}\) kilograms. By accurately quantifying the electron’s charge and mass, Millikan’s work cemented the particle’s status as a fundamental constituent of the atom.