Who Calculated the Charge on an Electron?
Use this interactive Millikan oil drop calculator to estimate elementary charge from experimental values, compare your result to the accepted constant, and learn why Robert A. Millikan is most often credited with measuring the electron’s charge.
Electron Charge Calculator
This calculator uses the core balancing relationship from the oil drop experiment: when the drop is suspended, electric force equals gravitational force, so q = mgd / V.
Accepted Reference
The accepted magnitude of the elementary charge is 1.602176634 × 10-19 C. Modern SI definitions treat this value as exact.
The chart compares your calculated value, the accepted elementary charge, and the total charge implied by your selected number of electrons.
Who Calculated the Charge on an Electron? The Expert Answer
The short answer is that Robert A. Millikan is the scientist most commonly credited with calculating and measuring the charge on the electron. His famous oil drop experiment, carried out in the early twentieth century, produced a convincing value for the elementary electric charge and demonstrated that electric charge exists in discrete units. When students ask, “Who calculated the charge on an electron?” textbooks, physics classrooms, and history of science references usually point to Millikan.
That said, the full historical answer is richer. Millikan did not work in a vacuum. The story of the electron’s charge includes several important scientists: J. J. Thomson, who discovered the electron and measured its charge-to-mass ratio; George Johnstone Stoney, who helped establish the conceptual idea of a fundamental unit of charge and coined the term “electron”; and Robert Millikan, who directly measured the elementary charge with enough precision to transform atomic physics.
Why Millikan Gets the Credit
Millikan’s achievement was not merely that he proposed a theory. He created a method that allowed physicists to infer the charge carried by individual oil droplets suspended in an electric field. By carefully adjusting the voltage across two metal plates, he made a droplet hover. At that moment, the upward electric force balanced the downward gravitational force. From that balance, the droplet’s charge could be determined.
The basic reasoning is elegant:
- A droplet with charge q inside an electric field E experiences electric force F = qE.
- For parallel plates, the electric field can be approximated as E = V/d, where V is voltage and d is plate separation.
- When the drop is stationary, electric force balances weight, so qV/d = mg.
- Rearranging gives q = mgd / V.
Millikan repeated this process across many droplets and found that measured charges were not arbitrary. Instead, they appeared as whole-number multiples of a smallest common value. That smallest value corresponded to the charge of a single electron. This was historically important because it gave strong evidence that electric charge is quantized, meaning it comes in indivisible packets.
The Role of J. J. Thomson
Millikan’s work built on earlier breakthroughs by J. J. Thomson. In 1897, Thomson’s cathode ray experiments established that cathode rays were made of negatively charged particles, later identified as electrons. Thomson measured the ratio of charge to mass, usually written as e/m, but he did not isolate the value of the charge alone with the precision Millikan later achieved.
This distinction matters. Thomson demonstrated the existence of the electron and estimated how much charge it carried relative to its mass. Millikan provided the measurement of the elementary charge itself. Once physicists had both e/m from Thomson and e from Millikan, they could compute the electron’s mass. That was a major milestone in modern physics.
| Scientist | Key Contribution | Approximate Date | Historical Importance |
|---|---|---|---|
| George Johnstone Stoney | Proposed a natural unit of electric charge and introduced the term “electron” | 1891 | Helped shape the concept of elementary charge before direct measurement |
| J. J. Thomson | Discovered the electron and measured charge-to-mass ratio | 1897 | Established the electron as a real subatomic particle |
| Robert A. Millikan | Measured the elementary charge using the oil drop experiment | 1909 to 1913 | Produced the classic value for the electron’s charge and confirmed quantization |
What the Oil Drop Experiment Actually Measured
A common misconception is that Millikan directly held a single electron in his apparatus and read off its charge. That is not exactly what happened. He measured the charge on tiny oil droplets that had gained or lost electrons. Because those charges came in integer multiples of a smallest unit, he inferred the value of the elementary charge. In practical terms, the experiment was powerful because the measurements converged on one basic constant.
In modern notation, the elementary charge is exactly:
e = 1.602176634 × 10-19 coulomb
For an electron, the charge is negative, so physicists often write:
qelectron = -1.602176634 × 10-19 C
Many educational references quote the magnitude only, since the question is often about the size of the fundamental charge rather than its sign.
Why This Discovery Was So Important
Knowing the charge on the electron changed physics in several ways. First, it made atomic theory more quantitative. Second, it helped establish that matter and electricity are linked through discrete subatomic particles. Third, it allowed the electron’s mass to be calculated when combined with Thomson’s charge-to-mass ratio. Finally, it influenced later developments in quantum mechanics, solid-state physics, chemistry, and electronics.
- It confirmed that charge is quantized.
- It provided a foundational physical constant.
- It improved understanding of atomic structure.
- It connected laboratory measurements to universal properties of matter.
- It made future precision physics possible.
How Accurate Was Millikan’s Result?
Millikan’s experimental value was remarkably close to modern accepted values, especially considering the equipment available in the early 1900s. His methodology was a landmark in precision measurement. Historians and physicists have also discussed how data selection and interpretation affected the presentation of his results, but that debate does not change the central scientific importance of the experiment. Millikan’s work remains one of the iconic measurements in the history of science.
| Quantity | Historical or Modern Figure | Value | Unit |
|---|---|---|---|
| Accepted elementary charge | Modern exact SI definition | 1.602176634 × 10-19 | C |
| Electron mass | Modern CODATA value | 9.1093837015 × 10-31 | kg |
| Thomson charge-to-mass ratio | Modern reference magnitude for electron | 1.75882001076 × 1011 | C/kg |
| Faraday constant | Charge per mole of electrons | 96485.33212 | C/mol |
Who “Calculated” Versus Who “Discovered”
People often use the words calculated, measured, discovered, and identified as if they all mean the same thing. In science history, those terms can point to different achievements:
- Discovered the electron: J. J. Thomson is usually credited.
- Coined the term electron: George Johnstone Stoney is usually credited.
- Measured or calculated the electron’s charge: Robert A. Millikan is usually credited.
That distinction helps answer exam questions and search queries accurately. If a student asks, “Who found the electron?” the answer is Thomson. If the question is, “Who calculated the charge on an electron?” the answer is Millikan.
How the Calculator Above Relates to the Real Experiment
The calculator on this page gives you a simplified way to explore the same logic Millikan used. You enter the droplet mass, plate spacing, and voltage. The script computes the total charge required to balance the droplet. If you tell the calculator how many electrons are assumed to be on the droplet, it divides the total charge by that number to estimate the elementary charge.
Real experiments involved additional corrections, careful observation of terminal velocities, and more sophisticated handling of drag and droplet size. The simplified formula still captures the central teaching idea: electric force can be balanced against weight, and from that balance a charge can be inferred.
Was Millikan the Only Scientist Involved?
No. Scientific discovery is almost always cumulative. Millikan’s contribution sits within a larger chain of ideas and experiments. Thomson’s work showed that electrons existed. Stoney helped frame the concept of elementary charge. Other experimental physicists improved electrical measurements and atomic theory. Even so, when historians of physics assign the specific accomplishment of measuring the elementary charge with the famous oil drop method, Millikan remains the principal figure.
Why Students Still Learn This Today
The oil drop experiment remains a staple in science education because it combines several core ideas in one elegant system:
- Force balance in mechanics
- Electric fields and potential difference
- Measurement uncertainty
- Quantization of physical properties
- The historical development of atomic theory
It is a perfect bridge between classical and modern physics. Students can understand the forces with basic mechanics, yet the conclusion points directly toward the discrete nature of the microscopic world.
Authoritative Sources for Further Reading
For deeper study, consult authoritative educational and government resources:
American Institute of Physics History of the Electron
NIST Reference on the Elementary Charge
Britannica biography of Robert A. Millikan
Final Takeaway
If you want the clearest and most accepted answer to the question “Who calculated the charge on an electron?” the answer is Robert A. Millikan. His oil drop experiment provided the classic measurement of the elementary charge and showed that electric charge comes in discrete units. J. J. Thomson discovered the electron, and George Johnstone Stoney helped define the concept and name, but Millikan is the scientist most closely associated with calculating the electron’s charge.
That is why Millikan’s name appears so often in textbooks, lectures, and exam questions. His measurement did more than assign a number to a particle. It helped reveal the architecture of matter itself.