The absolute value of the charge on the electron was first determined in which classic atomic physics experiment?

Introduction / Context:
This question belongs to the history of atomic structure and experimental physics. It asks which experiment measured the absolute charge of the electron, an essential constant in physics and chemistry known as the elementary charge.

Given Data / Assumptions:

• Several famous experiments are listed: cathode ray, oil drop, alpha scattering, neutron discovery and canal ray experiments.
• We are specifically interested in the experiment that gave the numerical value of the electron charge.
• We assume basic familiarity with the roles of these classic experiments.
• The elementary charge is about 1.6 * 10^-19 coulomb.

Concept / Approach:
J. J. Thomson's cathode ray work determined the charge to mass ratio (e/m) of the electron, not its absolute charge. Rutherford's alpha scattering experiment gave information about the nucleus. Chadwick's work identified the neutron, and Goldstein's canal ray experiment led to the discovery of positive rays and protons. The direct measurement of the electron charge was achieved by Robert A. Millikan in his oil drop experiment by carefully balancing electric and gravitational forces on tiny charged oil droplets.

Step-by-Step Solution:
1) Remember that the cathode ray experiment yielded the e/m ratio, not e alone.2) To find the absolute value of e, another independent measurement was required.3) Millikan's oil drop experiment involved spraying fine oil droplets into a chamber and subjecting them to an electric field.4) By adjusting the electric field so that the gravitational force on a charged droplet was balanced by electric force, Millikan could calculate the charge on individual droplets.5) He found that all measured charges were integer multiples of a smallest value, the elementary charge, thus determining the absolute charge of the electron.

Verification / Alternative check:
Science history clearly records that the numerical value of the elementary charge, approximately 1.6 * 10^-19 coulomb, comes from Millikan's oil drop experiment. Combining this value of e with Thomson's earlier value of e/m later allowed scientists to calculate the electron mass. No other experiment among the options directly measured the charge magnitude in this manner, which confirms that Millikan's experiment is the correct answer.

Why Other Options Are Wrong:
J. J. Thomson's cathode ray experiment: Established the existence of electrons and their e/m ratio but not the absolute value of e.
Rutherford's alpha scattering experiment: Provided evidence for a small, dense atomic nucleus, not electron charge.
Chadwick's neutron discovery experiment: Concerned neutral neutrons, not electron charge measurement.
Goldstein's canal ray experiment: Led to the discovery of positive rays and protons but did not measure electron charge.

Common Pitfalls:
Students sometimes confuse the electron discovery experiment with the charge measurement experiment and wrongly choose Thomson. It helps to remember that the oil drop experiment is always associated with measuring the elementary charge, while cathode ray work is linked to discovering the electron and finding e/m. Keeping separate mental tags for what each historic experiment achieved reduces such confusion in multiple choice questions.

Final Answer:
The absolute value of the electron charge was first determined in R. A. Millikan's oil drop experiment.