In 1896, Becquerel discovered that uranium ore emitted "rays". Marie Curie later theorized that the rays come from disintigrating atoms.
In 1897, J. J. Thompson devised the Cathode Ray Experiment. Thompson found that all metals he tested, when used as the cathode in his high voltage Cathode Ray tube, emitted identical particles that were attracted to the positive plate of the voltage source. He was able to determine the charge to mass ratio of these fundamental, negatively charged particles, which were later called electrons.
By balancing the electrical force on charged oil droplets in an electric field with the force of gravity, Robert Millikan was able to independently detemine the charge of the electron (-1.6x10-19 C) in his Oil Drop Experiment.
Later experiments involving Canal Rays demonstrated the existence of positively charged particles with charge to mass ratios that differed which gas molecules were used. This work lead to the discovery of the proton, whose charge (1.6x10-19 C) is equal in magnitude and opposite in sign to that of the electron.
In 1910, Ernest Rutherford interpreted results obtained by Giger and Marsden, when they exposed a thin film of gold foil to a beam of alpha particles. He concluded that all of the positive charge and most of the mass of an atom was concentrated in an extremely small region at its center. Rutherford named this region the nucleus.
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