In 1909, Robert Millikan (1868–1953) discovered that electric charge always
occurs as some integral multiple of a fundamental amount of charge e (see Section
25.7). In modern terms, the electric charge q is said to be quantized, where q is the
standard symbol used for charge as a variable. That is, electric charge exists as
discrete “packets,” and we can write q ! Ne, where N is some integer. Other experiments
in the same period showed that the electron has a charge "e and the proton
has a charge of equal magnitude but opposite sign #e. Some particles, such as
the neutron, have no charge.
From our discussion thus far, we conclude that electric charge has the following important
properties: