Charge, Elementary

(e), the smallest positive or negative electrical charge, whose value is e = (1.6021917 ± 0.0000070) x 10^–19 coulombs in the International System of Units, or e = (4.803250 ± 0.000021) x 10^–10 cm^3/2g^1/2sec^-1 in the centimeter-gram-second electrostatic system (esu). The first accurate measurements of the magnitude of the elementary charge were made in 1911 by R. Millikan.

As experience shows, the electrical charge transferred during interactions among bodies is always an integral multiple of the quantity e. Nearly all elementary particles have an electrical charge equal to +e or –e, or are uncharged (exceptions are some resonances, which have a charge that is a multiple of e). However, no strict physical law according to which the magnitude of the smallest electrical charge must always be equal to e has been established. In particular, hypothetical schemes in which a significant role is played by particles with charges that are multiples of e (so-called quarks) have been proposed in the theory of elementary particles, but particles with fractional electrical charges (in units of e) have not been observed in experiments.