Several simple experiments indicate that neutral substances, i.e. such that possess no overall charge, are composed of charged particles whose individual charges balance each other. For example, if a salt is dissolved in water, the conductivity of the resulting solution is larger than that of pure water. Historically, the first findings in this context (Elster and Geitel 1882; Hertz 1887) were those showing that a metal can emit charged particles when either heated (thermionic emission) or exposed to ultraviolet light (photoelectric effect).
- Franck J, Hertz G (1914) Über Zusammenstöße zwischen Elektronen und Molekülen des Quecksilberdampfes und die Ionisierungsspannung desselben. Verh Dtsch Phys Ges 16:457–467Google Scholar
- Huygens C (1690) Traite de la lumiere. Gauthier-Villars et Cie, ParisGoogle Scholar
- Newton I (1704) Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light. Also two treatises of the species and magnitude of curvilinear figures. Re-published 1998. Octavo Corporation, Palo Alto, CAGoogle Scholar
- Thomson JJ (1897) Cathode rays. The Electrician 39:104Google Scholar