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We examine the theoretical and experimental foundations of Coulomb's Law and review the various roles it plays not only in electromagnetism and electrodynamics, but also in quantum mechanics, cosmology, and thermodynamics. The many implications of Coulomb's Law draw attention to its fundamental importance within virtually all branches of physics and make this elementary yet profound law one of the most useful of all scientific tools.

The equality of the charges of the electron and the proton, and the charge neutrality of the neutron are of great significance in the fundamental theory of particles. This equality suggests a deep symmetry between leptons and quarks that is not yet revealed in other experiments. The electrical neutrality of bulk matter is a direct result of this characteristic of the fundamental charges, with important consequences for precise tests of fundamental physical laws and for electrical metrology. The question is of interest also in cosmology. In this paper, we discuss the experimental evidence for the equality of the fundamental electrical charges, its implications and the possibility of improved experiments.

The precise measurement of weak effects plays a pivotal role in metrology and in the determination of the fundamental constants. Hence, the possibility of new weak forces, and the related question of non-Newtonian behaviour of the gravitational force, have been of special interest to both measurement scientists and those involved in precise tests of physical laws. To date there is no compelling evidence for any deviations from the predictions of Newtonian gravity in the nonrelativistic weak-field regime. A significant literature on this question has developed over the past few years, and a host of experiments and theoretical scenarios have been discussed. Moreover, a very close relationship exists between the experimental methodologies used to determine the absolute value of the Newtonian gravitational constant G, and those employed in searches for new weak forces and for breakdowns in the inverse-square law of gravity. We have therefore prepared a new index of measurements of such effects, using the original bibliographic work of Gillies as a starting point, but also including citations to the appropriate theoretical papers in the field. The focus of the present version of the index is then studies of the "fifth force", measurements of gravitational effects on antimatter, searches for a spin-component in the gravitational force, and related phenomena.

This special issue on the physics and metrology of electrical charge attempts to provide the interested reader with an overview of the ways in which this fundamental property of matter has been studied and measured, both historically and in present times. Few topics in introductory physics and electrical engineering courses receive as much attention as does the nature and behaviour of electrical charge, and experimental tests of Coulomb's law are a staple of such curricula. The manipulation of electrical charge, even down to the level of single electrons, constitutes the currency of electrical metrology, while the effects of parasitic forces arising from spurious charges are the bane of virtually all who work in the realm of high precision experimentation. Moreover, basic questions about the equality, discreteness and possible fractional sizes of elementary charges lie at the foundation of modern physics on the one hand, while the control of charge carriers within electrical and electronic devices forms the core of essentially all of modern technology, on the other. The theme of the special issue is thus one of reviewing the scientific foundations of charge as a property of matter and as a tool for testing fundamental physical laws. The historical development of both aspects of this theme during the last two centuries has helped form the basis for modern electrical metrology. Therefore, it seemed timely to reassess the field with an eye towards future developments, especially since we find ourselves at the 250th anniversary of the period during which Benjamin Franklin carried out some of the first quantitative electrical measurements, in his colonial laboratory in Pennsylvania. To that end, the special issue contains articles on several aspects of electrical charge that have been the focus of intense study during the past several years. Coulomb's law is of course central to any discussions in electrical science, and two of the articles provide detailed descriptions of the experimental foundations for and the theoretical implications of it. The modern interpretation of possible deviations from exact inverse-square behaviour in Coulomb's law invokes a non-zero rest mass for the photon, and the limits on the sizes of such effects and the roles that they would play in physical theories are addressed in those articles. The other papers include a discussion of the state of knowledge regarding the electrical neutrality of bulk matter and proton/electron charge asymmetries, a description of a very high precision search for fractional electrical charges, a comparative overview of the physical analogies between the electromagnetic and gravitational forces, and a succinct historical study of Coulomb himself and the law and unit of charge which bear his name. During the past year, Metrologia has published two other topical issues on fundamental properties of matter: mass and density. The present special issue forms a companion document to them. It has been the goal of this special issue to capture the contemporary flavour of the work being done by physicists who seek to establish the exactness of the physical laws that serve as foundations for the further advancement of electrical metrology. Thanks are due to the several authors who took time to prepare these articles, the referees who reviewed and commented on them, to Professor Peter Martin and Dr Jeffrey Williams, former and present Editors of Metrologia, respectively, and to Dr Terry Quinn, Director Emeritus of the BIPM, for their advice, assistance and central roles in bringing the special issue to completion.

The Newtonian Gravitational Constant, G, has probably been measured more often but, interestingly, with less precision than any other physical constant of fundamental importance. In an effort that has spanned more than a century to connect gravitation to the other forces of nature, over 200 experiments on G have been completed and reported; but many of them have not been reported in what would now be considered to be the open literature. This paper is a third, more complete attempt to carry MacKenzie's and Poynting's bibliographies forward from the 1800's to the present; and thereby include as many as possible of the experimental results on G that have been obtained since 1900.

Decay time constants for magnetically-suspended rotors undergoing inertial spin-down have been measured at room temperature. Bearing-rotor interactions have been studied and evidence found that there is a driving mechanism capable of accelerating or decelerating a rotor with a moment of inertia of ≈1100 g cm2 at the 10-14 W level. The complicated rotor decay structure suggests that there exist unfavorable rotational frequencies at which mode coupling exists and leads to unavoidably large levels of angular momentum dissipation. The length of the rotors (≈20 cm) also produces a pendulum motion which at times results in ambiguities in the spin-down data due to interference with the normal operation of the optical-lever timing system.