TABLE OF CONTENTS Measured physical quantities are expressed in units. There are three systems of units commonly used in physics and technology. The Gaussian cgs system traditionally used in treatises and textbooks of physics; the international SI system, established by the 11th General Conference on Weights and Measures in 1960, now widely used in physical measurements and technological literature; besides, there is the atomic system used sometimes in some theoretical texts on microscopic physics. A system of units is chosen for convenience and clarity. A system of units once chosen affects not only the values of physical quantities obtained from experimental measurements, but also the forms of formulas relating various physical quantities. We may choose Maxwell equations with related formulas and Schr dinger equation for an atom with Z electrons to illustrate this point (Tables A.1 and A.2).
A glance at the tables listed below may convince us that the simplest formulation in use is the atomic system of units. This system of units by setting many universal constants, such as e (electron charge), m (electron mass), ? (? = h/2 ?, h Planck constant), and c (velocity of light in vacuo) to unity; thus it achieves simplicity at the expense of physical clarity. So it seems to be a reserved domain for the initiated, unsuitable for an introductory text on condensed matter physics. In comparison, the Gaussian formulation appears to be simpler in the case of Schr dinger equation for an atom, while the SI formulation appears...
