12.2.Magnetic Phenomena and Their Interpretation 227 to the north;see Figures 12.2(a)and (b).However,we will mostly utilize their moduli in the following sections and thus use light- face italic letters. B was called above to be the magnetic flux density in a mate- rial,that is,the magnetic flux per unit area.The magnetic flux is then defined as the product of B and area A,that is,by 中=BA. (12.7) Finally,we need to define the magnetic momentn(also a vector)through the following equation: M=0, (12.8) which means that the magnetization is the magnetic moment per unit volume. A short note on units should be added.This book uses SI units throughout.However,the scientific literature on magnetism(par- ticularly in the United States)is still widely written in electro- magnetic cgs(emu)units.The magnetic field strength in cgs units is measured in Oersted and the magnetic induction in Gauss. Conversion factors from SI into cgs units and for rewriting Eqs. (12.1)-(12.8)in cgs units are given in Appendix II. 12.2.Magnetic Phenomena and Their Interpretation We stated in the last section that different types of magnetism ex- ist which are characterized by the magnitude and the sign of the susceptibility (see Table 12.1).Since various materials respond so differently in a magnetic field,we suspect that several funda- mentally different mechanisms must be responsible for the mag- netic properties.We shall now attempt to unfold the multiplicity of the magnetic behavior of materials by describing some perti- nent experimental findings and giving some brief interpretations. 12.2.1 Ampere postulated more than one hundred years ago that so- Diamagnetism called molecular currents are responsible for the magnetism in solids.He compared these molecular currents to an electric cur- rent in a loop-shaped piece of wire which is known to cause a magnetic moment.Today,we replace Ampere's molecular cur- rents by orbiting valence electrons. To understand diamagnetism,a second aspect needs to be con- sidered.As explained in Chapter 10 a current is induced in a wire loop whenever a bar magnet is moved toward (or from)this loop.to the north; see Figures 12.2(a) and (b). However, we will mostly utilize their moduli in the following sections and thus use light￾face italic letters. B was called above to be the magnetic flux density in a mate￾rial, that is, the magnetic flux per unit area. The magnetic flux  is then defined as the product of B and area A, that is, by 
B A. (12.7) Finally, we need to define the magnetic moment m (also a vector) through the following equation: M
 V m , (12.8) which means that the magnetization is the magnetic moment per unit volume. A short note on units should be added. This book uses SI units throughout. However, the scientific literature on magnetism (par￾ticularly in the United States) is still widely written in electro￾magnetic cgs (emu) units. The magnetic field strength in cgs units is measured in Oersted and the magnetic induction in Gauss. Conversion factors from SI into cgs units and for rewriting Eqs. (12.1)–(12.8) in cgs units are given in Appendix II. We stated in the last section that different types of magnetism ex￾ist which are characterized by the magnitude and the sign of the susceptibility (see Table 12.1). Since various materials respond so differently in a magnetic field, we suspect that several funda￾mentally different mechanisms must be responsible for the mag￾netic properties. We shall now attempt to unfold the multiplicity of the magnetic behavior of materials by describing some perti￾nent experimental findings and giving some brief interpretations. Ampère postulated more than one hundred years ago that so￾called molecular currents are responsible for the magnetism in solids. He compared these molecular currents to an electric cur￾rent in a loop-shaped piece of wire which is known to cause a magnetic moment. Today, we replace Ampère’s molecular cur￾rents by orbiting valence electrons. To understand diamagnetism, a second aspect needs to be con￾sidered. As explained in Chapter 10 a current is induced in a wire loop whenever a bar magnet is moved toward (or from) this loop. 12.2.1 Diamagnetism 12.2 • Magnetic Phenomena and Their Interpretation 227 12.2 • Magnetic Phenomena and Their Interpretation