Wave Propagation and Group Velocity contains papers on group velocity which were published during the First World War and are missing in many libraries. It introduces three different definitions of velocities: the group velocity of Lord Rayleigh, the signal velocity of Sommerfeld, and the velocity of energy transfer, which yields the rate of energy flow through a continuous wave and is strongly related to the characteristic impedance. These three velocities are identical for nonabsorbing media, but they differ considerably in an absorption band. Some examples are discussed in the last chapter dealing with guided waves, and many other cases of application of these definitions are quoted. These problems have come again into the foreground, in connection with the propagation of radio signals and radar. Reflection in the Heaviside layers requires a real knowledge of all these different definitions. Group velocity also plays a very important role in wave mechanics and corresponds to the speed of a particle. The present book should be very useful to physicists and radio engineers and should give them a good basis for new discussions and applications.
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