In the past it has been customary to assume that earth wires in a transmission system have zero potential along their entire length. In the paper attention is drawn to the fact that this assumption is valid only if the electrical length of the spacing between towers is less than one quarter wavelength. A generalised theory is developed for the case of earth wires which are bonded to earth only at the towers and are treated as conductors between towers. The method is an extension of the theory of natural modes previously given, in which it is shown that a modified concept of natural modes satisfactorily explains the present case. A suitable procedure for digital computation is described, which draws special attention to problems different from those described previously. The results of numerical computation are presented in which it is shown that the discretely bonded earth wires act like selective filters. This is manifested by rapid changes in attenuation, impedance and modal vector distributions in the vicinity of frequencies which make the electrical length between towers even multiples of one half wavelength. The paper concludes by considering further problems still outstanding in developing a complete theory of multiconductor systems.
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