. . . "The exciter force Fexc, and also the exciter moment Mexc, can, in this case, as, in fact, shown schematically in FIGS. 4 and 6, each be in bi-directional form, or, however, also in uni-directional form, and are, in manner known to those skilled in the art, tuned as regards their amplitude e.g. by means of a current and/or voltage regulating circuit and as regards their frequency e.g. by means of a" .