other appropriate source known to those within the art which will provide a signal in the frequency range of interest the inverter circuits 15 and 16 are exemplary of many possible configurations which can produce the desired results the output from terminal 9 is inverted with respect to the original signal from oscillator 14 power inverter 15 maintains signal amplitude and generates an inverted signal which is applied to inverter 17 and the adjustable phase delay circuit 18 inverter 17 is essentially identical to inverter 16 the output from terminal 11 is inverted with respect to the output from terminal 9 phase delay circuit 18 inverts the signal twice so that the output is inverted with respect to the original oscillator signal the primary purpose of circuit 18 is to provide means for generating a signal which is out of phase with respect to the original input for the purpose of generating a circularly rotating field this phase angle should be 90 u00b0 circuit 18 also provides means for maintaining the circularity of the applied field by compensating for frequency dependent phase delays and imperfect electrode alignment power inverter 19 and inverters 20 and 21 are substantially identical to circuits 15 16 and 17 and are similarly arranged fig3 a is a graphic representation of the output signals from terminals 9 11 10 and 12 and are designated respectively u03c6 1 u03c6 3 u03c6 4 and u03c6 2 the subscripts.