The principles of the new polarity-sensing methods are based upon the differences between the positive and negative half waves of the signal that appear when the stator covered/stator exposed ratio of the mill head is large. Two types of the polarity discriminators are tested-a peak-comparing discriminator and a flip-flop discriminator. The latter is particularly stable if a voltage proportional to the peak value of the signal is supplied to the reference terminal of the shaping comparators (the floating reference method). Since mills with these polarity discriminators do not employ phase-sensitive detection, they can be driven by DC motors and they do not need an angular sensor. Hence the mills are simple, handy, inexpensive, easy to make, and can be battery operated.