| http://www.w3.org/ns/prov#value | - In another embodiment of the present invention, two kinds of control voltages at predetermined electric potentials are respectively input to the two D.C. power supplies, one of the two D.C. power supplies outputs a voltage equal to a sum of the two kinds of control voltages, and the other of the two D.C. power supplies outputs a voltage equal to a difference between the two kinds of control voltag
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