| 摘要 |
PURPOSE:To improve searching performance by containing receiving coils in negating coils for negation of the influence of excitation magnetic fields upon the receivng coils. CONSTITUTION:When electric current is supplied to a negating coil 3a, an exciting coil 1, and a negating coil 3b from an oscillator 7, an excitation magnetic field H1 is generated by the coil 1. If a metallic piece 8 exists in the axial direction of the magnetic field H1, induced current is generated in the piece 9 and a slight signal magnetic field H9 is generated. On the other hand, the current of the oscillator 7 flows in the coils 3a, 3b as well; therefore, negating magnetic fields H3a, H3b are generated. Since these magnetic fields are so constituted that their directions are reversed from the direction of the magnetic field H1, the magnetic field H1 is negated by the magnetic fields H3a, H3b. Therefore, receiving coils 2a, 2b detect only the signal magnetic field H9, and generate detection signals. An amplifier 8 delivers the output for the difference between the detection signals of the coils 2a and 2b. |
