| 摘要 |
PURPOSE:To obtain a high measuring accuracy, by arranging a material equivalent to an object to be inspected at a position separated from a reference coil by the liftoff of an active coil. CONSTITUTION:A probe body 3a contains an active coil 5 which causes a direct electromagnetic action directly upon a liner-covered tube 4 to obtain a change in the impedance corresponding to a linear layer thickness and a reference coil 6. The coil 5 is set to keep the distance from the surface of a liner layer 4b, namely, a lift off l constant. A plate 8 the same in the material as a base material layer of the tube 4 is arranged for the coil 6 through a spacer 7. With such an arrangement, the impedance ratio between the coils 5 and 6 is little varied with the frequency.
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