| 摘要 |
DC magnetizing pulses produce unidirectional flux in the object under test, advantageously into the saturation region thereof. The DC pulses have, or are accompanied by, an AC component for producing eddy currents in the object, and variations in the eddy currents are sensed and used to produce corresponding signals. In a preferred embodiment composite DC pulses having a relatively broad initial value followed by shorter DC pulses are intermittently applied to a primary coil wound around the core of a probe adapted to be inserted into a tube in situ. Small null coils provide an output signal. Quadrature detection and gating means yield outputs corresponding to sufficient saturation of an object, and free from transients due to the intermittent bursts of pulses. Flaw detector and comparator type apparatus are described, including bridge configurations in which the AC component is applied to the bridge. A differentiated output of a single null coil or a comparator coil may be used to give an indication of saturation.
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