ABSTRACT
The scope of eddy current testing is summarized in section 4.1. It should, however, be reiterated that (with few exceptions) the use of the method is restricted to testing samples made either wholly or partly from metal and then only for regions at or just below the surface. However, the technique has the distinct advantage that no contact with the object under test is required and that surface preparation is usually not necessary. As stated in section 4.1, the method consists of passing an alternating current through a coil so as to excite coaxially circular currents, i.e. eddy currents, in an electrically conducting object in its vicinity. The resulting changes in the components of the impedance of the coil are characteristic of the electrical and magnetic properties of that part of the object in which the eddy currents are flowing. Generally, a single coil is used but in some instances, to be discussed later, two coils are employed; one of these excites the eddy currents and the other detects them. The essential parts of eddy current equipment are the exciting oscillator, the measuring circuit, usually an ac inductance bridge, and the detecting coil (or coils). The object to be tested can be scanned either manually or with the aid of a mechanical device. In the latter instance, the signals denoting the positions of the coil are often synchronized with the output signals from the detector.
