What is the normal indication when using the ellipse method in eddy current testing and the test specimen matches the reference standard?

When using the ellipse method in eddy current testing, the normal indication occurs when the test specimen matches the reference standard. In this case, a straight horizontal line on a cathode ray tube indicates that there is no difference between the test specimen and the reference standard being used for comparison.

This straight line signifies that the characteristics of the test piece—such as conductivity, permeability, and geometry—align exactly with those of the reference, resulting in no detectable variations that would cause the indication to deviate from this horizontal baseline. The presence of a straight line is crucial for establishing that the material has no significant flaws or variations that would affect its performance.

Other options, such as an ellipse or a null reading, indicate varying conditions that do not represent the normal indication when a specimen matches the reference. An ellipse typically represents the interaction of signals that indicate variations or potential defects; hence it would not be the expected result in this scenario.