Which of the following is not a commonly used eddy current testing readout mechanism?

In the context of eddy current testing, the readout mechanisms are critical for interpreting the signals produced by the test. Among these options, a signal generator is not a readout mechanism but rather a device used to create the electrical signals that will be sent through the probe to induce eddy currents in the test material.

The other options represent different ways to visualize or record the eddy current signals. A meter can provide direct readings of the impedance changes detected, while a cathode ray tube (CRT) allows for a graphical representation of the signals, helping in the analysis of changes in response over time. Similarly, a strip-chart recorder provides a permanent paper record of the signal variations, which can be valuable for further analysis and documentation.

In summary, the signal generator's primary role is to produce signals for testing, rather than interpreting or displaying the results, which makes it the correct choice for the item that is not a commonly used readout mechanism in eddy current testing.