A length of tubing containing a notch running from end to end, when tested with an encircling self-reference differential coil system, will produce:

When testing a length of tubing that has a notch extending from end to end using an encircling self-reference differential coil system, the nature of the signal generated depends on how the notch interacts with the electromagnetic field created by the coils.

In this scenario, the notch essentially disrupts the uniformity of the tubing. A self-reference differential coil system is designed to detect changes in impedance or any discontinuities in the material being tested. Since a notch creates a significant discontinuity, the electromagnetic field is not adequately coupled through the material where the notch is present.

Therefore, the encircling coil detects little to no disruption because the magnetic field cannot effectively penetrate the area with the notch. This leads to the output signal from the coil indicating effectively no response, which can be interpreted as "no signal." In a properly functioning configuration, only uniform, continuous areas would generate signals, while the notch's presence leads to a lack of detectable signals, rendering it as "no signal."

Understanding this interaction helps explain the result seen during such a test, reinforcing the importance of recognizing material discontinuities in non-destructive testing methodologies.