A system that indicates only the magnitude of variations in total coil impedance is called:

The system that indicates only the magnitude of variations in total coil impedance is called the Impedance-magnitude system. This type of system focuses specifically on measuring the total impedance, which consists of both resistive and reactive components, but in this case, it does not provide information about the phase angle or any vector representation of the impedance.

Such systems are particularly useful in applications where understanding the amplitude of impedance changes is crucial, such as in monitoring the condition of coils in various electrical systems. By concentrating solely on the magnitude of the impedance, users can gain insights into the differences caused by factors like changes in temperature, material properties, or the physical condition of the coil, aiding in maintenance or performance assessments.

The other choices imply additional functionality or different focuses. The inductive reactance magnitude system suggests a focus only on inductive reactance rather than total impedance. A feedback-controlled system typically refers to a broader control mechanism and does not specifically pertain to measuring impedance. The impedance vector analysis system indicates a more comprehensive approach, which includes both magnitude and angle analysis. Thus, the Impedance-magnitude system is the most accurate description of the function in question.