When using an encircling coil with both primary and secondary windings, the excitation alternating current is applied to:

In an encircling coil, the primary winding is where the excitation alternating current is applied. This primary winding creates a magnetic field that induces a current in the secondary winding due to electromagnetic induction. The primary winding is responsible for generating the magnetic field needed for the process, while the secondary winding serves to detect the resulting induced currents, which are directly related to the properties of the material being tested.

Using this configuration, the primary winding effectively transfers energy to the secondary winding, allowing for the examination of the material's conductivity and other characteristics. Therefore, applying the excitation alternating current to the primary winding is fundamental to the mechanism of operation in encircling coil setups.