Core materials used in making transformers exhibit B-H characteristics due to magnetic saturation.
In order to determine the B-H characteristics, make the AC flow through the core, and then superimpose on the DC, then the second harmonic will produce on the secondary side.
Apply this principle to detect the second harmonic using the 2F mode of the lock-in amplifier. If this is fed back to the transformer as an inverse DC, the system will always be able to correct the lock-in amplifier so that it does not produce second harmonics. Therefore, the modified phase-locked amplifier DC current flow and DC equal to the lock-in amplifier by measuring the output, you can measure the current.