high resolution analog AC voltmeter mystery
From what I can tell (but the quality of the schematic is quite poor so this could be an incorrect interpretation) they use current sense resistors to turn the AC current into an AC voltage. This AC voltage is passed through the transformer (L1, L2 and L3). After this it is rectified and it's value is measured. Since the impedance of the voltage measurement circuit is so high, it doesn't influence the voltage drop due to the current.