A changing magnetic field will be present near the wires carrying AC. Voltage will be induced between the ends of a conductor placed in this field. We can explore this by connecting one end of a long wire to the measuring equipment.
Learn about the AC mains supply. Explore the phenomenon of propagation of AC through free space.
The amplitude of the pickup depends on the wattage of the electrical equipment operating nearby, distance to them and on the length of the wire used.