Fluke article on Hall Effect used on clamp meters. Link here
Hall Effect clamp meters can measure both ac and dc current up to the kilohertz (1000 Hz) range. Like current transformer types, Hall Effect clamp meters use rigid iron jaws to concentrate the magnetic field that encircles the conductor being measured.
Unlike current transformer clamp meters, the jaws are not wrapped by copper wires. Instead, the magnetic field generated by the conductor is focused across one or more gaps in the core after the jaws are clamped around the conductor. Notice the point where the jaw tips of a Hall Effect clamp meter meet.
A gap exists where the jaw tips of a Hall Effect clamp meter meet, creating an air pocket that the magnetic field (aka magnetic flux) must jump. This gap limits the magnetic flux so that the core cannot saturate.
In contrast, the jaws of an ac-only current transformer clamp are flush when closed. When opened, the tips of the jaws show bare metal core faces.
In that gap, covered by thin plastic molding, is a semiconductor known as a Hall Effect sensor—a transducer that varies its output voltage when responding to magnetic fields, in this case the magnetic field of the conductor or wire being measured. Its purpose is to measure magnetic flux directly. The output voltage from the sensor then amplified and scaled to represent the current flowing through the conductor that lies inside the jaws of the clamp.