Hall effect current measurement isn’t a direct measurement of electric current. Rather, the hall effect is caused by the magnetic fields that electric currents produce. Hall effect describes a change in resistance of a very thin conductive film when it has a magnetic field passing through it perpendicular to its surface. Thus, the hall effect is a direct measurement of the magnitude of a certain orientation of magnetic field. When the thin film is properly situated near a larger conductor, the magnetic field measurement can be turned into an accurate measurement of the current through the larger conductor. Reading unwanted nearby magnetic fields is possible, and strong magnets can saturate the hall effect sensors, but shielding measures can be taken to prevent those sources of error if necessary.
So the thin film is placed near a larger conductor, and the hall effect causes the resistance of the thin film to increase as a function of the magnetic field arising from the current through the larger conductor. This resistance is then converted into an output voltage by passing a separately sourced current through it, the signal is given low source impedance with a follower and/or amplifier, and we get an accurate measurement of the current without being in direct electrical contact with the conductor being measured. This ability to measure current without contact is highly advantageous in situations where the measurement has to be made at a portion of the circuit that is at a high voltage with respect to ground, and the current source can be designed to provide as much accuracy as necessary as well as providing outputs that are linear with respect to current.
