Hall effect current transducer schematic diagram
You have given no context or credit for the schematic (which is in violation of site policy so you should fix that) but it looks like a Hall-effect based current sensor.


  The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was discovered by Edwin Hall in 1879.


Sensors based on the Hall-effect run a controlled current through the sensor which is placed in the magnetic field to be measured. The voltage across the sensor is then amplified to give a signal proportional to the magnetic field strength.



Figure 1. A magnified view of the schematic.


The output amplifier.
The reference current amplifier with offset adjustment potentiometer.
The sensor.
The magnetic core (suggesting a torroidal current sensor).
The primary winding(s) which pass through the magnetic circuit.
Full-scale adjustment. (This is in the amplifier feedback circuit and adjusts the gain.)