[en] The plasma beta on axis of a high-β plasma column can be determined by combining measurements of the magnetic flux excluded by the plasma column and the plasma luminosity profile. The excluded flux is measured by a balanced loop-probe arrangement consisting of a small magnetic probe outside the discharge tube, which measures the external magnetic field, and a single-turn loop around the discharge tube which senses dphi/ dt inside the tube and is sensitive to the magnetic flux excluded by the plasma. With the signals from the loop and probe nulled without plasma, the integrated difference signal normalized to the integrated probe signal gives the net magnetic flux excluded by the plasma column or the effective area from which flux is completely excluded