First measurement of the magnetic birefringence of helium gas

Abstract Using a sensitive ellipsometer we have measured the Cotton-Mouton constant of helium at 514.5 nm. We have found that, at 1 atm and 0°C,C CM (He) = (3.5 ± 0.7) × 10 -20 G -2 cm -1 . Our experimental value is compatible within the errors with previous theoretical calculations.