A precision Langmuir film balance measuring system

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Abstract
A modified Langmuir film balance with a new type of pressure pick-up system is described. The pick-up system consists of a closed rectangular frame of Teflon-covered steel foil floating on the substrate surface. The deformation of the frame is measured by four strain gauges fixed to the frame in a symmetric arrangement. The gauges form a bridge circuit which is activated by a lock-in amplifier. The resolution of the film balance is better than 20 mu N m-1. By switching the sensitivity settings of the lock-in amplifier, one can measure between 1-70 mN cm-1 full scale range with the same device. The trough has been constructed of brass and massive Teflon parts. By avoiding the application of any adhesive the danger of impurity effects is minimised. The main advantage of the system is the extreme tightness. Thus both isotherms and isobars may be recorded at very low speed.