Comparative study on the maximum-pull and tear-off ring method for the measurement of surface tension

The ring method, due to its basic form, is often used in the classroom for an immediate and intuitive grasp of concept of surface tension. Yet, during the measuring process, the large and small results trouble the students. The large values are obtained from the maximum-pull, and the small from the tear-off moment. In this paper, we demonstrate a series of rings with different thicknesses to investigate these two moments. In the water measurement, by subtracting the weight of the liquid raised by the ring or replacing the ring-diameter with tearing-diameter for the moments of maximum pull and film breaking, respectively, a corrected surface tension is obtained. The same correction is also applied in the determination of Ethylene Glycol and ethanol, but only the moment of maximum pull is applicable. These experimental results are interesting, and will largely stimulate students to understand the concept of surface tension and the ring method of measurement.