Suresh Kumar

Abstract: This experiment safely provides following paradoxical observation to children not familiar with the concept of atmospheric pressure. Like all liquids, mercury stands at the same level in the two arms of a U‐tube. But, when air is pushed out of one of the arms, mercury level in it stands higher. The level difference is constant irrespective of inclination of the tube with vertical and length of vacuum space. It decreases if the apparatus is carried up in a building. Inference is clear and unambiguous that air pressure acting on lower mercury surface supports this level difference. This observation is also provided by the original Torricelli experiment and, therefore, it is always included in school curricula. However, it spreads toxic mercury vapor in air and cannot be demonstrated. Recently, we want to eliminate all use of mercury in schools. Hence, safe Torricelli experiment can be shown in class rooms by a motion picture. The actual experiment can be placed in a science resource center.

TL;DR: In this article, a 45°•90°•45° triangular prism is placed with its diagonal face downwards on a convex lens surface of large radius of curvature, and an air film at the tiny contact area is seen blue.