M-ST 3: Strength of surface tension of water

PIRA: Unknown

Equipment: glass tumbler (preferably rather large) located in a cabinet in Room. 4, cheesecloth in Set 5, Cabinet 1, Shelf 1 and an English card.

Procedure: Cut a piece of cheesecloth, either square or circle, so that when it is stretched over the mouth of the tumbler there is at least an inch of cloth extending beyond the rim at every point. Fill the tumbler completely with tap water, plce the cloth over the top making sure that it gets wet everywhere, and press the cloth outside the rim against the sides of the tumbler so that it clings there. Place the flattened fingers of one hand over the tumbler's mouth while with the other hand you pick up the tumbler and turn it upside down with a sort of circular motion designed to force the water away from the mouth during the operation. Then remove the hand over the mouth; the water stays in the tumbler.

If the tumbler is tilted so that the plane of the rim is not horizontal, water will flow out the low side and air will enter at the high side. This exchange can be stopped by righting the tumbler so that the rim is again horizontal. It is interesting to note that the atmospheric pressure is higher than that inside the tumbler, as indicated by the curvature of the cloth surface. If the experiment fails, it is usually because the cloth is drawn into the tumbler so far that the cloth loses contact at the rim.

The experiment can be successfully performed with an English card replacing the cheesecloth, but the warping of the card as it soaks up water makes the performance less reliable, and not nearly as dramatic.