**PURPOSE: **To demonstrate surface tension and to determine the value of the surface tension of water.

**DESCRIPTION: **This is the classic experiment for determination of the surface tension of water. A small pan is attached to the bottom of the spring onto which weights can be placed to determine the spring constant. Measure the equilibrium position of the rectangular frame when not immersed, and when completely immersed. The average should be taken with the wire just making surface contact. Slowly and smoothly lower the platform until separation occurs, and measure the displacement. The surface tension *T* is given by *T*=*F*/2*L*, where *F* is the force of the spring and *L* is the length of the wire (which is 8 cm for this apparatus). The spring constant *k* is 7.27 g/cm, so the force is *F*=*k*g*x*, where *x* is the extension of the spring and g is the acceleration of gravity.

**SUGGESTIONS: **Practice. This apparatus is sensitive.

**REFERENCES: ** (PIRA 2A10.32) See Demonstration Reference File for further information.

**EQUIPMENT: **Jolly balance, vertical adjusting platform, small weight set.

**SETUP TIME: **10 min.