Some of the techniques of viscometry were discussed in previous chapters. Many fluids, when subjected to a simple shear flow, develop not only shear stresses but also normal stresses. These normal stresses manifest themselves in the so-called “normal stress phenomena” which commonly occur in viscometric experiments. These stresses also materialize in many important flows. For example, when a rod (such as the shaft of a stirrer) is rotated about its axis perpendicular to the free surface of a Newtonian liquid, the liquid surface is depressed in the neighborhood of the rod, as a consequence of centrifugal forces that accompany the induced rotational flow. Some fluids, however, are observed to develop an elevated surface at the rod. This “rod climbing” effect is perhaps the best known normal stress phenomenon, and is called the “Weissenberg effect”.