Viscosity is a measurement which describes the relative ease or difficulty with which a fluid will flow. If one imagines a glass full of a LIQUID to be divided into very small individual layers, viscosity can be understood more easily. It is believed that when a liquid is poured, each layer acts in a manner to resist the flow of the layer above it. The more resistance offered by each layer, the more difficult the flow. Thus, the more viscous the liquid, the greater its viscosity.
One of the earliest methods of measuring the viscosity of a fluid was to observe the flow of the fluid through a small tube. A law and a fairly complicated formula were introduced by a scientist named Jean Poiseuille for measuring viscosity. The units of measurement are called poises in honor of this man. Although there have been refinements through the years, viscosity can, and still is, measured by the "flow-through-a-tube" method. In addition, however, electrical instruments are now used that contain a revolving spindle immersed in the liquid where viscosity is to be determined. These electric viscosimeters read directly in poises, or fractions of poises, called centipoises or millipoises. Viscosity measurements are very important in quality control of lubricants, in regulating the movement of liquids through pipes, and in analysing blood.