A capillary tube of very fine bore is connected by means of a rubber tube to a burette kept vertically. The capillary tube is kept horizontal as shown in Figure. The burette is filled with water and the pinch – stopper is removed.

The time taken for water level to fall from A to B is noted. If V is the volume between the two levels A and B, then volume of liquid flowing per second is V /t. If l and r are the length and radius of the capillary tube respectively, then

V/t = **πPr ^{4 }/ 8η∫ … …. (1)**

If ρ is the density of the liquid then the initial pressure difference between the ends of the tube is P_{1} = h_{1} ρg and the final pressure difference P_{2} = h_{2} ρg. Therefore the average pressure difference during the flow of water is P where

P = (P_{1} – P_{2})/2

= [(h_{1} – h_{2})/2]*ρg = hρg

Substituting the equation (1) we get,

V/t = **πhρgr ^{4 }/ 8η∫**

**Or, η = (πhρgr ^{4}t) / (8∫V)**