In this series of experimental tests freshly prepared floc- culant was placed in a cylindrical container. The container was a tube of glass with 15 mm internal diameter and 30 cm height which was sealed at both ends with rubber corks. The tube was small enough to be placed between the poles of the permanent magnet so that the bottom of the settling apparatus could be aligned accurately with the top of the magnet. The total initial concentration of iron was held constant at 10 g/l. Two sedimentation tests were carried out simultane ously — one with the magnet present and the other without. Before the test started the container was well shaken to ensure that the contents were well mixed and then a ruler was inserted into the cylinder to stop the fluids swirling around. A standard settling test was performed and the height h of the slurry water interface was measured as a function of time t.