The use of a magnetic separator for the separation of certain ores has long been a standard commercial practice and a laboratory scale instrument applying the same principles to the separation of mineral grains from a powdered rock has been designed by Hallimond. In this apparatus the mineral grains are spread on a rotating pan which carries them below the pole pieces of one or more electromagnets, the pole pieces being so designed that the grains pass through a strongly diverging magnetic field and so are subjected to a force proportional to their susceptibility. If this force exceeds the weight of the grains they are attracted and adhere to the pole pieces. The material thus collected is periodically discharged by interrupting the magnetic field and the grains then fall through a slot in the rotating pan into a receptacle below.