A method and apparatus for measuring the height of the density layer in a material bed layered according to densities, such as in a jigging machine operation, utilizes a free-floating measuring body having a transmitter means embedded therein and a probe mechanism for monitoring the relative position of the body in the material bed. The measuring body is made of a material having a density equal to the relative densities of materials forming the separating line between two density levels and is able to move with the materials during the grading process. The probe mechanism is housed in a guide tube which extends in the direction of the density gradient through the material bed.