Abstract: An improved instrument for measuring the relative "stickiness" of a drilling mud under well drilling condition is disclosed. It features a filtration member comprised of the material of the formation within which the well is drilled. After formation of a filter cake on the surface of the stone filtration member, a plunger is juxtaposed to the stone filtration member and the amount of torque required to rotate the plunger with respect thereto after juxtaposition for a fixed period of time is measured, to provide a relative indication of the adhesion of the plunger to the stone.