Counterweights, on the other hand, are lead.
The entire tonearm rides on a low-friction air bearing. A high-pressure (20-psi), low-flow pump sends air to a cylindrical manifold in the arm's bearing tube. Air, escaping through holes in the manifold, creates a cushion that totally supports the bearing tube. With air constantly escaping, pressure in the bearing is only about 7.5 psi. But bearing rigidity increases with the square of the air pressure, so the bearing is said to be quite rigid. The tonearm tracks linearly cross the record, keeping the cartridge or needle tangent to the record groove.