The ball mill is a rotating cylindrical or cylindrical-conical shell that is about half filled with balls of steel or ceramic. Its speed of rotation is enough such that the balls are caused to tumble over one another without causing cascading. The speed is usually less than 80% of the critical speed which would just cause the charge of balls and feed material to be centrifuged. The carrier medium is air in the continuous milling, which may be heated to avoid moisture which tends to cause clogging. The ball mill may also be used for wet grinding with water being used as the carrier medium. The balls size is chosen to suit the desired product size. The conical section of the mill causes the smaller balls to move towards the discharge end and accomplish the fine grinding. Tube mills are very long ball mills which are often compartmented by diaphragms, with balls graded along the length from large at the feed end to small at the discharge end.