A brief introduction is given to the formation of convection and "withdrawal" currents glass tanks as produced by heating one region of the top surface of the melt. The production of an S-shaped current when the top surface is not horizontal, due to withdrawing glass at one end and charging batch at the other, is explained. The effects on melting and refining are given. The following of erosion lines and the adding of isotopes as methods of determining the positions of the currents are briefly considered. The sinking pendulum and rudder methods for finding speed and direction are described in more detail. The pendulum is pushed with the current until it reaches a stop where it makes a contact at Pt points shielded from the glass by compressed air, the pressure of which is a measure of the pressure at that point in the melt.
