Learning or lurking?: Tracking the "invisible" online student

A crystal pulling apparatus is designed to generate a thermal gradient across the melt surface to prevent nucleation of stray crystals and production of floating debris to produce a high quality crystal, and has special provisions for observing the growth behavior and crystal dimension measurements. The apparatus includes a cylindrical chamber, a crucible disposed centrally within the chamber, a cylindrical heater surrounding the crucible, an insulation member disposed on the top section of the crucible, a first transparent plate and a second transparent plate for closing the center hole in the insulation member, a pull rod passing through the center hole of the transparent plates, a crystal illumination mechanism, a crystal size determination mechanism and an ambient atmosphere flowing mechanism. The crystal size determination mechanism is provided with a quartz prism, an infrared transmitting filter on a side wall of the chamber, a revolution count circuit, phase angle setting circuit, a CCD camera, an image processing section, and a crystal size determination device having a TV monitor.