The 274002 Glass Bayard-Alpert Hot Cathode Gauge features a single thoria-coated iridium filament, a <1 x 10-9 to 1 x 10-3 Torr vacuum measurement range, and a 3/4 inch Pyrex glass inlet port. 274 Ionization Vacuum Gauges provide good vacuum and pressure measurement at an economical cost. Bayard-Alpert (hot cathode) gauges provide pressure measurement based on the ionization of gas molecules by a constant flow of electrons from a filament. The number of ions created is directly proportional to the density of molecules. The ions are collected and measured by a vacuum gauge controller and converted to pressure based on the sensitivity of the gauge.