The bell-in-vacuum apparatus provides a direct demonstration of the relationship between sound transmission and air pressure. An electric bell powered by 4-6V AC or DC is suspended on a rubber cord inside a glass jar, with fine coiled wire connections running to terminals mounted in a rubber bung that seals the top of the jar. The assembled apparatus measures 22 x 15 cm and is designed to sit on a vacuum pump plate with a minimum diameter of 15 cm (pump plate not included). As air is progressively evacuated from the jar, the audible sound of the ringing bell diminishes, illustrating that sound cannot travel through a vacuum.