Without air or matter, sound waves cannot travel and therefore cannot be heard. This bell jar effectively demonstrates this concept using a battery operated buzzer housed inside the jar. When the buzzer is placed beneath the jar and a vacuum is created inside the jar, students will observe a total absence of sound. Once the air is returned to the jar, the sound of the buzzer can be heard once more. The activity requires a vacuum pump & plate, not included.
Features
The apparatus consists of an electric bell placed inside a bell jar. The bell jar is sealed on a pump plate, which can be connected to a vacuum pump. When the bell is turned on, it can be heard ringing. As air is pumped out of the jar, the sound of the bell becomes quieter until it is no longer audible, even though the bell's clapper can still be seen moving. This experiment proves that sound waves require a medium, such as air, to propagate.