PURPOSE: To show that sound, unlike light, does not travel in a vacuum.
DESCRIPTION: A bell is sealed in a jar. A small light bulb can also be sealed in the jar with the bell. Switch on a power supply to turn on the bell and start it ringing. The sound of the bell is heard. Turn on a vacuum pump and evacuate the jar. The sound becomes increasingly weaker as the pressure drops and is no longer audible when the jar is sufficiently pumped down. Note that a microphone cannot be used because the bell is electronic and the mic picks up the RF noise from the ringing of the small switching coil.
Alternatively, this demo can be performed using the metronome inside the large bell jar. This is better because the metronome is not electronic and a mic can be used inside the bell jar. The metronome runs off a spring.
There is some valid argument that this demo shows is due to an impedance effect under vacuum.
EQUIPMENT: Bell in a jar, as photographed.
Updated by Jun Qi in 3/24/2000