Aim of the experiment
To show that sound needs a material medium for its propagation
Glass jar, vacuum pump, electric bell.
Bell-jar experiment demonstrates that sound needs a medium for its propagation. Take an electric bell and an airtight glass bell jar connected to a vacuum pump. Suspend the bell inside the jar, and press the switch of the bell. You will be able to hear the bell ring. Now pump out the air from the glass jar. The sound of the bell will become fainter, and after some time, the sound will not be heard. This is so because almost all air has been pumped out. After some time, when less air is left inside the bell jar, you will hear a very feeble sound. And finally, when there is no air present inside the jar, there is no sound detectable. This shows that sound needs a material medium to travel.