Bell Jar Experiment

Materials Required:

 The Procedure:

   (a) As performed in a real lab: 

  1. Arrange the apparatus as shown in the figure.
  2. Connect the electric bell and the vacuum pump to the air tight glass bell jar.
  3. Insert key to complete the circuit.
  4. Reduce the air pressure inside the chamber by pumping out the air through the vacuum pump.
  5. As the air in the jar is being pumped out, the sound becomes fainter, although it is the same current that is passing through the bell.
  6. On reducing the air pressure further, the intensity of the sound reduces and stops.

   (b) As performed using the simulator:

  1. Click on the ‘Power on’ button to insert key in the circuit.
  2. The hammer hits the gong constantly and the sound of the electric bell is heard.
  3. Decrease the air pressure from 1 atm by moving the ‘air pressure’ slider.
  4. Listen to the intensity of sound heard from the bell jar, observe the hitting of the hammer on the gong and put a tick mark on the appropriate place in the observation column.


Air Pressure
Intensity of Sound
Very High High Low Very Low Very Feeble No Sound

We have observed that as the pressure inside the bell jar decreases, the intensity of sound heard from the jar gradually decreases and stops. However,  the bell is still continuing to ring.