Bell in Jar with Air pump

Quick Answer: The Bell in Jar with Air Pump is a sound demonstration in which an electric bell rings inside a sealed glass bell jar while air is pumped out. As the air is removed the ringing grows fainter even though the hammer can still be seen striking, showing that sound needs a material medium to travel.

Sound Needs a Medium

In the listing image, the bell in jar with air pump has a glass bell jar sealed on a base plate, an electric bell with its gong inside, terminals on the stopper for connecting the bell to a supply, and a hand-operated piston pump with a wooden handle joined to the base by a connecting tube. With air in the jar, switching on the bell gives a clear ring. The pump is then worked steadily to draw air out of the jar.

Sound is a mechanical wave: the vibrating gong pushes on neighbouring air molecules, which push on their neighbours, passing the vibration outwards as compressions and rarefactions. As the pump removes air, there are fewer molecules to carry the vibration and the sound reaching the students weakens, while they can still see the hammer striking. Letting air back in restores the ring, which makes a convincing before-and-after comparison.

It is worth telling students what to expect. A hand pump cannot produce a perfect vacuum, and some vibration also travels through the bell’s supports and wires into the base, so the sound fades to a faint ring rather than disappearing completely. That residual sound is itself evidence that solids carry sound too. The pump type, the jar size and the bell’s supply voltage should be confirmed at enquiry.

Specifications

Care & Handling

• Apply a thin film of vacuum grease where the jar rim meets the base plate, and keep the seal free of grit.
• Inspect the glass for chips and cracks before every use; never evacuate a damaged jar.
• Let air back in slowly rather than lifting the jar while it is under vacuum.
• Disconnect the bell supply after the demonstration and store the bell in jar with air pump in padding.

Teaching with the Bell Jar

• Showing that sound cannot travel through a vacuum
• Comparing the loudness of the bell before, during and after pumping
• Discussing why astronauts use radios and why space is silent
• Showing that light passes through the evacuated jar while sound does not

Why Choose LabEquip

Science teachers covering sound need a vacuum demonstration that the whole class can hear and see, and a hand pump avoids the need for an electric vacuum pump. LabEquip lists this set in its STEM kits range with the Organ Pipe Demonstration Model for the lessons on pitch and resonance that follow. Contact us through the contact page.

Frequently Asked Questions

Why does the bell sound fainter as air is pumped out?

Sound travels as vibrations passed from molecule to molecule. Removing air leaves fewer molecules to pass on the vibration, so less sound energy reaches the jar wall and the listeners, and the ringing becomes quieter.

Why can the bell still be heard faintly?

A hand pump leaves some air in the jar, and vibrations also travel through the bell’s mounting and wires into the base and bench. The faint remaining sound shows that solids carry sound as well as air.

Why can we still see the bell when we cannot hear it?

Light is an electromagnetic wave and needs no medium, so it passes through the vacuum unaffected. Sound is a mechanical wave and does need a medium. The jar shows both facts at once.

How should the jar be sealed?

Spread a thin, even layer of vacuum grease on the base plate where the rim sits, press the jar down with a slight twist, and check that the connecting tube is tight. A good seal lets the pump lower the pressure noticeably within a short time.

Is it safe to pump air out of a glass jar?

Yes, provided the jar is undamaged and made for the purpose. Outside air pressure pushes inward on the evacuated jar, so a cracked or chipped jar could implode. Inspect the glass each time and keep students at a sensible distance.

What does the demonstration tell us about space?

Space is very close to a vacuum, so sound cannot travel through it. Astronauts working outside a spacecraft talk by radio, because radio waves, like light, cross a vacuum easily.

Last Updated: September 2026