Bell Jar With Stopper

Quick Answer: A bell jar with stopper is a dome-shaped glass cover with an open, flat rim at the bottom and a stoppered opening at the top. Placed on a flat plate it encloses an experiment, and the top opening admits a tube, wire, thermometer or suspended object.

What the Top Opening Adds

A plain bell jar is a closed dome. The stoppered version has a neck at the top closed by a bung or stopper, and that small change makes it far more flexible. A bored bung can carry a glass tube, a thermometer or electrical leads, so an electric bell can hang inside for the sound-in-a-vacuum demonstration, or a Y-shaped tube with two balloons can be fixed in place for a model of breathing.

In the lung model, the open base of the jar is covered with a stretched rubber sheet that represents the diaphragm. Pulling the sheet down increases the volume inside and lowers the pressure, and the balloons, standing in for lungs, inflate through the tube; pushing the sheet up makes them deflate. It is one of the most familiar biology demonstrations of how breathing works, and it is built from exactly this kind of jar.

For vacuum demonstrations the jar stands on the flat plate of a vacuum pump, sealed with a thin smear of vacuum grease on the rim, and a balloon, a marshmallow or a ringing bell inside shows the effect of removing air. The listing does not give size, rim finish or glass type, and the rim must be flat and smooth to seal on a pump plate.

Specifications

Applications

• Bell jar lung model with balloons and a rubber diaphragm in biology lessons on breathing
• Demonstrating that sound needs a medium, using an electric bell hung from the stopper
• Plant experiments, such as covering a potted plant with a dish of potassium hydroxide solution to show that photosynthesis needs carbon dioxide
• Keeping dust off delicate apparatus or samples on the bench

Care & Handling

• Place a mesh guard or cage around the jar, or tape it, before evacuating, as any glass vessel under vacuum can implode.
• Inspect the rim and dome for chips and scratches before every vacuum run.
• Apply only a thin film of vacuum grease to the rim, and wipe it off after use.
• Lift the jar with both hands around the body, not by the top stopper.

Why Choose LabEquip

Physics and biology departments often share one bell jar between vacuum demonstrations and the lung model, so a stoppered version that takes tubes and wires is the more useful purchase. It is supplied through LabEquip’s chemistry lab equipment section; send the size you need via the contact page.

Frequently Asked Questions

What is the stopper on a bell jar for?

It closes an opening at the top of the dome. With a bored bung, the opening can carry a glass tube, a thermometer or electrical wires into the jar, which is how the lung model and the ringing bell demonstration are set up.

How do I make an airtight seal on the vacuum plate?

Clean the rim and the plate, spread a very thin, even film of vacuum grease on the rim, and seat the jar with a small turning movement. Pumping then pulls it down firmly against the plate.

How does the bell jar lung model work?

A rubber sheet over the open base acts as the diaphragm and balloons on a Y-tube act as lungs. Pulling the sheet down enlarges the space in the jar, pressure falls, and air flows in through the tube to inflate the balloons.

Why does the bell go quiet when air is pumped out?

Sound needs a medium such as air to travel. As air is removed, fewer molecules carry the vibrations to the glass, so the sound fades even though the bell can still be seen ringing. A little sound always passes through the wires and supports.

Is it safe to evacuate a glass bell jar?

It can be done safely with precautions. Use a jar intended for vacuum, check it for damage, put a guard or mesh around it, and keep students at a distance. Let air back in slowly through the valve before lifting the jar.

Can a bell jar be used for plant experiments?

Yes. A potted plant covered by the jar, with a dish of potassium hydroxide solution to absorb carbon dioxide, is a classic way of showing that leaves need carbon dioxide to make starch. Seal the base on a glass plate with petroleum jelly.

Last Updated: September 2026