A Leyden jar is a device used to store static electric charge. It can be used to conduct many experiments with electricity such as creating a spark across a gap.
The diagram below shows the parts of a Leyden jar that can be constructed using easily-available material.
Be careful not to be too ambitious with the amount of charge stored and do not carry out this demonstration with the charge passing through your hands if you have a weak heart! Try not to carry out this demonstration without adult supervision and make sure that you are clear of any furniture that might hurt you should the discharge cause you to crash into nearby furniture. Finally, do this demonstration at your own risk!
- Plastic cylinder with lid (e.g. soap bottle, film canister)
- Brass or steel sinker
- Metal paper clips
- Aluminum foil
- Adhesive tape
- PVC rod
- Cut a rectangular piece of aluminum foil of width equal to three-quarters of the height of the plastic cylinder, and length equal to the circumference of the cylinder.
- Stick it onto the outer surface of the cylinder with adhesive tape, leaving about a centimeter of gap between foil and the base. Make sure that the aluminum foil remains smooth.
- Pierce a hold in the middle of the lid of the plastic cylinder and put the hook of the sinker through it. String 3 to 4 metal paper clips together and hang them from the hook.
- Fill up three-quarters of the plastic cylinder with saltwater.
- Put the lid on with the paper clips dipping into the saltwater.
- Stick one end of the conducting wire onto the foil and the other end to a metal tap. The purpose of this wire is to earth the foil. Alternatively, you can get someone to hold on to the Leyden jar with his hands around the foil while you charge up the jar.
- Now the Leyden jar is ready for use. To charge up the Leyden jar, you can either place it near the screen of a cathode ray tube TV, or rub a PVC rod with wool, making sure that the surface of the rod that is being rubbed gets real close to the sinker. You should be able to hear a crackling sound as charges fly through the air to the Leyden jar.
- Note that if your environment is humid, switching on either the air conditioner or dehumidifier will make charging up the Leyden jar easier.
- To discharge the Leyden jar, you can either give yourself a mild electric shock by placing your finger on the sinker (if you can take it) or using the loose end of the conducting wire to do so. Turn off your lights and you should be able to see a spark before the conducting wire touches the sinker.
The Leyden jar serves as a capacitor – a device that stores charges. When charges are deposited via the sinker, the saltwater receives an electric charge which induces an opposite charge on the aluminum foil outside. The plastic keeps the charges apart. When the conducting wire touches the sinker, it provides a path for the charges to flow.
It can be charged up by rubbing a PVC tube with a piece of wool. The pipe should be placed near the door knob (made of brass). One member of the group will hold the curved surface of the jar with both hands while the next member will hold his forearm, and join hands with everyone else. The last person in the line will then touch the door knob, thereby completing the circuit loop.