The Franklin's bell is a very simple electrostatic experiment you can do with just two soda cans, the pull tab from one of the cans, a thread or string and a straw. You'll also need some way to electrically charge at least one of the cans. In the photo on the left below a Van de Graaff generator is being used for charging and on the right a Wimshurst machine. Once charged, the pull tab will rapidly swing back and forth between the cans, making a ringing sounds as it hits them.
How to make a Franklin's bell
The photo below shows the parts you'll needed. You'll have to empty at least one of the soda cans since you'll need a pull tab (the pull tab is the metal ring-like part that you pull on to open the can.) The thread can be string instead or fishing line or any fairly flexible thread-like thing that is not electrically conductive (e.g. electrical wire will not do.) Instead of a straw you use anything non-electrically conductive (e.g. a plastic knife.) In the photo below I'm using clear packing tape but any tape will do.
See the photos below for the steps.
Step 1. Tie one end of the thread to the middle of the straw. Using a straw, or any uniformly shaped object, is good because once the whole thing is finished you'll still be able to slide the thread along the straw to better position the pull tab between the cans.
Step 2. Tie the pull tab to the thread such that the pull tab will hang about halfway down between the cans.
Step 3. Tape one end of the straw to the top of one of the cans.
Next you need to connect a source of electrical charge.
Using a Van de Graaff generator as the charge source
For a Van de Graaff generator, as shown in the photo below, take a piece of wire and strip some of the insulation off of both ends. Tape one end to the top of one can such that the bare copper of the wire is in physical contact with the can (preferably not with the paint, but that still works.) In the photo below that's the can on the right. Arrange the can so that the other end of the wire is touching the dome of the Van de Graaff.
The other wire is optional, but it will work better with it. Tape the end of another wire to the other can and connect the other end to whatever the Van de Graaff is connecting to for its bottom brush. That may be Earth ground or in the case of the photo below, it's the shorter dome on the bottom.
Adjust the spacing of the pull tab between the cans such that the each can is separated from the pull tab by around 1/8 of an inch. Turn on the Van de Graaff machine and the bell should start ringing. If it doesn't then adjust the distance between the cans and the pull tab. You can also slide the thread along the straw.
Using a Wimshurst machine as the charge source
For a Wimshurst machine get two wires and strip the insulation off of both ends of each. As shown in the photo below, tape one end of one wire to the top of one can and tape one end of the other wire to the top of the other can. Get two wires with alligator clips on their ends (you can get these at electronics stores like Radio Shack.) Clip one end of each to the other ends of the wires you'd just taped to the cans and clip the other ends to the metal of the arms for the Wimshurst machine's spark gap.
Adjust the spacing of the pull tab between the cans such that the each can is separated from the pull tab by around 1/8 of an inch. Start turning the Wimshust machine disks and the bell should start ringing. If it doesn't then adjust the distance between the cans and the pull tab.
Video showing how to make a Franklin's bell (includes demos)
This video shows step-by-step how to make this Franklin's bell and shows how to set up a few different types of charge sources followed by demonstrations.
Video showing how a Franklin's bell works (includes a demo)
This video explains in detail how the Franklin's bell works, including some of the history.