What is this object?
a a musical instrument?
b a dishwasher?
c an electrical device for generating charge?
It's a glass armonica, a musical instrument invented by Benjamin Franklin in the 18th century. Inspired by a performer playing a set of wine glasses filled with different amounts of water, Franklin created an instrument that made a similar sound but was easier to control. The armonica is made up of a number of glass bowls of different sizes mounted on a spindle, as seen in the picture. This is rotated using a foot pedal, attached to a flywheel. The bowls were colour coded to indicate which notes they played. The performer would rub his fingers around the rim of the bowls to create eerie music.
So popular was the instrument that even Mozart composed music for it.
Although it is possible to make music by rubbing your finger (moistened with water or vinegar) gently around the rim of a wine glass it can be dangerous if the glass breaks. However a great classroom activity is to take a plastic bottle, remove the lid and cut off the bottom. Cover the cut edge with tape. Fill a plastic jug with water. Lower the bottom of the bottle into the water. Using a spoon, lightly tap the bottle. Move the bottle up and down in the water, ensuring that the end is always covered in water, and the notes will change. (An even better sound can be produced if you blow across the top of the bottle.) A variation of this is to start with only a small amount of water in the jug. As you tap the bottle add more water and the note will change the water reaches the top of the bottle.
A great website where you can play and hear a glass armonica is www.fi.edufranklinmusicianvirtualarmonica.html
Sophie Duncan is a physicist and programme manager at Planet Science (formerly Science Year) www.planetscience.com