Part of the show Naked Science Question and Answer
What you Need
A few wineglasses - ideally some cheap and some expensive
What to do
Try pinging the glasses, see how long they keeps ringing for
Pick the one that rang for the longest.
Wet your finger, then gently rub it around the rim of the glass - don't press too hard or you could break the glass.
- What happens?
Try adding some water to the glass, try again.
- Is there any difference?
What may happen
You should find that any glass that will ring for more than 3-4 seconds will start to vibrate and produce an eerie note as you move your finger around it. It may take a while to get the knack but it should work.
Different glasses will produce different notes and will work differently well, you should find that the glasses that ring for longest after you ping them are easiest to play.
If you add water you should find that the pitch of the note goes down.
Why does it happen?
These effects are all to do with a principle called resonance. Some things have a speed that they vibrate at really well, for example a swing - just by wobbling your legs at the right speed you can build up a great big swing and have lots of fun.
Your finger is doing something similar to the glass, as you slide it around, it will tend to stick, then slip, then stick then slip. Some glasses have a speed at which they will vibrate really really well, if this sticking and slipping is at about the same speed as this the vibration will build up enough that you can start to hear it as the eerie note.
Some glasses are better at vibrating than others, cheap ones tend to have minute flaws in their structure that rub against one another while the glass is vibrating causing it to loose energy - a bit like putting your feet down on a swing. In this case you are never going to get it to vibrate well. The best glasses are large crystal wineglasses that haven't been decorated.
If you add water to the glass it essentially makes the glass heavier so it takes it longer to vibrate back and forth so vibration is slower and therefore the pitch is lower.
It may make you go mad
Instruments based on this very principle were very popular at the beginning of the 19th century, called Glass Harmonicas or Armonicas they consisted of a series of tuned glass bowls which the player could rub to produce different notes. Although after a while it was belived that the beautiful eerie sounds could make people particularly women go mad....
Although it was later discovered that they were marking the notes with lead paint, and the players were licking their fingers to play the insrtument, and therefore eating lots of poisonous lead paint, which was probably more of the problem.