This week's experiment is a very old one that has entertained and educated people for hundreds of years.
You will need:
- One or more glasses. They should be made of glass and the thinner the glass is, the better they will work.
- Water
First, wash your hands. Now, go do it again, and use soap this time. Your fingers must be very clean for this trick to work well.
Fill the glass about half-full of water. Dip one finger into the water, and begin rubbing it lightly around the top edge of the glass. Move your finger round and round the edge of the glass, pressing lightly. After a few circles, the glass should begin to make a ringing sound. You may have to vary the pressure a bit, harder or softer, to get it to really sing. Once it starts, you should be able to get it to sing very easily.
The glass sings because of vibrations caused by your finger. At first, your finger sticks to the glass. When enough pressure builds up, your finger overcomes the stickiness and jumps forward. As soon as it touches the glass, it sticks again, until the pressure is strong enough for it to jump again. This happens very quickly, over and over. This stick-slide-stick-slide makes the glass vibrate. As we have seen in past experiments, things the vibrate produce a sound. The pattern of stick and slide is also the reason your finger must be very clean. The oils on your fingers keep it from sticking to the glass, and it will not sing.
Once you get the glass to sing, you can change the water level to change the tone of the glass. Does adding more water to the glass make the sound higher or lower? With some experimentation, you can line up several glasses with different water levels and actually play songs on them. Benjamin Franklin took this idea a bit farther. He built a musical instrument which he called the glass harmonica. It was made up of different sized glasses that fit together. All of the glasses fit sideways into a trough of water. By spinning the glasses, you could place your hand on different places and play different tones.
Have a wonder-filled week.
