The resonance tube
Resonance in a closed tube is usually studied by means of the apparatus shown in Fig. 29.6. Raising or lowering the water reservoir increases or decreases the effective
length of the air column in the glass tube. The same figure also illustrates a simpler form of apparatus known as the resonance jar. A glass or metal tube stands in a tall jar full of water, and the length of the air column is varied by raising or lowering the tube.
Starting with a very short air column, a vibrating fork is held over the mouth of the tube, and the length of the column is then gradually increased. Strong resonance occurs when the column reaches a certain critical length. This is called the first position of resonance. If the length of the column is now increased still further a second position of resonance is obtained when the column is approximately three times as long. Note that the resonance in this position is not so strong as in the first position.