Page:Elementary Text-book of Physics (Anthony, 1897).djvu/186

172 This lowest tone of the tube is called the fundamental, and the others are called overtones, or harmonics. These simple relations between the length of the tube and length of the wave are only realized when the tubes are so narrow that the air particles lying in a plane cross-section are all actuated by the same movement. This is never the case at the open end of the tube, and the distance from this end to the first node is, therefore, always less than a quarter wave length.

145. Modes of Exciting Vibrations in Tubes.—If a tuning-fork be held in front of the open mouth of a tube of proper length, the sound of the fork is strongly reinforced by the vibration of the air in the tube. If we merely blow across the open end of a tube, the agitation of the air may, by the reaction of the returning reflected pulses, be made to assume a regular vibration of the proper rate and the column made to sound. In organ pipes a mouthpiece of the form shown in Fig. 54 is often employed. The thin sheet of air projected against the thin edge is thrown into vibration. Those elements of this vibration which correspond in frequency with the pitch of the pipe are strongly reinforced by the action of the stationary wave set up in the pipe, and hence the tone proper to the pipe is produced. Sometimes reeds are used, as shown in Fig. 54a. The air escaping from the chamber $$a$$ through the passage $$c$$ causes the reed $$r$$ to vibrate. This alternately closes and opens the passage, and so throws into vibration the air in the pipe. If the reed be stiff, and have a determined period of vibration of its own, it must be tuned to suit the period of the air-column which it is intended to set in vibration. If the reed be very flexible, it will accommodate itself to the rate of vibration of the air-column, and may then serve to produce various tones, as in the clarionet.

In instruments like the cornet and bugle the lips of the player