EXPT. 3.-Smoke one side of a strip of glass by holding it over the flame of burning turpentine or camphor. Make a light pointed style of thin sheet-brass or wire: fix this to the end of one prong of a tuning-fork, as shown in Fig. 4. A bristle attached with wax may be used instead, but is apt to Fig. 4.-WAVE-Line traced by FORK. bend or break off. Strike the fork, and immediately draw the style lightly but quickly over the smoked glass. After a little practice you will be able to produce a beautifully regular wavy line on the glass. This 'wave-line' is a record of the motion of the prong in its own handwriting. 8. Musical Sounds and Noises.-By musical sounds are meant such as are produced by the human voice, the violin, the organ, and other instruments used for musical purposes. It is not possible to draw any very sharp distinction between a musical sound and a noise. The sounds produced by certain instruments used in military bands—e.g. drums and triangles cannot strictly be called musical, and yet they do not destroy the harmony of the music, but rather strengthen and brighten it. We may, however, broadly distinguish between the two classes of sounds as follows. A noise is produced by confused and non-periodic movements,—movements which are irregular both in respect of time and strength: whereas a musical sound is produced by regular and periodic vibrations. Our business lies entirely with the latter class of sounds, and we shall in general use the word sound as denoting a musical sound. 9. Pitch. The long thin spring used in Expt. I gave out no sound while vibrating. A certain rate of vibration must be reached before any audible sound is produced. By gradually shortening the spring you can make it vibrate more rapidly, until at last it begins to produce a deep sound or a note of 'low pitch.' By still further shortening the spring (or by using a shorter and thicker one) you can go on increasing the rate of vibration this will make the note still higher or 'raise the pitch.' Thus the pitch of a note depends upon the rate of vibration. Further proofs of this will be given later on. 10. Intensity of Sound.—The intensity or loudness of a sound depends upon the amplitude of vibration of the sounding body. You can verify this statement sufficiently well for present purposes by watching a vibrating string while it is coming to rest as the amplitude of vibration diminishes so the sound gradually dies out. In the case of a tuning-fork it can be verified by a slight modification of Expt. 3. But there are other circumstances that affect the intensity of the sound produced by an instrument. A vibrating string (or a tuning-fork) does not offer a large surface to the air: it cannot directly set any large mass of air into vibration, and consequently it produces only a feeble sound. But if it is made to impart its vibrations to an elastic body presenting a larger surface to the air (generally a thin wooden board called a 'sounding-board'), the intensity of the sound is greatly increased. This principle is employed in the construction of most stringed instruments, such as violins and pianos. Tuning-forks, also, when used for experimental purposes, are generally mounted on sounding-boxes (Fig. 5), which greatly strengthen the sound. Fig. 5. EXPT. 4.-Take hold of a tuning-fork by the stem and set it into vibration. You hear only a feeble sound as long as the stem is held in the hand: but the sound swells out much more loudly when it is lightly pressed against a table or door-panel; or, better still, against its own sounding-box. CHAPTER II WAVE-MOTION 11. Water-Waves. -You cannot begin the study of wave-motion better than by examining carefully the waves which travel over the surface of a pool of water when a stone is dropped into the middle of it. Where the stone drops there is at first produced a depression, which immediately begins to spread outwards from the centre of disturbance in the form of a circular trough. The disturbance which thus travels to the edge of the pool is caused, not by any bodily motion of the water outwards, but by a downward motion of the water-particles, which spreads outwards from point to point. This downward motion is followed by a swing upwards, producing a crest, which follows the trough and travels after it with the same velocity across the surface of the pool. By, this succession of moving troughs and crests is produced a series of ripples or water-waves. The distance between one crest and the next, or between one trough and the next, is called a wave-length (Arts. 15 and 16). That the water itself does not move in the direction in which the waves travel is shown by the behaviour of chips of wood or bits of straw lying on its surface. These simply rise and fall, floating idly on the surface of the water and showing no tendency (unless the disturbance be very violent) to move forward in the direction in which the waves are travelling. 12. Two kinds of Waves.-Observe that in the case of water-waves the motion of the particles is an up-and-down motion; whereas the waves themselves move horizontally across the surface of the water. Such waves are called transverse waves, because they are produced by motion which is transverse or at right angles to the direction in which the waves are propagated. Strictly speaking, the motion of the water particles is circular, but all that we wish to do here is to point out the broad distinction between the two classes of waves, which are called respectively transverse and longitudinal waves. 13. Transverse Waves are such as are produced by a vibratory motion of particles executed in a direction at right angles to that in which the waves are propagated. As an instance of the production of a transverse wave may be mentioned the sudden jerk which a bargeman sends along a rope when he wishes it to clear an obstacle in its path. Get a rope a few yards in length and lay it straight along the floor. Take hold of one end and, by jerking it rapidly from side to side, send a series of right- and left-handed pulses along it. If an instantaneous photograph of the wave were taken it would present an appearance like that of the wave-line in Expt. 3. The waves appear to travel along the rope in the direction of its length: but the appearance is simply due to a vibration of each part of the rope which is executed transversely (or at right angles to its length) and which is communicated from each part of the rope to the next. 14. Longitudinal Waves are such as are produced by a vibratory motion of particles executed along the line in which the waves are propagated. If you look down from a hillside upon a cornfield when a light summer breeze is blowing over it you will see a good example of wave-motion. The only motion of which the ears of corn are capable is a slight swinging motion; but as the breeze sweeps along the motion is transmitted from one ear to the next and from this again to its neighbour. Thus a certain state of things (ears of corn tightly packed together) is transmitted from point to point, and every gust of wind produces a wave of condensation which skims across the surface of the field. The waves are not wholly longitudinal. There is some little transverse motion: for each cornstalk swings like an inverted pendulum. But in the main the motion of the ears takes place in the line along which the wave travels. Observe that the characteristic of wave-motion is the transmission of a certain state of things or state of motion without any corresponding transmission of matter. The best illustration of longitudinal waves is afforded by the behaviour of a spiral coil of wire. When EXPT. 5. Make a spiral of thick copper wire by winding it tightly round a curtain-pole or a thick glass tube. drawn out the spiral should be about two yards long.1 Hang it up by a series of double threads, as shown in Fig. 6, the double suspension being for the purpose of preventing any side swing. By means of such a 'wave-machine' you can study the mode of propagation of waves of condensation as well as waves of rarefaction. A wave of rarefaction is produced by taking hold of one end of the spiral, pulling it out in the direction of the axis with a smart jerk, and then letting it go. 1 A common defect of spiral coils used for illustrating wave-motion is that they are made too small and slight: when this is the case it is difficult to follow a wave as it quickly passes along the coil. It is really worth while taking trouble to make a good spiral, for it can be used afterwards to illustrate reflection, interference, and stationary vibration. I find that the following dimensions (recommended by Weinhold) are very suitable, viz.-Diameter of wire, 2 mm.; diameter of spiral, 7 cm. ; number of turns, 72; length of completed spiral, 2 metres; length of threads 60 cm. |