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ON A VIBRATING SOLID, LIQUID, OR GASEOUS BODY
BEING ALWAYS THE ORIGIN OF SOUND.
In this chapter it is shown that the mechanical actions, which finally result in giving us the sensation of sound, always have their origin in some vibrating body, and that this vibrating body may be either solid, liquid, or gaseous.
EXPERIMENTS WITH A TUNING-FORK.
At the music-dealer's buy two forks marked “Philharmonic A.” These two forks must be rigorously in tune with one another. Buy also another fork marked “Philharmonic C.” For the present we only need one of the A-forks; the others will be used in future experiments.
EXPERIMENT 22.—Get a match, and spreading two of your fingers apart rest it upon both. Hold the fork in the right hand, and strike the end of one prong squarely and smartly on the knee, or on a piece of thick paper folded over the edge of the table. Now bring the fork up under the match. The instant the match is touched it flies into the air as if knocked away by a sudden blow.
EXPERIMENT 23.–Fill a tumbler brimful of water, and, starting the fork once more, hold it over the water so that the ends of the prongs touch the surface; imme
diately two tiny showers of spray will fly off on either side. This makes a startling experiment when seen magnified upon the screen (see “ Light,” page 79). A blow struck on the match, and the water dashed violently out of the glass, show that the tuning-fork is in motion, that it vibrates or quivers when it is struck. Strike it once more and bring it to the ear, and you hear a clear sound, a smooth and pleasant musical note. We conclude that the motion must be the cause of the sound, for the sound ceases when the fork ceases to quiver.
EXPERIMENT 24.—Put a small piece of wax against the broad face of the end of a prong of the fork, and stick against this wax the flat head of a tack. The point of the tack should be slightly rounded by a file. Place a piece of tin-foil on a napkin or piece of cloth, then vibrate the fork and draw the point of the tack quickly along the surface of the foil. The series of dots now seen on the foil show that the prong moved to and from the foil as you drew the fork over its surface.
EXPERIMENT 25.–Fig. 21 represents a square block of wood made by nailing several pieces of board of the same size one upon the other. At one side, near the bottom, is the tuning-fork driven into a hole in the block, so that it will be supported in a horizontal position about 4 inch (6 millimetres) from the table. A slender triangular bit of tinsel is fixed with wax on the end of one of the prongs to serve as a pen. To make the fork sound in this position we need a hammer or drumstick, made by slipping a piece of rubber tubing over a stout wire. It is, however, always better to vibrate a fork by drawing a violin-bow over one of its prongs.
Get a piece of clean glass 3 inches (7.6 centimetres) wide and about 8 inches (20.4 centimetres) long. Smoke
this on one side with the apparatus described in Experiment 12, and then slip it, smoked side up, under the tuning-fork, lifting the fork so that the tinsel-pen on its end will not touch the glass. Next, lay a straight strip of wood, A B, Fig. 21, beside the glass and fasten it down with brads. This is to serve as a guide in sliding the plate under the fork.
Bend the tinsel-pen down so that it just touches the glass. While the fork is vibrating, slide the glass quickly along the guide, taking pains to move it at the same speed all the time.
Hold the glass up to the light, and you will see a delicate wavy line, a sinusoidal trace. We cannot see the minute movements of the fork, yet here we have a picture of these movements. We see and readily read and understand its own handwriting. Placed in the water-lantern, this trace made by the fork may be exhibited on a magnified scale before many persons. To preserve this autograph of the fork, flow spirit-varnish over the smoked side of the glass, and then the picture may be handled without injuring it.
You must have observed that the fork gave a clear musical sound; and here we must make a distinction between a musical sound and mere noise. A noise is also a sound, but an irregular sound. It also is caused by vibrations, but the vibrations are irregular, now fast, now slow, confused and disordered. A musical sound is always caused by vibrations, simple or compound, which regularly repeat themselves, like those of the tuning-fork which you have just examined. For convenience, we will call all sounds made by regular vibrations “sounds." All of our experiments will have to do with musical sounds, and will finally lead to music itself.
EXPERIMENT WITH A VIBRATING TUNING-FORK AND A
EXPERIMENT 26.–Fig. 22 represents the fork inclined by placing a book under one edge of the block. small ball, about the size of a pea, out of cork, and dip it in spirit-varnish ; when dry, fasten it to a fibre of fine floss silk. Take the end of the fibre in the left hand so that the ball will hang free, and then with the hammer in the right hand strike the fork a smart blow. At once bring the ball to rest against the foot of the prong, just above where it joins the handle, Here the ball rests quiet against the fork. Now slowly raise the ball, keeping it close to the fork. Immediately it begins to tremble. Raise it higher, and it darts away in little jerks and jumps. Lift it higher, and it becomes still more agitated, and near the end of the fork it is dashed violently away, as shown in Fig. 22; it falls back and is dashed away again.
Now take the fork out of the block, and bring the end of its handle against the cork ball. The ball trembles.
These experiments show that there are places of rest above the crotch of the fork, and that the prongs swing to and fro about these places of rest, while the handle, or foot of
the fork, vibrates up and down. Experiment 60 will teach us yet more as to the manner in which the prongs vibrate. In the above experiments, a small bead, hung by the silk fibre, sometimes works as well as the cork ball.
At the hardware-store you can buy a piece of sheetbrass 4 inch (3 millimetres) thick and 6 inches (15 centimetres) square. Take care to get a flat piece, and, if it is not perfectly flat, have it hammered out flat at the brass-worker's. Then let the brass-worker cut it into a