Tuning fork

“ Loading such a fork with a bit of wax, so as to throw it slightly out of unison, and bringing it, when agitated, near the tube in which the flame is singing, the beats and the leaps of the flame occur at the same intervals. When the fork is placed over a resonant jar, all of you can hear the beats, and see at the same time the dancing of the flame. By changing the load upon the tuning-fork, or by slightly altering the size of the flame, the rate at which the beats succeed each other may be altered; but in all cases the jumps address the eye at the moments when the beats address the ear. ”

“ Rust affects a fork but very little: the effect being to slightly flatten it. Tuning-forks have been used to construct a keyboard instrument, but the paucity of harmonic upper partial tones causes a monotonous quality of tone. An account of the combination of tuning-forks into a Tonometer for the accurate measurement of pitch will be found under Scheibler, the inventor. ”

“ A tuning-fork affords a good example of interference. When that instrument vibrates, its two branches alternately recede from and approach one another; each communicates its vibrations to the air, and a musical note is the consequence. If the fork be held upright about a foot from the ear, and turned round its axis while vibrating, at every quarter revolution the sound will scarcely be heard, while at the intermediate points it will be strong and clear. This phenomenon arises from the interference of the undulations of air coming from the two branches of the fork. ”

“ To free the fundamental tone from an overtone, you draw your bow across the fork at the place where the node is required to form the latter. In the third mode of division there are two nodes on each prong and one at the bottom; in the fourth division there are two nodes on each prong and two at the bottom; while in the fifth division there are three nodes on each prong and one at the bottom. The first overtone of the fork requires, according to Chladni, 6-1/4 times the number of vibrations of the fundamental tone.
It is easy to elicit the overtones of tuning-forks. ”

“ C is a steel tuning fork. The secondary circuit is closed through a telephone receiver. These wires are extended so that the receiver is too far distant for the tuning fork to be heard through the air. When the switch S is closed the tuning fork acts as the interrupter for the primary circuit, and it interrupts according to its time of vibration. If, for instance, the fork gives the tone of middle C on the piano it vibrates 256 times a second. It interrupts the primary circuit 256 times a second. It induces an alternating current of the same frequency in the secondary circuit. ”

“ If two tones, produced by two tuning-forks, of equal pitch, are produced one near each ear, there is a uniform single sensation; if one of the tuning-forks be made to revolve round its axis in such a way that its tone increases and diminishes in intensity, neither fork is heard continuously, but both sound alternately, the fixed one being only audible when the revolving one is not. ”

“ For doing this the tuning fork carries two insulating ebonite or ivory strips, B B (Fig. 3) , which, at every oscillation, abut against vertical brass springs, r. Each of these latter is located in front of the platinized point of a screw, v, which is affixed to a small metallic tongue. The springs and tongues are insulated from each other, and are mounted on a piece which may be moved by a screw, V, so as to cause the springs of the strips, B B', to approach or recede according to the amplitude of the instrument's vibrations. ”

“ Now if the motor be thrown out of synchrony with the tuning-fork, even if it makes only 39 or 41 instead of 40 revolutions per second, the disturbance is shown instantly, because looking at the illuminated tuning-fork one observes it jumping spasmodically. ”

“ As the vibrations of the tuning fork are known, it is only necessary, in order to determine the interval of time between each exposure, to count the number of photographs taken successively during a complete vibration of the tuning fork. As a result of experiment, however, it has been found that the ear can be trained to determine with astonishing accuracy the speed at which the apparatus revolves—and consequently the number of pictures taken per second—by the succession of sparks produced by a tuning fork the vibrations of which are known. ”

“ Various other means may be employed to throw the air into a state of periodic motion. A stretched string pulled aside and suddenly liberated imparts vibrations to the air which succeed each other in perfectly regular intervals. A tuning-fork does the same. When a bow is drawn across the prongs of this tuning-fork, Fig. 20, the resin of the bow enables the hairs to grip the prong, which is thus pulled aside. ”

“ When a tuning-fork receives a blow and is made to rest upon a piano-forte during its vibration, every string which, either by its natural length or by its spontaneous subdivisions, is capable of executing corresponding vibrations, responds in a sympathetic note. ”

“ Where an oscillation arc or a vacuum tube is used the condenser is charged as rapidly as it is discharged and the result is the oscillations are sustained as shown at D.
About Mechanical Tuning.--A tuning fork is better than a spring or a straight steel bar for setting up mechanical vibrations. As a matter of fact a tuning fork is simply a steel bar bent in the middle so that the two ends are parallel. ”

“ If one of the branches of a vibrating tuning-fork be brought near the embouchure of a flute, the lateral apertures of which are stopped so as to render it capable of producing the same sound as the fork, the feeble and scarcely audible sound of the fork will be augmented by the rich resonance of the column of air within the flute, and the tone will be full and clear. The sound will be found greatly to decrease by closing or opening another aperture ”

“ The first mode of vibration gives the " fundamental tone, and the succeeding modes are termed " overtones." The whole series forms the series of odd harmonics. A " stopped pipe " in an organ is a pipe of this type, and both the fundamental and the overtones may occur simultaneously when it is blown.
We may illustrate the successive modes of vibration by using as pipe a tall cylindrical jar, and as exciter a vibrating tuning-fork held over the mouth. The length of the pipe may be varied by pouring in water, and this is done until we get maximum resonance of the pipe to the fork. ”

“ If by the force of our fingers we deflect the ends of the prongs of a tuning-fork and then let go, the prongs will return to their normal position after performing a number of vibrations of gradually diminishing amplitude. The state of rest is reached when the energy of bending, produced by the work of our fingers, has been expended, partly in overcoming the internal friction in the tuning-fork, partly in overcoming the reactions of the surrounding medium, the air ”

“ If the spider is not at the centre of the web at the time that the fork is applied, it cannot tell which way to go until it has been to the centre to ascertain which radial thread is vibrating, unless of course it should happen to be on that particular thread or on a stretched supporting thread in contact with the fork. ”

“ With others the exciting tremors must be very rapid, and the sound consequently shrill. Not one of these four tuning-forks, which vibrate 256 times, 320 times, 384 times, and 512 times respectively in a second, has any effect upon the flame from our iron nipple. But, besides their fundamental tones, these forks, as you know, can be caused to yield a series of overtones of very high pitch. The vibrations of this series are 1,600, 2,000, 2842,400, and 3,200 per second, respectively. The flame jumps in response to each of these sounds ”

“ The vibration of tuning forks induces alternate conditions of the air that surrounds them, if in open atmosphere; but quite a different action presents itself when the forks are exercised in resonating tubes, set to thirds of the mass chord they represent. Then high vortex action is the instant result. Vibrators cannot be set promiscuously in tubes, and get such results, any more than a musician can render a musical composition on the violin before tuning it. The conditions under which light is evolved negatize whatever is associated with undulation, as this word is understood by physicists. ”

“ If the rates of vibration of the two tuning-forks at the two stations could be maintained precisely the same, the two arms would rotate in synchronise, but as this uniform vibration cannot be exactly preserved for any length of time, a means is provided whereby the rate of vibration of either of the forks can be slowed down, so as to retard the rate of rotation of one or other of the arms. This is effected by means of “correcting” segments, of which there are six sets containing three each. ”

“ Or, if the same plate be moved in contact with two tuning-forks, we shall, by comparing the number of sinuosities in the one trace with that in the other, be enabled to assign the ratio of the corresponding numbers of vibrations per second. Thus, if the one note be an octave higher than the other, it will give double the number of waves in the same distance. The motion of the plate may be simply produced by dropping it between two vertical grooves, the tuning-forks being properly fixed to a frame above. ”

“ Even when making such observations with an alternator one is apt to be mistaken. When a circuit is connected to an alternator there are an indefinite number of values for capacity and self-induction which, in conjunction, will satisfy the condition of resonance. So there are in mechanics an infinite number of tuning forks which will respond to a note of a certain pitch, or loaded springs which have a definite period of vibration. ”

“ The combined overhanging weight of the cast-iron fork, the mirror and tube is so great, that without a very perfect relief-friction system the instrument could not be moved in right ascension with any approach to practical ease. But a hollow steel float, 10 ft. in diameter, is bolted to the upper end of the polar axis just below the fork. This float dips into a tank filled with mercury so that practically the entire instrument is floated by the mercury, leaving only sufficient pressure on the bearings to ensure that the pivots will remain in contact with them. ”

“ This will be understood by inspecting Fig. 107, where A A' shows a lever fork as if in contact with both banks, and the jewel pin, represented at B B'', just passes the angle a c' of the fork. The circle described by the jewel pin B is indicated by the arc e. It is well to put a slight friction under the balance rim, in order that we can try the freedom of the guard pin. As a rule, all the guard pin needs is to be free and not touch the roller. The entire point, as far as setting the fork and bankings is concerned, is to have the fork and roller action sound. ”

“ A fork of frequency 256 was used as the source. The energy of this fork with a given amplitude of vibration could be calculated from its dimensions and elasticity, and the amplitude was observed by measuring with a microscope the line into which the image of a starch grain on the prong was drawn by the vibration. The rate of loss of energy was calculated from the rate of dying down of the vibration. This rate of loss for each amplitude was determined (1) when the fork was vibrating alone, and (2) when a resonator was placed with its mouth under the free ends of the fork. ”

“ When the action is very lively, the rod immediately commences a second revolution; and so it goes on as long as the operator walks over the exciting surface of ground.
It is to be understood that the operator does not grasp the handles of the fork so tightly but that they may turn in his hands. If, indeed, he tries to prevent this, and the fork is only of hazel twig, the rotatory force is so strong as to twist it at the handles and crack the bark, and finally, fracture the wood itself. ”

“ Two-pronged Fork.—A very quickly worked insertion, fringe, or edging may be made by means of a two-pronged fork. The little implement can be manufactured quite easily at home. For a fringe, a wooden fork is necessary, with one wide and one narrow prong. The usual fork is of thick steel wire and varies in size, but as steel wire is too hard for the unskilled worker to [Pg 166] manipulate, a softer metal would be more satisfactory. ”

“ If the fork has slightly greater frequency, then a white line will not quite reach the next place while the fork is making its swing in and out, and the waves will travel against the motion of the cylinder. ”

“ Let us, as a practical example, take up the consideration of a short fork. To represent this in our model we take a lever as shown at Fig. 99, with the elongated slot for the pallet staff at g. To facilitate the description we reproduce at Fig. 102 the figure just mentioned, and also employ the same letters of reference. We fancy everybody who has any knowledge of the lever escapement has an idea of exactly what a "short fork" is, and at the same time it would perhaps puzzle them a good deal to explain the difference between a short fork and a roller too small. ”