Tuning fork

Consider the continuous oscillations of a tuning fork. These oscillations generate successive compressions and rarefactions outward through the air. The human ears, w hen receiving these pressure variations, transfer them to the brain, where they are interpreted as sound. Therefore, the phenomenon of sound is a pressure variation in a fixed point in the air or in another elastic medium, such as water, gas, or solid. 

A tuning fork gives a pure tone, that is, a pressure variation represented by a sinusoid curve (Fig. 9.60). 

FIGURE 9.60 Sound-waves generation by a tuning fork. 

Stimm-gabel, /. tuning fork, -recht, n. suffrage, franchise. 

An example of resonance is a tuning fork. If you activate a tuning fork by striking it sharply, the fork vibrates rapidly. As long as it is held suspended, the vibration decays with time. However, if you place it on a desktop, the fork could potentially excite the natural frequency of the desk, which would dramatically amplify the vibration energy. 

It is well-known, for instance, that if one applies tension at properly timed intervals to pull a stretched wire or string, it begins to oscillate laterally. Lord Rayleigh performed an experiment of this kind by attaching a stretched wire to a prong of a tuning fork when the latter... 

Let us now turn our attention to the application of the sound wave to a liquid since this is the medium of importance to the practising chemist. The sound wave is usually introduced to the medium by either an ultrasonic bath or an ultrasonic horn (see Chapter 7). In either case, an alternating electrical field (generally in the range 20-50 kHz) produces a mechanical vibration in a transducer, which in turn causes vibration of the probe (or bottom of the bath) at the applied electric field frequency. The horn (or bath bottom) then acts in a similar manner to one prong of a tuning fork. As in the case of air, the molecules of the liquid, under the action of the applied acoustic field, will vibrate about their mean position and an acoustic pressure (P = P sin 2k ft) will be superimposed upon the already ambient pressure (usually hydrostatic, Pjj) present in the liquid. The total pressure, P, in the liquid at any time, t, is given by Eq. 2.4. 

In our previous discussions (tuning fork in air) we pointed out that sound was a form of energy. The particles of the medium were set into vibratory motion and thereby possessed kinetic energy. This energy was derived from the wave itself Using this principle we can deduce the energy (and hence intensity) associated with our applied ultrasonic field. 

In all forms of Meotallsm there is ia evidence the phenomena of Mental Vibration. Vibration is not confined to the gross material substance of the universe, hut b equally in evidence in the Mind Stuff with which all space id filled. Mind 5tutf it the elementary material of which the great Cosmic World Brain is composed When one thinks, feels, or wills, there is manifest vibration just as truly as in ths vibration of the atom or the tuning-fork Each kind of thought, feeling or emotion has its own rate of vibration, or keynote. 

In the ripple method a series of ripples is caused to travel over the surface of the liquid, the ripples being formed by means of an an electrically driven tuning fork dipping into the liquid. If viewed by means of intermittent illumination conveniently arranged by periodic interception of the light by interposition of a screen attached to one limb of the fork, apparently stationary waves may be observed and the mean wave length readily determined. 

The concept of resonance was introduced into quantum mechanics by Heisenberg16 in connection with the discussion of the quantum states of the helium atom. He pointed out that a quantum-mechanical treatment somewhat analogous to the classical treatment of a system of resonating coupled harmonic oscillators can be applied to many systems. The resonance phenomenon of classical mechanics is observed, for example, for a system of two tuning forks with the same characteristic frequency of oscillation and attached to a common base, hich... 

Time base Crystal or atomic Oscillator Escapement, motor, tuning fork Pyrotechnic burning rate Oscillator Rate of chemical reaction... 

The conductance of SAMs produced from an alkanethiol and various aromatic and OPE thiols was measured using a tuning fork-based scanning probe microscope (SPM).45 The experimental set-up is illustrated in Figure 5.8. NDR effects were seen in some of the SAMs from the OPEs while the alkanethiolate SAM did not exhibit NDR behavior. 

FIGURE 5.8. The experimental setup for measuring the conductance of SAMs using a tuning fork-based scanning probe microscope (SPM). 

F. J. Giessibl, High speed force sensors for force microscopy and profilometry utilizing a quartz tuning fork, Appl. Phys. Lett. 73(26), 3956 (1998). 

S. Hembacher, F. J. Giessibl and J. Mannhart, Evaluation of a force sensor based on a quartz tuning fork for operation at low temperatures and ultrahigh vacuum, Appl. Surf. Sci. 188, 445 (2002). 

When the application involves foaming, one must detect both the liquid-foam interface and foam level. Radiation sensors can detect the liquid-foam interface and TDR transmitters, or conductance and RF switches can detect the foam level if it is conductive. In the case of heavier foams, vibrating or tuning-fork switches and beta-radiation gauges have been used in some cases, optical or thermal switches have also been successful. 

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