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rod:see English units of measurementEnglish units of measurement,
principal system of weights and measures used in a few nations, the only major industrial one being the United States. It actually consists of two related systems—the U.S.
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(also rod cell), a photoreceptor of the human and lower vertebrate eye. The rod cells respond to faint light. They and the cone cells are located in the outermost part of the retina. The cells consist of a basal synapse (connected with deeper-lying retinal cells), the nucleus, an internal segment containing ergastoplasm, the myoid (a contractile element of the rod cells), the ellipsoid (a mass of mitochondria), and an external segment made up of disks. A connective fiber with nine pairs of threads typical of cilia and departing from a pair of centrioles unites the internal and external segments. The disks of the outer segment, which are composed partly of visual pigment, are formed by an invagination of the cytoplasmic membrane. At the retinal periphery, there are more rod cells than cone cells. The retinas of nocturnal and crepuscular animals contain only rods.
REFERENCEVinnikov, la. A. Tsitologicheskie i molekuliarnye osnovy retseptsii. Leningrad, 1971.
O. G. STROEVA
in the theory of oscillations, an elastic solid body whose length greatly exceeds its transverse dimensions. When a rod is excited, for example, by an impact, free oscillations arise in the rod. The oscillatory displacements of the particles of the rod may be directed either along the rod’s axis—longitudinal oscillations—or perpendicular to the axis—torsional and flexural oscillations. For torsional oscillations, any cross section of the rod is twisted with respect to an adjacent cross section. For flexural oscillations, the points of the axis of the rod are displaced in a transverse direction, and fibers parallel to and lying on various sides of the axis undergo tensile and compressive strains.
Any oscillation of a rod may be represented as the sum of the simplest sinusoidals of the various types of natural oscillations in the rod. The frequencies f of the oscillations depend on the length l of the rod, the density p of the material, the shape and area S of the cross section, the elastic reaction to the given type of deformation, and the conditions of attachment of the rod’s ends. For example, for longitudinal oscillations of a free rod,
where E is Young’s modulus and n is an integer corresponding to the number of the harmonic component. For torsional oscillations of a round free rod,
where G is the shear modulus. In the case of flexural oscillations, the natural frequencies do not form a harmonic series, since the rate of propagation of flexural waves is dependent on frequency. For a rod secured at both ends,
where I is the moment of inertia of the cross section with respect to a neutral axis of the rod and the coefficient αn assumes the values α1 = 4.73, α2 = 7.85,. . . The form of the free oscillations of the rod depends on which of the free oscillations are found in the spectrum; this, in turn, is determined by the method of excitation.
Under the action of a sinusoidal driving force the rod oscillates at the frequency of the force f (forced oscillations). When the frequency of the force coincides with one of the rod’s natural frequencies, the phenomenon of resonance occurs.
The practical importance of oscillations of a rod is varied. Any beam in a structural design may be considered as a rod on whose natural frequencies the strength of the structure depends. Dangerous oscillations arising along a ship’s length because of an engine imbalance may be considered as oscillations of a rod. Rods are used in some musical instruments, such as xylophones. A tuning fork is a curved rod with two free ends.
REFERENCESMorse, P. Kolebaniia i zvuk. Moscow-Leningrad, 1949. (Translated from English.)
Strutt, J. W. (Lord Rayleigh). Teoriia zvuka, 2nd ed., vol. 1. Moscow, 1955. (Translated from English.)