SUMMARY / RELATED TOPICS

Tuning fork

A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs formed from a U-shaped bar of elastic metal. It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, emits a pure musical tone once the high overtones fade out. A tuning fork's pitch depends on the mass of the two prongs, they are traditional sources of standard pitch for tuning musical instruments. The tuning fork was invented in 1711 by British musician John Shore, Sergeant trumpeter and lutenist to the court. A tuning fork is a fork-shaped acoustic resonator used in many applications to produce a fixed tone; the main reason for using the fork shape is that, unlike many other types of resonators, it produces a pure tone, with most of the vibrational energy at the fundamental frequency. The reason for this is that the frequency of the first overtone is about 52/22 = 25/4 = ​6 1⁄4 times the fundamental. By comparison, the first overtone of a vibrating string or metal bar is one octave above the fundamental, so when the string is plucked or the bar is struck, its vibrations tend to mix the fundamental and overtone frequencies.

When the tuning fork is struck, little of the energy goes into the overtone modes. It is easier to tune other instruments with this pure tone. Another reason for using the fork shape is that, when it vibrates in its principal mode, the handle vibrates up and down as the prongs move apart and together. There is a node at the base of each prong; the handle motion is small, so the user can hold the fork by the handle without damping the vibration, but the handle can still transmit the vibration to a resonator, which amplifies the sound of the fork. The user strikes the fork, presses the handle against a wooden box resonator, table top, bridge of a musical instrument, or just behind their ear. String players sometimes use bone conduction, holding the stem of the fork in their teeth, so that both hands are free. If just held in open air, the sound of a tuning fork is faint; the sound waves from each prong are 180° out of phase with the other, so at a distance from the fork they interfere and cancel each other out.

If a sound-absorbing sheet is slid in between the prongs of a vibrating fork, reducing the waves reaching the ear from one prong, the volume increases, due to a reduction of this cancellation. Commercial tuning forks are tuned to the correct pitch at the factory, the pitch and frequency in hertz is stamped on them, they can be retuned by filing material off the prongs. Filing the ends of the prongs raises the pitch, while filing the inside of the base of the prongs lowers it; the most common tuning fork sounds the note of A = 440 Hz, the standard concert pitch that many orchestras use. That A is the pitch of the violin's second string, the first string of the viola, an octave above the first string of the cello. Orchestras between 1750 and 1820 used A = 423.5 Hz, though there were many forks and many different pitches. Standard tuning forks are available that vibrate at all the pitches within the central octave of the piano, other pitches. Well-known tuning fork manufacturers include both of Sheffield, England.

Tuning fork pitch varies with temperature, due to a slight decrease in the modulus of elasticity of steel with increasing temperature. A change in frequency of 48 parts per million per °F is typical for a steel tuning fork; the frequency decreases with increasing temperature. Tuning forks are manufactured to have their correct pitch at a standard temperature; the standard temperature is now 20 °C. The pitch of other instruments is subject to variation with temperature change; the frequency of a tuning fork depends on its dimensions and what it's made from: f = 1.875 2 2 π l 2 E I ρ A where: f is the frequency the fork vibrates at in hertz. 1.875 is the smallest positive solution of coscosh = −1. L is the length of the prongs in metres. E is the Young's modulus of the material. I is the second moment of area of the cross-section in metres to the fourth power. Ρ is the density of the material. A is the cross-sectional area of the prongs in square metres; the ratio I/A in the equation above can be rewritten as r2/4 if the prongs are cylindrical with radius r, a2/12 if the prongs have rectangular cross-section of width a along the direction of motion.

Tuning forks have traditionally been used to tune musical instruments, though electronic tuners have replaced them. Forks can be driven electrically by placing electronic oscillator-driven electromagnets close to the prongs. A number of keyboard musical instruments use principles similar to tuning forks; the most popular of these is the Rhodes piano, in which hammers hit metal tines that vibrate in the magnetic field of a pickup, creating a signal that drives electric amplification. The earlier, un-amplified dulcitone, which used tuning forks directly, suffered from low volume; the quartz crystal that serves as the timekeeping element in modern quartz clocks and watches is in the form of a tiny tuning fork. It vibrates at a frequency of 32,768 Hz in the ultrasonic ran

Jihad of Construction

Jihad of Construction or Construction Jihad, or Jihad was one of Organizations of the Iranian Revolution. The organization began as a movement of volunteers to help with the 1979 harvest, but soon was institutionalized and took on a broader, more developmental role in the countryside, it was involved with road building, piped water, clinics and irrigation canals. It provides "extension services, loans," etc. to small farmers. During the Iran-Iraq war, the organization held a combat engineering responsibility, they were active in various operation of the war, most notably in Operation Fath ol-Mobin, Operation Beit-ol-Moqaddas, Operation Kheibar, Operation Dawn 8. Iranian leader Ayatollah Khomeini has called them the "trench-less trench-makers"; the organization engaged in development activities overseas in Tanzania and Lebanon, Sudan and Sierra Leone, Albania. It was active in Pakistan, Bosnia; the title for a Jihad member is Jahādgar. The title for the commanders is Sardār-e Jahādgar. In 2001 it was merged with the Agriculture Ministry to form the Ministry of Agriculture Jihad.

411th Engineering Group of Borujerd

Robert Lansing (state senator)

Robert Lansing was an American lawyer and politician from New York. He was the son of Judge Sanders G. Catherine Lansing, he did not graduate. In 1817, he removed to Watertown, studied law there with Egbert Ten Eyck, was admitted to the bar in 1820, he was District Attorney of Jefferson County from 1826 to 1833. On December 22, 1831, he married Maria Hubbard, they had several children, among them John Lansing, the father of U. S. Secretary of State Robert Lansing, he was a member of the New York State Senate from 1832 to 1835, sitting in the 55th, 56th, 57th and 58th New York State Legislatures. Afterwards he practiced law in partnership with brother-in-law George C. Sherman. On February 2, 1841, Lansing married a sister of his first wife, he was again District Attorney of Jefferson County from 1845 to 1846. He was buried at the Brookside Cemetery in Watertown. Chancellor John Lansing, Jr. and State Treasurer Abraham G. Lansing were his uncles. Lansing was his first cousin; the New York Civil List compiled by Franklin Benjamin Hough Lansing genealogy at RootsWeb Robert Lansing at Find a Grave