Jewelry

Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower caratage, typically 22k, 18k, 14k or 10k, contain higher percentages of copper, or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color. Eighteen-carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen-carat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police, as well as other, badges. Blue gold can be made by alloying with iron and purple gold can be made by alloying with aluminium, although rarely done except in specialized jewelry. Blue gold is more brittle and therefore more difficult to work with when making jewelry. Fourteen and eighteen carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with palladium or nickel. White 18-carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silvery in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe. Alternative white gold alloys are available based on palladium, silver and other white metals,[12] but the palladium alloys are more expensive than those using nickel. High-carat white gold alloys are far more resistant to corrosion than are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects.

Gold

Gold (pronounced /ˈɡoʊld/) is a chemical element with the symbol Au (from Latin: aurum, "shining dawn", hence adjective, aureate) and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is dense, soft, shiny and the most malleable and ductile pure metal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Gold is one of the coinage metals and has served as a symbol of wealth and a store of value throughout history. Gold standards have provided a basis for monetary policies. It also has been linked to a variety of symbolisms and ideologies.




A total of 165,000 tonnes of gold have been mined in human history, as of 2009.[1] This is roughly equivalent to 5.3 billion troy ounces or, in terms of volume, about 8,500 cubic meters, or a 20.4m cube.



Although primarily used as a store of value, gold has many modern industrial uses including dentistry and electronics. Gold has traditionally found use because of its good resistance to oxidative corrosion and excellent quality as a conductor of electricity.



Chemically, gold is a transition metal and can form trivalent and univalent cations in solutions. Compared with other metals, pure gold is chemically least reactive, but it is attacked by aqua regia (a mixture of acids), forming chloroauric acid, but not by the individual acids, and by alkaline solutions of cyanide. Gold dissolves in mercury, forming amalgam alloys, but does not react with it. Gold is insoluble in nitric acid, which dissolves silver and base metals. This property is exploited in the gold refining technique known as "inquartation and parting". Nitric acid has long been used to confirm the presence of gold in items, and this is the origin of the colloquial term "acid test", referring to a gold standard test for genuine value.

Magnetic metallic elements

Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as magnetic). Because of the way their regular crystalline atomic structure causes their spins to interact, some metals are (ferro)magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring (ferro)magnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various man made products, all based, however, on naturally magnetic elements.

Magnetization and demagnetization

Heating the object above its Curie temperature, allowing it to cool in a magnetic field and hammering it as it cools. This is the most effective method, and is similar to the industrial processes used to create permanent magnets.


Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the Earth's magnetic field and which are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant residual magnetism. A magnetic field much stronger than the Earth's can be generated inside a solenoid by passing direct current through it.

Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction.

Magnetized materials can be demagnetized in the following ways:



Heating a magnet past its Curie temperature; the molecular motion destroys the alignment of the magnetic domains. This always removes all magnetization.

Hammering or jarring: the mechanical disturbance tends to randomize the magnetic domains. Will leave some residual magnetization.

Placing the magnet in an alternating magnetic field with an intensity above the materials coercivity and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools and erase credit cards and hard disks, and degaussing coils used to demagnetize CRTs.

Magnetic materials

The term magnet is typically reserved for objects that produce their own persistent magnetic field even in the absence of an applied magnetic field. Only certain classes of materials can do this. Most materials, however, produce a magnetic field in response to an applied magnetic field; a phenomenon known as magnetism. There are several types of magnetism, and all materials exhibit at least one of them.




The overall magnetic behavior of a material can vary widely, depending on the structure of the material, and particularly on its electron configuration. Several forms of magnetic behavior have been observed in different materials, including:



Ferromagnetic and ferrimagnetic materials are the ones normally thought of as magnetic; they are attracted to a magnet strongly enough that the attraction can be felt. These materials are the only ones that can retain magnetization and become magnets; a common example is a traditional refrigerator magnet. Ferrimagnetic materials, which include ferrites and the oldest magnetic materials magnetite and lodestone, are similar to but weaker than ferromagnetics. The difference between ferro- and ferrimagnetic materials is related to their microscopic structure, as explained below.

Paramagnetic substances such as platinum, aluminium, and oxygen are weakly attracted to a magnet. This effect is hundreds of thousands of times weaker than ferromagnetic materials attraction, so it can only be detected by using sensitive instruments, or using extremely strong magnets. Magnetic ferrofluids, although they are made of tiny ferromagnetic particles suspended in liquid, are sometimes considered paramagnetic since they cannot be magnetized.

Diamagnetic means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances such as carbon, copper, water, and plastic are even more weakly repelled by a magnet. The permeability of diamagnetic materials is less than the permeability of a vacuum. All substances not possessing one of the other types of magnetism are diamagnetic; this includes most substances. Although force on a diamagnetic object from an ordinary magnet is far too weak to be felt, using extremely strong superconducting magnets diamagnetic objects such as pieces of lead and even mice [7] can be levitated so they float in mid-air. Superconductors repel magnetic fields from their interior and are strongly diamagnetic.

There are various other types of magnetism, such as spin glass, superparamagnetism, superdiamagnetism, and metamagnetism

Magnetic moment

The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m. It is a vector field, rather than just a vector (like the magnetic moment), because different areas in a magnet can be magnetized with different directions and strengths (for example, because of domains, see below). A good bar magnet may have a magnetic moment of magnitude 0.1 A·m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter. Such a large value explains why magnets are so effective at producing magnetic fields.

Magnetic field

Main article: Magnetic field


The magnetic field (usually denoted B) is a vector field. The magnetic field vector at a given point in space is specified by two properties:



1.Its direction, which is along the orientation of a compass needle.

2.Its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction.

In SI units, the strength of the magnetic field is given in teslas.

Magnet

This article is about objects and devices that produce magnetic fields. For a description of magnetic materials, see magnetism. For other uses, see Magnet (disambiguation).




Iron filings that have oriented in the magnetic field produced by a bar magnet

Magnetic field lines of a solenoid which are similar to a bar magnet as illustrated above with the iron filingsA magnet (from Greek μαγνήτις λίθος magnḗtis líthos, Magnesian stone) is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials like iron and attracts or repels other magnets.



A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.



Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron which can be magnetized but don't tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials which are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity whereas "soft" materials have low coercivity.



An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops. Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.



The overall strength of a magnet is measured by its magnetic moment, while the local strength of the magnetism in a material is measured by its magnetization.

Hertz

This article is about the unit of frequency. For other uses, see Hertz (disambiguation).


"MHZ" redirects here. MHZ is also the IATA airport code for RAF Mildenhall in Suffolk, England.

"MHz" redirects here. MHz may also refer to MHz Networks.

"Megahertz" redirects here. For the German NDH industrial metal band, see Megaherz.

Hertz

Standard: SI derived unit

Quantity: Frequency

Symbol: Hz

Named after: Heinrich Hertz

Expressed in: 1 Hz =

SI base units 1/s



The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon.[1] One of its most common uses is the description of sine wave, particularly those used in radio and audio applications.

X-ray laser active media

Most often used media include highly ionized plasma created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance to the Sah equation, the most stable electron configurations are neon-like with 10 electrons remaining and nickel-like with 28 electrons remaining. The electron transitions in highly ionized plasma usually correspond to energies in the order of 100s eV.
• Capillary plasma discharge medium: In this setup, a several centimeters long capillary made of resistant material (e. g. alumina) confines a high current, sub microsecond electrical pulse in low-pressure gas. The Lorentz force causes further compression of the plasma discharge (see pinch). A pre-ionisation electric pulse and/or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating at 47 nm).


• Solid slab target medium: After being hit by optical pulse, the target emits highly excited plasma. Again, a longer prepulse is often used for the plasma creation and a second, shorter and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes, a sheared excitation pulse may be needed (GRIP - grazing incidence pump). The refractive index gradient causes the amplified pulse to bend from the target surface. This can be compensated using curved target or multiple targets in series.

• Plasma excited by optical field: At optical densities high enough to cause effective electron tunelling or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize the gas without contact with any capillary or target. Usually a collinear setup is used, enabling to synchronize pump and signal pulses.

Alternative amplifying medium is the relativistic electron beam in free electron laser.

A completely different approach to X-ray generation is the high-harmonic generation.

X-ray laser

X-ray laser is a device that uses stimulated emission to generate or amplify the electromagnetic radiation in the near X-ray or extreme ultraviolet region, usually in the order of several nanometer or tens of nm.


Because of high gain in the medium, short upper state lifetime (1 - 100 ps) and problems associated with construction of X-ray mirrors, the X-ray laser usually operates without any resonator. The emitted radiation based on amplified spontaneous emission has relatively low spatial coherence. The line is mostly Doppler broadened (which depends on the ion temperature).

As the common laser transitions between electronic or vibrational states correspond to energies up to 10 eV, a different active medium is needed