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Katherine Johnson’s mathematical calculations helped guide some of America’s most important early space missions while she confronted the racial and gender barriers faced by Black women in twentieth-century America.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
Permalloy (/pɜːrm.ʌ.lɔɪ/[1]) is a nickel–iron magnetic alloy, with about 80% nickel and 20% iron content. Invented in 1914 by physicist Gustav Elmen at Bell Telephone Laboratories,[2] it is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields. Commercial permalloy alloys typically have relative permeability of around 100,000, compared to several thousand for ordinary steel.[3]
In addition to high permeability, its other magnetic properties are low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, allowing it to be used in thin films where variable stresses would otherwise cause a ruinously large variation in magnetic properties. Permalloy's electrical resistivity can vary as much as 5% depending on the strength and the direction of an applied magnetic field. Permalloys typically have the face-centered cubic crystal structure with a lattice constant of approximately 0.355 nm in the vicinity of a nickel concentration of 80%. A disadvantage of permalloy is that it is not very ductile or workable, so applications requiring elaborate shapes, such as magnetic shields, are made of other high permeability alloys such as mu metal. Permalloy is used in transformer laminations and magnetic recording heads.
Permalloy was initially developed in the early 20th century for inductive compensation of telegraph cables.[4] When the first transatlantic submarine telegraph cables were laid in the 1860s, it was found that the long conductors caused distortion which reduced the maximum signalling speed to only 10–12 words per minute.[4] The right conditions for transmitting signals through cables without distortion were first worked out mathematically in 1885 by Oliver Heaviside.[5] It was proposed by Carl Emil Krarup in 1902 in Denmark that the cable could be compensated by wrapping it with iron wire, increasing the inductance and making it a loaded line to reduce distortion. However, iron did not have high enough permeability to compensate a transatlantic-length cable. After a prolonged search, permalloy was discovered in 1914 by Gustav Elmen of Bell Laboratories, who found it had higher permeability than silicon steel.[2] Later, in 1923, he found its permeability could be greatly enhanced by heat treatment.[6] A wrapping of permalloy tape could reportedly increase the signalling speed of a telegraph cable fourfold.[4]
This method of cable compensation declined in the 1930s, but by World War II many other uses for Permalloy were found in the electronics industry.
Other compositions of permalloy are available, designated by a numerical prefix denoting the weight percentage of nickel in the alloy, for example "45 permalloy" means an alloy containing 45% nickel, and 55% iron by weight. "Molybdenum permalloy" is an alloy of 81% nickel, 17% iron and 2% molybdenum. The latter was invented at Bell Labs in 1940. At the time, when used in long distance copper telegraph lines, it allowed a tenfold increase in maximum line working speed.[5] Supermalloy, at 75% Ni, 20% Fe, and 5% Mo, is also well known for its high performance as a "soft" magnetic material, characterized by high permeability and low coercivity.
Due to its high magnetic permeability and low coercivity, Permalloy is often used in applications that require efficient magnetic field generation and sensing.[7] It exhibits low energy loss, which is beneficial for improving the performance of magnetic sensors, transformers, and inductors.[8] Permalloy is also used in the production of magnetic shielding materials, which help protect electronic equipment from external magnetic interference.[9]
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
Permalloy () is a nickel–iron magnetic alloy, with about 80% nickel and 20% iron content. Invented in 1914 by physicist Gustav Elmen at Bell Telephone Laboratories, it is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields. Commercial permalloy alloys typically have relative permeability of around 100,000, compared to several thousand for ordinary steel. In addition to high permeability, its other magnetic properties are low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, allowing it to be used in thin films where variable stresses would otherwise cause a ruinously large variation in magnetic properties. Permalloy's electrical resistivity can vary as much as 5% depending on the strength and the direction of an applied magnetic field. Permalloys typically have the face-centered cubic crystal structure with a lattice constant of approximately 0.355 nm in the vicinity of a nickel concentration of 80%. A disadvantage of permalloy is that it is not very ductile or workable, so applications requiring elaborate shapes, such as magnetic shields, are made of other high permeability alloys such as mu metal. Permalloy is used in transformer laminations and magnetic recording heads.
Plated-wire memory is a variation of magnetic-core memory developed by Bell Laboratories in 1957. Its primary advantage was that it could be assembled by machine, which potentially led to lower prices than magnetic core, which was almost always assembled by hand. Instead of threading individual ferrite cores on wires, plated-wire memory used a grid of wires coated with a thin layer of iron–nickel alloy (permalloy). The magnetic field normally stored in the ferrite core was instead stored on the wire itself. Operation was generally similar to core memory, with the wire itself acting as the data line, and the magnetic domains providing the individual bit locations defined by address (word) lines running on either side of (and perpendicular to) the data wire. Early versions operated in a destructive read mode, requiring a write after read to restore data. Non-destructive read mode was possible, but this required much greater uniformity of the magnetic coating. Improvements in semiconductor RAM chips provided the higher storage densities and higher speeds needed for large-scale application such as mainframe computers, replacing previous types of memory, including both core and plated-wire memory. Plated-wire memory has been used in a number of applications, typically in aerospace. It was used in the UNIVAC 1110 and UNIVAC 9000 series computers, the Viking program that sent landers to Mars, the Voyager space probes, a prototype guidance computer for the Minuteman-III, the Space Shuttle Main Engine controllers, the KH-9 Hexagon reconnaissance satellite, and in the Hubble Space Telescope.
Thin-film memory is a high-speed alternative to magnetic-core memory developed by Sperry Rand in a government-funded research project. Instead of threading individual ferrite cores on wires, thin-film memory consisted of 4-micrometer thick dots of permalloy, an iron–nickel alloy, deposited on small glass plates by vacuum evaporation techniques and a mask. The drive and sense lines were then added using printed circuit wiring over the alloy dots. This provided very fast access times in the range of 670 nanoseconds, but was very expensive to produce. In 1962, the UNIVAC 1107, intended for the civilian marketplace, used thin-film memory only for its 128-word general register stack. Military computers, where cost was less of a concern, used larger amounts of thin-film memory. Thin film was also used in a number of high-speed computer projects, including the high-end of the IBM System/360 line, but general advances in core tended to keep pace.
Sendust is a magnetic metal powder that was invented by Hakaru Masumoto at Tohoku Imperial University in Sendai, Japan circa 1936 as an alternative to permalloy in inductor applications for telephone networks. Sendust composition is typically 85% iron, 9% silicon and 6% aluminium. The powder is sintered into cores to manufacture inductors. Sendust cores have high magnetic permeability (up to 140 000), low loss, low coercivity (5 A/m) good temperature stability and saturation flux density up to 1 T. Due to its chemical composition and crystallographic structure Sendust exhibits simultaneously zero magnetostriction and zero magnetocrystalline anisotropy constant K1. Sendust is harder than permalloy, and is thus useful in abrasive wear applications such as magnetic recording heads.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →The Greenwood District.