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In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
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
In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity (which helps prevent eddy currents). Moreover, because of its comparatively low losses at high frequencies, ferrite is extensively used for the cores of RF transformers and inductors in applications such as switched-mode power supplies and ferrite loopstick antennas for AM radio receivers.
Ferrites are ceramic compounds of the transition metals with oxygen, which are ferrimagnetic but non-conductive. Ferrites that are used in transformer or electromagnetic cores contain iron oxides combined with nickel, zinc, and/or manganese compounds. They have a low coercivity and are called "soft ferrites" to distinguish them from "hard ferrites", which have a high coercivity and are used to make ferrite magnets. The low coercivity means the material's magnetization can easily reverse direction while dissipating very little energy (hysteresis losses); at the same time, the material's high resistivity prevents eddy currents in the core, another source of energy loss. The most common soft ferrites are:
For applications below 5 MHz, MnZn ferrites are used; above that, NiZn is the usual choice. The exception is with common mode inductors, where the threshold of choice is at 70 MHz.[1]
As any given blend has a trade-off of maximum usable frequency, versus a higher mu value, within each of these sub-groups, manufacturers produce a comprehensive range of materials for different applications blended to give either a high initial (low frequency) inductance or lower inductance and higher maximum frequency, or for interference suppression ferrites, an extensive frequency range, but often with a very high loss factor (low Q).
It is essential to select the suitable material for the application, as the correct ferrite for a 100 kHz switching supply (high inductance, low loss, low frequency) is quite different from that for an RF transformer or ferrite rod antenna, (high frequency, low loss, but lower inductance), and different again from a suppression ferrite (high loss, broadband)
Ferrites were invented in 1930 by Yogoro Kato and Takeshi Takei at the Tokyo Institute of Technology.[2] TDK began mass production of ferrite cores in 1937 for radio equipment; the development of ferrite materials and their applications was recognized as an IEEE Milestone in 2009.[3]
There are two broad applications for ferrite cores that differ in size and frequency of operation: signal transformers, which are of small size and higher frequencies, and power transformers, which are of large size and lower frequencies. Cores can also be classified by shape, such as toroidal, shell, or cylindrical cores.
The ferrite cores used for power transformers work in the low-frequency range (1 to 200 kHz usually[4]) and are relatively large in size, can be toroidal, shell, or shaped like the letters 'C', 'D', or 'E'. They are useful in all kinds of electronic switching devices – especially power supplies from 1 Watt to 1000 Watts maximum, since more robust applications are usually out of range of ferritic single core and require grain-oriented lamination cores.
The ferrite cores used for signals have a range of applications from 1 kHz to many MHz, perhaps as much as 300 MHz, and have found their main application in electronics, such as in AM radios and RFID tags.
Ferrite rod aerials (or antennas) are a type of small magnetic loop (SML) antenna[5][6] ubiquitous in AM radio broadcast band transistor radios. However, they began to be used in vacuum tube ("valve") radios in the 1950s. They are also helpful in very low frequency (VLF) receivers,[7] and can sometimes give good results over most of the shortwave frequencies (assuming a suitable ferrite is used). They consist of a coil of wire wound around a ferrite rod core (usually several inches longer than the coil). This core effectively concentrates the magnetic field of the radio waves[8] to give a stronger signal than could be obtained by an air core loop antenna of comparable size, although still not as strong as the signal that could be obtained with a good outdoor wire aerial.
Other names include "loopstick antenna", "ferrod", and "ferrite-rod antenna". "Ferroceptor" [9] is an older alternative name for a ferrite rod aerial, mainly used by Philips where the ferrite core would be called a "Ferroxcube" rod (a brand name acquired by Yageo from Philips in the year 2000). The short terms "ferrite rod" or "loop-stick" sometimes refer to the coil-plus-ferrite combination that takes the place of both an external antenna and the radio's first tuned circuit or just the ferrite core itself (the cylindrical rod or flat ferrite slab).
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In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity (which helps prevent eddy currents). Moreover, because of its comparatively low losses at high frequencies, ferrite is extensively used for the cores of RF transformers and inductors in applications such as switched-mode power supplies and ferrite loopstick antennas for AM radio receivers.
A ferrite bead – also called a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite choke – is a type of choke that suppresses high-frequency electronic noise in electronic circuits. Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices.
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.
TIFRAC (Tata Institute of Fundamental Research Automatic Calculator) was the first computer developed in India, at the Tata Institute of Fundamental Research in Mumbai. Initially a TIFR Pilot Machine was developed in the 1950s (operational in 1956). Based on the IAS machine design, the development of the final machine was started in 1955 and was formally commissioned (and named TIFRAC, by Jawaharlal Nehru) in 1960. The full machine was in use until 1965. TIFRAC included 2,700 vacuum tubes, 1,700 germanium diodes and 12,500 resistors. It had 2,048 40-bit words of ferrite core memory. This machine was an early adopter of ferrite core memory. The main assembly of TIFRAC, which had vacuum tubes was housed in a massive steel rack measuring 18 feet x 2.5 feet x 8 feet. It was fabricated from modules of 4 feet x 2.5 feet x 8 feet. Each module had steel doors on either side for accessing the circuits. A cathode-ray tube display system was developed to serve as an auxiliary output to the computer for analogue and digital display of both graphs and alpha-numeric symbols. A manual console served as the input/output control unit of the computer. The software of TIFRAC were written in a series of commands of 0s and 1s (machine code). A British-built HEC 2M computer, happened to be the first digital computer in India, which was imported and installed in Indian Statistical Institute, Kolkata, during 1955. Prior to that, this institute had developed a small analog computer in 1953, which is technically the first computer in India.
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 →Brown v. Board of Education in 1954.
