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Movements, leaders, victories and the continuing fight for equality.
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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
In 1968 Shirley Chisholm became the first Black woman elected to the United States Congress. In 1972 she launched a campaign for the Democratic presidential nomination, breaking another political barrier.
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 surveying and construction, the laser level is a control tool consisting of a rotating laser beam projector that can be affixed to a tripod. The tool is leveled according to the accuracy of the device and projects a fixed red or green beam in a plane about the horizontal and/or vertical axis.[1]
The concept of a laser level has been around since at least the early 1970s,[2] the original spinning-mirror design laser plane and line level was patented by the late 1980s,[3] and the compact lens-based laser line level (as produced by many tool manufacturers today) was patented in the late 1990s. It was invented by Oscar Soliz in the late 1960s, who later sold the rights to it.[4]
A rotary laser level is a more advanced laser level in that it spins the beam of light fast enough to give the effect of a complete 360 degree horizontal or vertical plane, thus illuminating not just a fixed line, but a horizontal plane.[5] The laser beam projector employs a rotating head with a mirror for sweeping the laser beam about a vertical axis. If the mirror is not self-leveling, it is provided with visually readable level vials and manually adjustable screws for orienting the projector. A staff carried by the operator is equipped with a movable sensor, which can detect the laser beam and gives a signal when the sensor is in line with the beam (usually an audible beep). The position of the sensor on the graduated staff, also known as a grade rod, or story pole, allows comparison of elevations between different points on the terrain. Most laser levels are used in the construction industry.
A tower-mounted laser level is used in combination with a sensor on a wheel tractor-scraper in the process of land laser leveling to bring land (for example, an agricultural field) to near-flatness with a slight grade for drainage.
Cross laser levels and line laser levels are types of modern electronic laser levels commonly used in construction and home projects. Instead of a rotating laser, they have one or several stationary line lasers that is levelled by a spirit level and/or plumb bob to simultaneously display precise horizontal or vertical illuminated lines on surfaces. Cross laser levels have largely replaced line laser levels.[6] While line laser levels can be handy for things like hanging pictures, cross laser levels have a much wider range of uses, including painting and tiling.[6] The most versatile type is the 3x360-degree cross laser levels, which draw horizontal and vertical lines on all walls. They can be manufactured with cross lasers in one, two or three planes:
The light beam can span, for example, 90, 180 or 360 degrees of each of these planes, with small interruptions due to the posts that protect the glass housings around the lasers. A 360 degree laser level is sometimes confused with a rotary laser level.
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Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
In surveying and construction, the laser level is a control tool consisting of a rotating laser beam projector that can be affixed to a tripod. The tool is leveled according to the accuracy of the device and projects a fixed red or green beam in a plane about the horizontal and/or vertical axis.
A line laser level is a tool combining a spirit level and/or plumb bob to balance a stationary line laser to display a precise horizontal or vertical illuminated line on a surface the line laser level is laid against. Line laser levels are used wherever precise verticals and horizontals are required, typically in the construction and cabinetry industries. Some models are inexpensive enough for do-it-yourself applications. A cross laser level or cross line laser level utilizes pairs of line lasers to project both horizontal and vertical lines simultaneously. They can be manufactured with cross line lasers in one, two or three planes: XY plane: Draws lines on the floor plane XZ plane: Draws lines on the forward and backwall YZ plane: Draws lines on the side walls The light beam can span, for example, 90, 180 or 360 degrees of each of these planes, with small interruptions due to the posts that protect the glass housings around the lasers. A 360 degree laser level is sometimes confused with a rotary laser level. Cross laser levels have largely replaced line laser levels. While line laser levels can be handy for things like hanging pictures, cross laser levels have a much wider range of uses, including painting and tiling. The most versatile type is the 3x360-degree cross laser levels, which draw horizontal and vertical lines on all walls.
Low-level laser therapy (LLLT), cold laser therapy, or photobiomodulation (PBM) is a photochemical reaction-based medical treatment that applies low-level (low-power) lasers or light-emitting diodes (LEDs) to the surface of the body without damaging tissue. Proponents claim that this treatment stimulates healing, relieves pain, and enhances cell function. Sometimes termed as low-level red-light therapy (LLRL), its effects appear to be limited to a specific range of wavelengths. Its effectiveness is under investigation. Several such devices are cleared by the United States Food and Drug Administration (FDA). The therapy may be effective for conditions such as juvenile myopia, rheumatoid arthritis, and oral mucositis.
A nitrogen laser is a gas laser operating in the ultraviolet range (typically 337.1 nm) using molecular nitrogen as its gain medium, pumped by an electrical discharge. The wall-plug efficiency of the nitrogen laser is low, typically 0.1% or less, though nitrogen lasers with efficiency of up to 3% have been reported in the literature. The wall-plug efficiency is the product of the following three efficiencies: electrical: TEA laser gain medium: This is the same for all nitrogen lasers and thus has to be at least 3% inversion by electron impact is 10 to 1 due to Franck–Condon principle energy lost in the lower laser level: 40% optical: More stimulated emission than spontaneous emission
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 →Madam C.J. Walker
