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Pentacene
Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings. This highly conjugated compound is an organic semiconductor. The compound generates excitons upon absorption of ultra-violet (UV) or visible light; this makes it very sensitive to oxidation. For this reason, this compound, which is a purple powder, slowly degrades upon exposure to air and light. Structurally, pentacene is one of the linear acenes, the previous one being tetracene (four fused benzene rings) and the next one being hexacene (six fused benzene rings). In August 2009, a group of researchers from IBM published experimental results of imaging a single molecule of pentacene using an atomic force microscope. In July 2011, they used a modification of scanning tunneling microscopy to experimentally determine the shapes of the highest occupied and lowest unoccupied molecular orbitals. In 2012, pentacene-doped p-terphenyl was shown to be effective as the amplifier medium for a room-temperature maser.
  • 3.2K
  • 12 Oct 2022
Topic Review
Air Mass
In astronomy, air mass or airmass is a measure of the amount of air along the line of sight when observing a star or other celestial source from below Earth's atmosphere (Green 1992). It is formulated as the integral of air density along the light ray. As it penetrates the atmosphere, light is attenuated by scattering and absorption; the thicker atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies when nearer the horizon appear less bright than when nearer the zenith. This attenuation, known as atmospheric extinction, is described quantitatively by the Beer–Lambert law. "Air mass" normally indicates relative air mass, the ratio of absolute air masses (as defined above) at oblique incidence relative to that at zenith. So, by definition, the relative air mass at the zenith is 1. Air mass increases as the angle between the source and the zenith increases, reaching a value of approximately 38 at the horizon. Air mass can be less than one at an elevation greater than sea level; however, most closed-form expressions for air mass do not include the effects of the observer's elevation, so adjustment must usually be accomplished by other means. Tables of air mass have been published by numerous authors, including Bemporad (1904), Allen (1976), and Kasten and Young (1989).
  • 5.3K
  • 11 Oct 2022
Topic Review
MSU Temperature Measurements
Microwave sounding unit temperature measurements have been obtained from the troposphere since 1979, when they were included within NOAA weather satellites. By comparison, the usable balloon (radiosonde) record begins in 1958 but has less geographic coverage and is less uniform. Satellites do not measure temperature. They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature. The resulting temperature profiles depend on details of the methods that are used to obtain temperatures from radiances. As a result, different groups that have analyzed the satellite data have obtained different temperature trends. Among these groups are Remote Sensing Systems (RSS) and the University of Alabama in Huntsville (UAH). The satellite series is not fully homogeneous - the record is constructed from a series of satellites with similar but not identical instrumentation. The sensors deteriorate over time, and corrections are necessary for satellite drift in orbit. Particularly large differences between reconstructed temperature series occur at the few times when there is little temporal overlap between successive satellites, making intercalibration difficult. To compare to the trend from the surface temperature record (approximately +0.07 °C/decade over the past century and +0.15-0.16 °C/decade since 1979) it is most appropriate to derive trends for the part of the atmosphere nearest the surface, i.e., the lower troposphere. Doing this, through January 2012: An alternative adjustment introduced by Fu et al. (2004) finds trends (1979–2011) of +0.14 °C/decade when applied to the RSS data set and +0.11 °C/decade when applied to the UAH data set . Using the T2 channel (which include significant contributions from the stratosphere, which has cooled), Mears et al. of Remote Sensing Systems (RSS) find (through January 2012) a trend of +0.082 °C/decade. Spencer and Christy of the University of Alabama in Huntsville (UAH), find a smaller trend of +0.046 °C/decade. A no longer updated analysis of Vinnikov and Grody found +0.20 °C/decade (1978–2005). Another satellite temperature analysis is provided by NOAA/NESDIS STAR Center for Satellite Application and Research and use simultaneous nadir overpasses (SNO) to remove satellite intercalibration biases yielding more accurate temperature trends. The SNO analysis finds a 1979-2011 trend of +0.128 °C/decade for T2 channel. The satellite records have the advantage of greater spatial coverage, whereas the radiosonde record is longer. There have been complaints of data problems with both records, and difficulty reconciling climate model predictions with the observed data.
  • 1.3K
  • 09 Oct 2022
Topic Review
Gravity Well
A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space – the more massive the body, the deeper and more extensive the gravity well associated with it. The Sun is very massive, relative to other bodies in the Solar System, so the corresponding gravity well that surrounds it appears "deep" and far-reaching. The gravity wells of asteroids and small moons, conversely, are often depicted as very shallow. Anything at the center of mass of a planet or moon is considered to be at the bottom of that celestial body's gravity well, and so escaping the effects of gravity from such a planet or moon (to enter outer space) can be likened to "climbing out of the gravity well". The deeper a gravity well is, the more energy any space-bound "climber" must use to escape it. In astrophysics, a gravity well is specifically the gravitational potential field around a massive body. Other types of potential wells include electrical and magnetic potential wells. Physical models of gravity wells are sometimes used to illustrate orbital mechanics. Gravity wells are frequently confused with embedding diagrams used in general relativity theory, but the two concepts are distinctly separate and not directly related.
  • 13.3K
  • 12 Oct 2022
Topic Review
Erowid
Erowid, also called Erowid Center, is a 501(c)(3) non-profit educational organization that provides information about psychoactive plants and chemicals as well as activities and technologies that can produce altered states of consciousness such as meditation, lucid dreaming, transcranial magnetic stimulation, and electroceuticals. Erowid documents legal and illegal substances, including their intended and adverse effects. Information on Erowid's website is gathered from diverse sources including published literature, experts in related fields, and the experiences of the general public. Erowid acts as a publisher of new information as well as a library for the collection of documents and images published elsewhere.
  • 3.7K
  • 09 Oct 2022
Topic Review
Comparison Between Esperanto and Ido
Ido, like Esperanto, is a constructed international auxiliary language. L. L. Zamenhof published his constructed language under the pseudonym Dr Esperanto in 1887. It immediately became popular, but soon people were making suggestions as to how they thought it might be improved. Zamenhof responded by making a list of possible changes to Esperanto and in 1894 put them before the Esperanto community. If accepted they would create what Dr Zamenhof called "a Reformed Esperanto". This proposed reformed Esperanto is sometimes referred to as Esperanto 1894. However, when the Esperanto community was invited to vote on whether to adopt the proposals, they rejected the proposals by a large majority. Ido was created around a quarter of a century after Esperanto. The name Ido means "offspring" in Esperanto and was so named by its creators because it was a development of Esperanto. The creation of Ido led to a schism between those who believed that Esperanto should be left as it was and those who believed that it had what they perceived as inherent flaws which made it not quite good enough to be the world's international auxiliary language. Those who opposed change maintained that it was endless tinkering that had led, in their opinion, to the decline of Volapük a once popular constructed language that had predated Esperanto's publication by a few years. They would also surely have pointed out that Dr Zamenhof's reform proposals of 1894 had been rejected by popular vote. The languages of Esperanto and Ido remain close, and largely mutually intelligible, like two dialects of the same language. Just as dialects of a language are quite often sources of new words for that language through literature, so Ido has contributed many neologisms to Esperanto (especially in poetic substitutes for long words using the mal- prefix). One study conducted with 20 college students at Columbia University circa 1933 suggests that Esperanto's system of correlative words is easier to learn than Ido's. Two other studies by the same researchers suggest no significant overall difference in difficulty of learning between Esperanto and Ido for educated American adults, but the sample sizes were again small: in the two tests combined, only 32 test subjects studied Ido. The researchers concluded that additional comparative studies of Esperanto and Ido are needed.
  • 2.1K
  • 09 Oct 2022
Topic Review
M939 Series 5-Ton 6X6 Truck
The M939 Truck is a 5-ton 6×6 U.S. military heavy truck. The basic cargo versions were designed to transport a 10,000 pounds (4,500 kg) cargo load over all terrain in all weather. Designed in the late 1970s to replace the M39 and M809 series of trucks, it has been in service ever since. The M939 evolved into its own family of cargo trucks, dump trucks, semi-tractors, vans, wreckers, and bare chassis/cabs for specialty bodies. 44,590 in all were produced.
  • 9.2K
  • 10 Oct 2022
Topic Review
Workplace Respirator Testing
Respirators, also known as respiratory protective equipment (RPE) or respiratory protective devices (RPD), are used in some workplaces to protect workers from air contaminants. Initially, respirator effectiveness was tested in laboratories, but in the late 1960s it was found that these tests gave misleading results regarding the level of protection provided. In the 1970s, workplace-based respirator testing became routine in industrialized countries, leading to a dramatic reduction in the claimed efficacy of many respirator types and new guidelines on how to select the appropriate respirator for a given environment.
  • 1.3K
  • 12 Oct 2022
Topic Review
Virtual LAN
A virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2). LAN is the abbreviation for local area network and in this context virtual refers to a physical object recreated and altered by additional logic. VLANs work by applying tags to network frames and handling these tags in networking systems – creating the appearance and functionality of network traffic that is physically on a single network but acts as if it is split between separate networks. In this way, VLANs can keep network applications separate despite being connected to the same physical network, and without requiring multiple sets of cabling and networking devices to be deployed. VLANs allow network administrators to group hosts together even if the hosts are not directly connected to the same network switch. Because VLAN membership can be configured through software, this can greatly simplify network design and deployment. Without VLANs, grouping hosts according to their resource needs the labor of relocating nodes or rewiring data links. VLANs allow devices that must be kept separate to share the cabling of a physical network and yet be prevented from directly interacting with one another. This managed sharing yields gains in simplicity, security, traffic management, and economy. For example, a VLAN can be used to separate traffic within a business based on individual users or groups of users or their roles (e.g. network administrators), or based on traffic characteristics (e.g. low-priority traffic prevented from impinging on the rest of the network's functioning). Many Internet hosting services use VLANs to separate customers' private zones from one other, allowing each customer's servers to be grouped in a single network segment no matter where the individual servers are located in the data center. Some precautions are needed to prevent traffic "escaping" from a given VLAN, an exploit known as VLAN hopping. To subdivide a network into VLANs, one configures network equipment. Simpler equipment might partition only each physical port (if even that), in which case each VLAN runs over a dedicated network cable. More sophisticated devices can mark frames through VLAN tagging, so that a single interconnect (trunk) may be used to transport data for multiple VLANs. Since VLANs share bandwidth, a VLAN trunk can use link aggregation, quality-of-service prioritization, or both to route data efficiently.
  • 798
  • 09 Oct 2022
Topic Review
Dhyāna Sutras
The Dhyāna sutras (Chinese: 禪経) or "meditation summaries" (Chinese: 禪要) are a group of early Buddhist meditation texts which are mostly based on the Yogacara[note 1] meditation teachings of the Sarvāstivāda school of Kashmir circa 1st-4th centuries CE. Most of the texts only survive in Chinese and were key works in the development of the Buddhist meditation practices of Chinese Buddhism.
  • 2.4K
  • 09 Oct 2022
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