Monthly Archives: September 2016
iPhoneOgraphy – 30 Sep 2016 (Day 274/366)
The vascular cambium (plural cambia) is a plant tissue located between the xylem and the phloem in the stem but not in the root of a vascular plant, and is the source of both the secondary xylem growth (inwards, towards the pith [material at the center of plant, often dead and/or deteriorated, that is composed of parenchyma tissue]) and the secondary phloem growth (outwards [to the bark, rough or smooth, of the plant]). It is a cylinder of unspecialized meristem cells that divide to give new cells which then specialize to form secondary vascular tissues.
Vascular cambia are found in dicots and gymnosperms but not monocots, which usually lack secondary growth. A few leaf types also have a vascular cambium.Vascular cambium does not transport water, minerals, or dissolved food through the plant. It does, however, produce the phloem and xylem, which do perform these functions.
For successful grafting, the vascular cambia of the rootstock and scion must be aligned so they can grow together. In wood, the vascular cambium is the obvious line separating the bark and wood.
The cambium present between primary xylem and primary phloem is called intrafasicular cambium. At the time of secondary growth, cells of meduallary rays, in a line with intrafasicular cambium, become meristematic and form interfascicular cambium. The intrafascicular and interfascicular cambiums, therefore, represent a continuous ring which bisects the primary xylem and primary phloem and is known as cambium ring. The vascular cambium then produces secondary xylem on the inside of the ring, and secondary phloem on the outside, pushing the primary xylem and phloem apart.
Shot & Edited using iPhone 6+
iPhoneOgraphy – 29 Sep 2016 (Day 273/366)
True flies are insects of the order Diptera, the name being derived from the Greek di = two, and ptera = wings. Insects of this order use only a single pair of wings to fly, the hindwings being reduced to club-like balancing organs known as halteres. Diptera is a large order containing an estimated 1,000,000 species including horse-flies, crane flies, hoverflies and others, although only about 150,000 species have been described.
Flies have a mobile head, with a pair of large compound eyes, and mouthparts designed for piercing and sucking (mosquitoes, black flies and robber flies), or for lapping and sucking in the other groups. Their wing arrangement gives them great manoeuvrability in flight, and claws and pads on their feet enable them to cling to smooth surfaces. Flies undergo complete metamorphosis; the eggs are laid on the larval food-source and the larvae, which lack true limbs, develop in a protected environment, often inside their source of their food. The pupa is a tough capsule from which the adult emerges when ready to do so; flies mostly have short lives as adults.
Diptera is one of the major insect orders and are of considerable ecological and human importance. Flies are important pollinators, second only to the bees and their Hymenopteran relatives. Flies may have been among the evolutionarily earliest pollinators responsible for early plant pollination. Fruit flies are used as model organisms in research, but less benignly, mosquitoes are vectors for malaria, dengue, West Nile fever, yellow fever, encephalitis, and other infectious diseases, and houseflies spread food-borne illnesses. Flies can be annoyances especially in some parts of the world where they can occur in large numbers, buzzing and settling on the skin or eyes to bite or seek fluids. Larger flies such as tsetse fly and screwworm cause significant economic harm to cattle. Blowfly larvae, known as gentles, and other dipteran larvae, known more generally as maggots, are used as fishing bait and as food for carnivorous animals. They are used in medicine in debridement to clean wounds.
Flies are adapted for aerial movement and typically have short and streamlined bodies. The first tagma of the fly, the head, bears the eyes, the antennae, and the mouthparts (the labrum, labium, mandible, and maxilla make up the mouthparts). The second tagma, the thorax, bears the wings and contains the flight muscles on the second segment, which is greatly enlarged; the first and third segments have been reduced to collar-like structures, and the third segment bears the halteres, which help to balance the insect during flight. The third tagma is the abdomen consisting of 11 segments, some of which may be fused, and with the 3 hindermost segments modified for reproduction.
Flies have a mobile head with a pair of large compound eyes on the sides of the head, and in most species, three small ocelli on the top. The compound eyes may be close together or widely separated, and in some instances are divided into a dorsal region and a ventral region, perhaps to assist in swarming behaviour. The antennae are well-developed but variable, being thread-like, feathery or comb-like in the different families. The mouthparts are adapted for piercing and sucking, as in the black flies, mosquitoes and robber flies, and for lapping and sucking as in many other groups. Female horse-flies use knife-like mandibles and maxillae to make a cross-shaped incision in the host’s skin and then lap up the blood that flows. The gut includes large diverticulae, allowing the insect to store small quantities of liquid after a meal.
For visual course control, flies’ optic flow field is analyzed by a set of motion-sensitive neurons. A subset of these neurons is thought to be involved in using the optic flow to estimate the parameters of self-motion, such as yaw, roll, and sideward translation. Other neurons are thought to be involved in analyzing the content of the visual scene itself, such as separating figures from the ground using motion parallax. The H1 neuron is responsible for detecting horizontal motion across the entire visual field of the fly, allowing the fly to generate and guide stabilizing motor corrections midflight with respect to yaw. The ocelli are concerned in the detection of changes in light intensity, enabling the fly to react swiftly to the approach of an object.
Like other insects, flies have chemoreceptors that detect smell and taste, and mechanoreceptors that respond to touch. The third segments of the antennae and the maxillary palps bear the main olfactory receptors, while the gustatory receptors are in the labium, pharynx, feet, wing margins and female genitalia, enabling flies to taste their food by walking on it. The taste receptors in females at the tip of the abdomen receive information on the suitability of a site for ovipositing. Flies that feed on blood have special sensory structures that can detect infrared emissions, and use them to home in on their hosts, and many blood-sucking flies can detect the raised concentration of carbon dioxide that occurs near large animals. Some tachinid flies (Ormiinae) which are parasitoids of bush crickets, have sound receptors to help them locate their singing hosts.
Diptera have one pair of fore wings on the mesothorax and a pair of halteres, or reduced hind wings, on the metathorax. A further adaptation for flight is the reduction in number of the neural ganglia, and concentration of nerve tissue in the thorax, a feature that is most extreme in the highly derived Muscomorpha infraorder. Some species of flies are exceptional in that they are secondarily flightless. The only other order of insects bearing a single pair of true, functional wings, in addition to any form of halteres, are the Strepsiptera. In contrast to the flies, the Strepsiptera bear their halteres on the mesothorax and their flight wings on the metathorax. Each of the fly’s six legs has a typical insect structure of coxa, trochanter, femur, tibia and tarsus, with the tarsus in most instances being subdivided into five tarsomeres. At the tip of the limb is a pair of claws, and between these are cushion-like structures known as pulvilli which provide adhesion.
The abdomen shows considerable variability among members of the order. It consists of eleven segments in primitive groups and ten segments in more derived groups, the tenth and eleventh segments having fused. The last two or three segments are adapted for reproduction. Each segment is made up of a dorsal and a ventral sclerite, connected by an elastic membrane. In some females, the sclerites are rolled into a flexible, telescopic ovipositor.
Flies are capable of great manoeuvrability during flight due to the presence of the halteres. These act as gyroscopic organs and are rapidly oscillated in time with the wings; they act as a balance and guidance system by providing rapid feedback to the wing-steering muscles, and flies deprived of their halteres are unable to fly. The wings and halteres move in synchrony but the amplitude of each wing beat is independent, allowing the fly to turn sideways. The wings of the fly are attached to two kinds of muscles, those used to power it and another set used for fine control.
Flies tend to fly in a straight line then make a rapid change in direction before continuing on a different straight path. The directional changes are called saccades and typically involve an angle of 90°, being achieved in 50 milliseconds. They are initiated by visual stimuli as the fly observes an object, nerves then activate steering muscles in the thorax that cause a small change in wing stroke which generate sufficient torque to turn. Detecting this within four or five wingbeats, the halteres trigger a counter-turn and the fly heads off in a new direction.
Flies have rapid reflexes that aid their escape from predators but their sustained flight speeds are low. Dolichopodid flies in the genus Condylostylus respond in less than 5 milliseconds to camera flashes by taking flight. In the past, the deer bot fly, Cephenemyia, was claimed to be one of the fastest insects on the basis of an estimate made visually by Charles Townsend in 1927. This claim, of speeds of 600 to 800 miles per hour, was regularly repeated until it was shown to be physically impossible as well as incorrect by Irving Langmuir. Langmuir suggested an estimated speed of 25 miles per hour.
Although most flies live and fly close to the ground, a few are known to fly at heights and a few like Oscinella (Chloropidae) are known to be dispersed by winds at altitudes of up to 2000 ft and over long distances. Some hover flies like Metasyrphus corollae have been known to undertake long flights in response to aphid population spurts.
Males of fly species such as Cuterebra, many hover flies, bee flies (Bombyliidae) and fruit flies (Tephritidae) maintain territories within which they engage in aerial pursuit to drive away intruding males and other species. While these territories may be held by individual males, some species form leks with many males aggregating in displays. Some flies maintain an airspace and still others form dense swarms that maintain a stationary location with respect to landmarks. Many flies mate in flight while swarming.
iPhoneOgraphy – 28 Sep 2016 (Day 272/366)
An electric bell is a mechanical bell that functions by means of an electromagnet. When an electric current is applied, it produces a repetitive buzzing or clanging sound. Electric bells have been widely used at railroad crossings, in telephones, fire and burglar alarms, as school bells, doorbells, and alarms in industrial plants, but they are now being widely replaced with electronic sounders.
Fire alarm bells are divided into two categories: vibrating, and single-stroke. On a vibrating bell, the bell will ring continuously until the power is cut off. When power is supplied to a single-stroke bell, the bell will ring once and then stop. It will not ring again until power is turned off and on again. These were frequently used with coded pull stations.
The interrupter bell evolved from various oscillating electromechanical mechanisms which were devised following the invention of the electromagnet by William Sturgeon in 1823. One of the first was the oscillating electric wire invented by James Marsh in 1824. This consisted of a wire pendulum dipping into a mercury trough, suspended between the poles of an electromagnet. When current was passed through the wire, the force of the magnet made the wire swing sideways, out of the mercury, which broke the current to the magnet, so the wire fell back. The modern electric bell mechanism had its origin in vibrating “contact breaker” or interrupter mechanisms devised to break the primary current in induction coils. Vibrating “hammer” interrupters were invented by Johann Philipp Wagner (1839) and Christian Ernst Neeff (1847), and was developed into a buzzer by Froment (1847). John Mirand around 1850 added a clapper and gong to make the standard electric bell for use as a telegraph sounder. Other types were invented around that time by Siemens and Halske and by Lippens. The polarized (permanent magnet) bell used in telephones, which appeared about 1860, had its beginning in the polarized relay and telegraph developed by Werner Siemens around 1850.
iPhoneOgraphy – 27 Sep 2016 (Day 271/366)
A telephone keypad is a keypad that appears on a “Touch Tone” telephone. It was standardised when the dual-tone multi-frequency system in the new push-button telephone was introduced in the 1960s, which gradually replaced the rotary dial. The invention of the keypad is attributed to John E. Karlin, an industrial psychologist at Bell Labs. The contemporary keypad is laid out in a 4×3 grid, although the original DTMF system in the new keypad had an additional column for four now-defunct menu selector keys (see Autovon). Most keypads have a “*” key (called star or asterisk) on the bottom left and a “#” (called hash, pound, or other names) on the bottom right.
When used to dial a telephone number, pressing a single key will produce a dual-tone multi-frequency signaling pitch consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the low frequency, and the column determines the high frequency. For example, pressing the ‘1’ key will result in a sound composed of both a 697 and a 1209 hertz (Hz) tone.
The layout of the digits is different from that commonly appearing on calculators and numeric keypads. This layout was chosen after extensive human factors testing from Bell Labs. At the time (late 1950s), mechanical calculators were not widespread, and few people had experience with them. (Indeed, calculators were only just starting to settle on a common layout; a 1955 paper says “Of the several calculating devices we have been able to look at… Two other calculators have keysets resembling [the layout that would become the most common layout…. Most other calculators have their keys reading upward in vertical rows of ten,” while a 1960 paper just five years later refers to today’s common layout as “the arrangement frequently found in ten-key adding machines”.) In any case, Bell Labs’ testing found that the layout used today, with 1-3 on the top, was slightly faster than the calculator layout with 1-3 on the bottom; however, it’s not apparent whether the two layouts were ever compared directly against each other. One advantage to the layout is that the same letter associations from rotary dial phones appears in alphabetic ordering, although it’s not clear whether this was considered as a factor in choosing the now-standard layout.
The “*” is called the “star key” or “asterisk key”. (Technically it should always have six points, as shown here, but it’s conventionally typed on computers with the plain asterisk “*”, which usually has five points in sans-serif typefaces.) “#” is called the “number sign”, “pound key”, “hash key”, hex key, “octothorpe”, “gate” or “square”, depending on one’s nationality or personal preference. (The Greek symbols alpha and omega had been planned originally.) These can be used for special functions.
When designing or selecting a new phone, publishing or using phone words, one should be aware that there have been multiple standards for the mapping of letters (characters) to numbers (keypad layouts, as with keyboard layout) on telephone keypads over the years.
The system used in Denmark was different from that used in the U.K., which was different from the U.S. and Australia. The use of alphanumeric codes for exchanges was abandoned in Europe when international direct dialling was introduced in the 1960s, because, for example, dialling VIC 8900 on a Danish telephone would result in a different number to dialling it on a British telephone. At the same time letters were no longer put on the dials of new telephones.
Letters did not re-appear on phones in Europe until the introduction of mobile phones, and the layout followed the new international standard ITU E.161/ISO9995-8. The ITU established an international standard (ITU E.161) in the mid-1990s, and that should be the layout used for any new devices. There is a standard, ETSI ES 202 130, that covers European languages and other languages used in Europe, published by the independent ETSI organization in 2003 and updated in 2007. Work describing some principles of the standard is available.
Since many newer smartphones, such as PalmPilot and BlackBerry, have full alphanumeric keyboards instead of the traditional telephone keypads, the user must execute additional steps to dial a number containing convenience letters. On certain BlackBerry devices, a user can press the Alt key, followed by the desired letter, and the device will generate the appropriate DTMF tone.
iPhoneOgraphy – 26 Sep 2016 (Day 270/366)
A USB flash drive, also known as a USB drive, USB stick, USB key, USB, and a variety of other names, is a data storage device that includes flash memory with an integrated USB interface. USB flash drives are typically removable and rewritable, and physically much smaller than an optical disc. Most weigh less than 30 grams (1.1 oz). As of January 2013, drives of up to 512 gigabytes (GB) were available. A one-terabyte (TB) drive was unveiled at the 2013 Consumer Electronics Show and became available later that year. Storage capacities as large as 2 TB are planned, with steady improvements in size and price per capacity expected. Some allow up to 100,000 write/erase cycles, depending on the exact type of memory chip used, and have a 10-year shelf storage time.
USB flash drives are often used for the same purposes for which floppy disks or CDs were once used, i.e., for storage, data back-up and transfer of computer files. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because they have no moving parts. Additionally, they are immune to electromagnetic interference (unlike floppy disks), and are unharmed by surface scratches (unlike CDs). Until about 2005, most desktop and laptop computers were supplied with floppy disk drives in addition to USB ports, but floppy disk drives have become obsolete after widespread adoption of USB ports and the larger USB drive capacity compared to the 1.44 MB 3.5-inch floppy disk.
USB flash drives use the USB mass storage device class standard, supported natively by modern operating systems such as Windows, Linux, OS X and other Unix-like systems, as well as many BIOS boot ROMs. USB drives with USB 2.0 support can store more data and transfer faster than much larger optical disc drives like CD-RW or DVD-RW drives and can be read by many other systems such as the Xbox 360, PlayStation 3, DVD players, automobile entertainment systems, and in a number of handheld devices such as smartphones and tablet computers, though the electronically similar SD card is better suited for those devices.
A flash drive consists of a small printed circuit board carrying the circuit elements and a USB connector, insulated electrically and protected inside a plastic, metal, or rubberized case which can be carried in a pocket or on a key chain, for example. The USB connector may be protected by a removable cap or by retracting into the body of the drive, although it is not likely to be damaged if unprotected. Most flash drives use a standard type-A USB connection allowing connection with a port on a personal computer, but drives for other interfaces also exist. USB flash drives draw power from the computer via the USB connection. Some devices combine the functionality of a portable media player with USB flash storage; they require a battery only when used to play music on the go.
iPhoneOgraphy – 25 Sep 2016 (Day 269/366)
The toothbrush is an oral hygiene instrument used to clean the teeth and gums and tongue that consists of a head of tightly clustered bristles mounted on a handle, which facilitates the cleansing of hard-to-reach areas of the mouth.
Toothpaste, which often contains fluoride, is commonly used in conjunction with a toothbrush to increase the effectiveness of tooth brushing. Toothbrushes are available with different bristle textures, sizes and forms. Most dentists recommend using a toothbrush labelled “soft”, since hard bristled toothbrushes can damage tooth enamel and irritate the gums. The predecessor of the toothbrush, the chew stick, first appeared in Egypt and Babylonia, and the earliest bristle toothbrush, the direct predecessor to the modern toothbrush, originated in China. Toothbrushes were introduced to Europe through merchants and travelers in East Asia by the 17th century. DuPont introduced the nylon toothbrush in the 1930s.
A variety of oral hygiene measures have been used since before recorded history prior to the toothbrush. This has been verified by various excavations done all over the world, in which chew sticks, tree twigs, bird feathers, animal bones and porcupine quills were recovered.
The predecessor of the toothbrush is the chew stick. Chew sticks were twigs with a frayed end used to brush against the teeth, while the other end was used as a toothpick. The earliest chew sticks were discovered in Babylonia in 3500 BC, an Egyptian tomb dating from 3000 BC, and mentioned in Chinese records dating from 1600 BC. The Greeks and Romans used toothpicks to clean their teeth and toothpick-like twigs have been excavated in Qin Dynasty tombs. Chew sticks remain common in Africa; the rural Southern United States – and in the Islamic world the use of chewing stick Miswak is considered a pious action, and has been prescribed to be used before every prayer five times a day. Miswak has been used by Muslims since 7th Century AD.
The first bristle toothbrush, resembling the modern toothbrush, was found in China during the Tang Dynasty (619–907) and used hog bristle. The bristles were sourced from hogs living in Siberia and northern China because the colder temperatures provided firmer bristles. They were then attached to a handle manufactured from bamboo or bone, forming a toothbrush. In 1223, Japanese Zen master Dōgen Kigen recorded on Shōbōgenzō that he saw monks in China clean their teeth with brushes made of horse-tail hairs attached to an ox-bone handle. The bristle toothbrush spread to Europe, brought back from China to Europe by travellers. It was adopted in Europe during the 17th century. The earliest identified use of the word toothbrush in English was in the autobiography of Anthony Wood, who wrote in 1690 that he had bought a toothbrush from J. Barret. Europeans found the hog bristle toothbrushes exported from merchants in China too firm, and preferred softer bristle toothbrushes manufactured from horsehair. Mass-produced toothbrushes, made with horse or boar bristle, continued to be imported to England from China until the mid-20th century.
In Europe, William Addis of England is believed to have produced the first mass-produced toothbrush, in 1780. In 1770, he had been jailed for causing a riot; while in prison he decided that the method used to clean teeth – at the time rubbing a rag with soot and salt on the teeth – was ineffective and could be improved. To that end, he saved a small animal bone left over from the meal he had eaten the previous night, into which he drilled small holes. He then obtained some bristles from one of his guards, which he tied in tufts that he then passed through the holes in the bone, and which he finally sealed with glue. After his release, he started a business that would manufacture the toothbrushes he had built, and he soon became very rich. He died in 1808, and left the business to his eldest son, also called William, and it stayed in family ownership until 1996. Under the name Wisdom Toothbrushes the company now manufactures 70 million toothbrushes per year in the UK. By 1840 toothbrushes were being mass-produced in England, France, Germany, and Japan. Pig bristle was used for cheaper toothbrushes, and badger hair for the more expensive ones.
The first patent for a toothbrush was granted to H. N. Wadsworth in 1857 (US Patent No. 18,653) in the United States, but mass production in the United States only started in 1885. The rather advanced design had a bone handle with holes bored into it for the Siberian boar hair bristles. Unfortunately, animal bristle was not an ideal material as it retains bacteria and does not dry well, and the bristles often fell out. In addition to bone, sometimes handles were made of wood or ivory. In the United States, brushing teeth did not become routine until after World War II, when American soldiers had to clean their teeth daily.
During the 1900s, celluloid handles gradually replaced bone handles in toothbrushes. Natural animal bristles were also replaced by synthetic fibers, usually nylon, by DuPont in 1938. The first nylon bristle toothbrush, made with nylon yarn, went on sale on February 24, 1938. The first electric toothbrush, the Broxodent, was invented in Switzerland in 1954. As of the turn of the 21st century, nylon had come to be widely used for the bristles, and the handles were usually molded from thermoplastic materials.
Johnson & Johnson, a leading medical-supplies firm, introduced the “Reach” toothbrush in 1977. It differed from previous toothbrushes in three ways: First, it had an angled head, similar to dental instruments, to reach back teeth; second, the bristles were concentrated more closely than usual to clean each tooth of potentially cariogenic (cavity-causing) materials; and third, the outer bristles were longer and softer than the inner bristles, to clean between teeth. The Reach toothbrush was the first to have a specialized design intended to increase its effectiveness. Other models, from other manufacturers, soon followed; each of these had unique design features intended to be, and promoted as being, more effective than the basic toothbrush design that had been employed for years.
In January 2003 the toothbrush was selected as the number one invention Americans could not live without according to the Lemelson-MIT Invention Index.
iPhoneOgraphy – 24 Sep 2016 (Day 268/366)
A fan is a machine used to create flow within a fluid, typically a gas such as air. The fan consists of a rotating arrangement of vanes or blades which act on the fluid. The rotating assembly of blades and hub is known as an impeller, a rotor, or a runner. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors and internal combustion engines. Fans produce flows with high volume and low pressure (although higher than ambient pressure), as opposed to compressors which produce high pressures at a comparatively low volume. A fan blade will often rotate when exposed to a fluid stream, and devices that take advantage of this, such as anemometers and wind turbines, often have designs similar to that of a fan.
Typical applications include climate control and personal thermal comfort (e.g., an electric table or floor fan), vehicle and machinery cooling systems, ventilation, fume extraction, winnowing (e.g., separating chaff of cereal grains), removing dust (e.g. in a vacuum cleaner), drying (usually in combination with heat) and to provide draft for a fire. While fans are often used to cool people, they do not actually cool air (if anything, electric fans warm it slightly due to the warming of their motors), but work by evaporative cooling of sweat and increased heat convection into the surrounding air due to the airflow from the fans. Thus, fans may become ineffective at cooling the body if the surrounding air is near body temperature and contains high humidity.
The word fan comes from Middle English, winnowing fan, from Old English fann and from Latin vannus.
iPhoneOgraphy – 23 Sep 2016 (Day 267/366)
A bug zapper, more formally called an electrical discharge insect control system, electric insect killer or (insect) electrocutor trap, is a device that attracts and kills flying insects that are attracted by light. A light source attracts insects to an electrical grid, where they are electrocuted by touching two wires with a high voltage between them. The name comes from the characteristic onomatopoeic zap sound produced when an insect is electrocuted.
In its October 1911 issue, popular Mechanics magazine had a piece showing a model “fly trap” that used all the elements of a modern bug zapper, including electric light and electrified grid. The design was implemented by two unnamed Denver men and was conceded to be too expensive to be of practical use. The device was 10 by 15 inches (25 by 38 cm), contained 5 incandescent light bulbs, and the grid was 1/16-inch (1.59 mm) wires spaced 1/8-inch (3.17 mm) apart with a voltage of 450 volts. Users were supposed to bait the interior with meat.
According to the US Patent and Trademark Office, the first bug zapper was patented in 1932 by William M. Frost; the patent was filed Jan 14 1928, and the devices were first sold in 1928.
Separately, Dr. William Brodbeck Herms (1876–1949), a professor of parasitology at the University of California, had been working on large commercial insect traps for over 20 years for protection of California’s important fruit industry. In 1934 he introduced the electronic insect killer that became the model for all future bug zappers.
iPhoneOgraphy – 22 Sep 2016 (Day 266/366)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. There are a large number of spring designs; in everyday usage the term often refers to coil springs.
When a spring is compressed or stretched from its resting position, it exerts an opposing force approximately proportional to its change in length (this approximation breaks down for larger deflections). The rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection curve. An extension or compression spring’s rate is expressed in units of force divided by distance, for example lbf/in or N/m. A torsion spring is a spring that works by twisting; when it is twisted about its axis by an angle, it produces a torque proportional to the angle. A torsion spring’s rate is in units of torque divided by angle, such as N.m/rad or ft.lbf/degree. The inverse of spring rate is compliance, that is: if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N. The stiffness (or rate) of springs in parallel is additive, as is the compliance of springs in series.
Springs are made from a variety of elastic materials, the most common being spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication. Some non-ferrous metals are also used including phosphor bronze and titanium for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current (because of its low electrical resistance).
Simple non-coiled springs were used throughout human history, e.g. the bow (and arrow). In the Bronze Age more sophisticated spring devices were used, as shown by the spread of tweezers in many cultures. Ctesibius of Alexandria developed a method for making bronze with spring-like characteristics by producing an alloy of bronze with an increased proportion of tin, and then hardening it by hammering after it was cast.
Coiled springs appeared early in the 15th century, in door locks. The first spring powered-clocks appeared in that century and evolved into the first large watches by the 16th century.
In 1676 British physicist Robert Hooke discovered Hooke’s law which states that the force a spring exerts is proportional to its extension.
In classical physics, a spring can be seen as a device that stores potential energy, specifically elastic potential energy, by straining the bonds between the atoms of an elastic material.
Hooke’s law of elasticity states that the extension of an elastic rod (its distended length minus its relaxed length) is linearly proportional to its tension, the force used to stretch it. Similarly, the contraction (negative extension) is proportional to the compression (negative tension).
This law actually holds only approximately, and only when the deformation (extension or contraction) is small compared to the rod’s overall length. For deformations beyond the elastic limit, atomic bonds get broken or rearranged, and a spring may snap, buckle, or permanently deform. Many materials have no clearly defined elastic limit, and Hooke’s law can not be meaningfully applied to these materials. Moreover, for the superelastic materials, the linear relationship between force and displacement is appropriate only in the low-strain region.
Hooke’s law is a mathematical consequence of the fact that the potential energy of the rod is a minimum when it has its relaxed length. Any smooth function of one variable approximates a quadratic function when examined near enough to its minimum point as can be seen by examining the Taylor series. Therefore, the force—which is the derivative of energy with respect to displacement—will approximate a linear function.
iPhoneOgraphy – 21 Sep 2016 (Day 265/366)
An EXIT sign is a device in a public facility (such as a building, aircraft or boat) denoting the location of the closest emergency exit in case of fire or other emergency.
Most relevant codes (fire, building, health or safety) require exit signs to be permanently lit.
Exit signs are designed to be absolutely unmistakable and understandable to anyone. In the past this generally meant exit signs that show the word “EXIT” or the equivalent in the local language, but increasingly exit signs around the world are in pictogram form, with or without text supplement.
Early exit signs were generally either made of metal and lit by a nearby incandescent light bulb or were a white glass cover with EXIT written in red that fit directly over a single-bulb light fixture. The inherent flaws with these designs were that, in a fire, the power to the light often failed. In addition, the fixtures could be difficult to see in a fire where smoke often reduced visibility, despite being relatively bright. The biggest problem was the exit sign being hardly distinguishable from an ordinary safety lighting fixture commonly installed above doors in the past. The problem was partially solved by using red-tinted globes instead.
Better signs were soon developed that more resembled today’s modern exit sign, with an incandescent bulb inside a rectangular-shaped box that backlit the word “EXIT” on one or both sides. Being larger than its predecessors, this version of the exit sign solved some of the visibility problem. The sign was only useful as long as main power remained on.
As battery-backup systems became smaller and more efficient, some exit signs began to use a dual-power system. Under normal conditions, the exit sign was lit by mains power and the battery was in a charge state. In the event of a power outage, the battery would supply power to light the sign. It continued to discharge until mains power returned to the unit or the battery was no longer able to provide sufficient power to light the sign. Early battery-backup systems were big, heavy, and costly. Modern systems are lightweight, can be installed virtually anywhere, and are integrated into the fixture, rather than requiring a separate box. As batteries improved, so did the amount of time that a fixture could remain lit on batteries.
While exit signs were more visible due to large letters, even a 60-watt bulb shown through a plastic or glass cover (see image), could appear somewhat dim under certain conditions. With the development of fluorescent lamp and light-emitting diode technology, exit signs could be made even brighter to show up in the limited visibility of a fire situation, or use less electricity. LED signs combine a large number of bright light-emitting diodes to illuminate the sign from inside. Fluorescent lamps work in the same way as incandescent bulbs, back-lighting both sides of an exit fixture from within. Because an exit sign is constantly lit, fluorescent bulbs need to be changed more often than LEDs, although the almost nonexistent on/off cycles extend the life of fluorescent lamps significantly. Generally, LEDs have a very long life, and may last for 10 years or more of continuous use, although the brightness may diminish. . Incandescent bulbs are still in use, because they are cheap and common, even though they use more electricity and require more or less frequent replacement. Bulbs lit 24/7 will have a greatly extended lifespan.
Decades ago, radioluminescent and phosphorescent signs that require no electricity have also been developed. These have been around since the 1970s. Radio Luminescence uses the radioactive decay of tritium to light the sign, while phosphorescence uses light-emitting phosphors to glow in the dark. While both of these signs meet California State Fire Marshall standards, where practical, electricity is used in the vast majority of signs.