Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is hard and ductile. Pure nickel, powdered to maximize the reactive surface area, shows a significant chemical activity, but larger pieces are slow to react with air under standard conditions because an oxide layer forms on the surface and prevents further corrosion. So, pure native nickel is found in Earth's crust only in tiny amounts in ultramafic rocks, in the interiors of larger nickel–iron meteorites that were not exposed to oxygen when outside Earth's atmosphere. Meteoric nickel is found in combination with iron, a reflection of the origin of those elements as major end products of supernova nucleosynthesis. An iron–nickel mixture is thought to compose Earth's outer and inner cores. Use of nickel has been traced as far back as 3500 BCE. Nickel was first isolated and classified as a chemical element in 1751 by Axel Fredrik Cronstedt, who mistook the ore for a copper mineral, in the cobalt mines of Los, Hälsingland, Sweden.
The element's name comes from a mischievous sprite of German miner mythology, who personified the fact that copper-nickel ores resisted refinement into copper. An economically important source of nickel is the iron ore limonite, which contains 1–2% nickel. Nickel's other important ore minerals include pentlandite and a mixture of Ni-rich natural silicates known as garnierite. Major production sites include the Sudbury region in Canada, New Caledonia in the Pacific, Norilsk in Russia. Nickel is oxidized by air at room temperature and is considered corrosion-resistant, it has been used for plating iron and brass, coating chemistry equipment, manufacturing certain alloys that retain a high silvery polish, such as German silver. About 9% of world nickel production is still used for corrosion-resistant nickel plating. Nickel-plated objects sometimes provoke nickel allergy. Nickel has been used in coins, though its rising price has led to some replacement with cheaper metals in recent years. Nickel is one of four elements that are ferromagnetic at room temperature.
Alnico permanent magnets based on nickel are of intermediate strength between iron-based permanent magnets and rare-earth magnets. The metal is valuable in modern times chiefly in alloys. A further 10% is used for nickel-based and copper-based alloys, 7% for alloy steels, 3% in foundries, 9% in plating and 4% in other applications, including the fast-growing battery sector; as a compound, nickel has a number of niche chemical manufacturing uses, such as a catalyst for hydrogenation, cathodes for batteries and metal surface treatments. Nickel is an essential nutrient for some microorganisms and plants that have enzymes with nickel as an active site. Nickel is a silvery-white metal with a slight golden tinge, it is one of only four elements that are magnetic at or near room temperature, the others being iron and gadolinium. Its Curie temperature is 355 °C; the unit cell of nickel is a face-centered cube with the lattice parameter of 0.352 nm, giving an atomic radius of 0.124 nm. This crystal structure is stable to pressures of at least 70 GPa.
Nickel belongs to the transition metals. It is hard and ductile, has a high for transition metals electrical and thermal conductivity; the high compressive strength of 34 GPa, predicted for ideal crystals, is never obtained in the real bulk material due to the formation and movement of dislocations. The nickel atom has two electron configurations, 3d8 4s2 and 3d9 4s1, which are close in energy – the symbol refers to the argon-like core structure. There is some disagreement. Chemistry textbooks quote the electron configuration of nickel as 4s2 3d8, which can be written 3d8 4s2; this configuration agrees with the Madelung energy ordering rule, which predicts that 4s is filled before 3d. It is supported by the experimental fact that the lowest energy state of the nickel atom is a 3d8 4s2 energy level the 3d8 4s2 3F, J = 4 level. However, each of these two configurations splits into several energy levels due to fine structure, the two sets of energy levels overlap; the average energy of states with configuration 3d9 4s1 is lower than the average energy of states with configuration 3d8 4s2.
For this reason, the research literature on atomic calculations quotes the ground state configuration of nickel as 3d9 4s1. The isotopes of nickel range in atomic weight from 48 u to 78 u. Occurring nickel is composed of five stable isotopes. Isotopes heavier than 62Ni cannot be formed by nuclear fusion without losing energy. Nickel-62 has the highest mean nuclear binding energy per nucleon of any nuclide, at 8.7946 MeV/nucleon. Its binding energy is greater than both 56Fe and 58Fe, more abundant elements incorrectly cited as having the most tightly-bound nuclides. Although this would seem to predict nickel-62 as the most abundant heavy element in the universe, the high rate of photodisintegration of nickel in stellar interiors causes iron to be by far the most abundant. Stable isotope nickel-60 is the daughter product of the extinct radionuclide 60Fe, whi
Vallabhbhai Patel, popularly known as Sardar Patel, was an Indian politician. He served as the first Deputy Prime Minister of India, he was an Indian barrister and statesman, a senior leader of the Indian National Congress and a founding father of the Republic of India who played a leading role in the country's struggle for independence and guided its integration into a united, independent nation. In India and elsewhere, he was called Sardar, meaning "chief" in Hindi and Persian, he acted as Home Minister during the political integration of India and the Indo-Pakistani War of 1947. Patel was raised in the countryside of state of Gujarat, he was a successful lawyer. He subsequently organised peasants from Kheda and Bardoli in Gujarat in non-violent civil disobedience against the British Raj, becoming one of the most influential leaders in Gujarat, he was appointed as the 49th President of Indian National Congress, organising the party for elections in 1934 and 1937 while promoting the Quit India Movement.
As the first Home Minister and Deputy Prime Minister of India, Patel organised relief efforts for refugees fleeing from Punjab and Delhi and worked to restore peace. He led the task of forging a united India integrating into the newly independent nation those British colonial provinces, "allocated" to India. Besides those provinces, under direct British rule 565 self-governing princely states had been released from British suzerainty by the Indian Independence Act of 1947. Threatening military force, Patel persuaded every princely state to accede to India, his commitment to national integration in the newly independent country was total and uncompromising, earning him the sobriquet "Iron Man of India". He is remembered as the "patron saint of India's civil servants" for having established the modern all-India services system, he is called the "Unifier of India". The Statue of Unity, the world's tallest statue, was dedicated to him on 31 October 2018, 182 metres in height. Patel's date of birth was never recorded.
He belonged to the Leuva Patel Patidar community of Central Gujarat, although the Leuva Patels and Kadava Patels have claimed him as one of their own. Patel travelled to attend schools in Nadiad and Borsad, living self-sufficiently with other boys, he reputedly cultivated a stoic character. A popular anecdote recounts that he lanced his own painful boil without hesitation as the barber charged with doing it trembled; when Patel passed his matriculation at the late age of 22, he was regarded by his elders as an unambitious man destined for a commonplace job. Patel himself, harboured a plan to study to become a lawyer and save funds, travel to England, become a barrister. Patel spent years away from his family, studying on his own with books borrowed from other lawyers, passing his examinations within two years. Fetching his wife Jhaverba from her parents' home, Patel set up his household in Godhra and was called to the bar. During the many years it took him to save money, Patel – now an advocate – earned a reputation as a fierce and skilled lawyer.
The couple had a daughter, Maniben, in 1904 and a son, Dahyabhai, in 1906. Patel cared for a friend suffering from the Bubonic plague when it swept across Gujarat; when Patel himself came down with the disease, he sent his family to safety, left his home, moved into an isolated house in Nadiad. Patel practised law in Godhra and Anand while taking on the financial burdens of his homestead in Karamsad. Patel was the first chairman and founder of "Edward Memorial High School" Borsad, today known as Jhaverbhai Dajibhai Patel High School; when he had saved enough for his trip to England and applied for a pass and a ticket, they were addressed to "V. J. Patel," at the home of his elder brother Vithalbhai, who had the same initials as Vallabhai. Having once nurtured a similar hope to study in England, Vithalbhai remonstrated his younger brother, saying that it would be disreputable for an older brother to follow his younger brother. In keeping with concerns for his family's honour, Patel allowed Vithalbhai to go in his place.
In 1909 Patel's wife Jhaverba was hospitalised in Bombay to undergo major surgery for cancer. Her health worsened and, despite successful emergency surgery, she died in the hospital. Patel was given a note informing him of his wife's demise as he was cross-examining a witness in court. According to witnesses, Patel read the note, pocketed it, continued his cross-examination and won the case, he broke the news to others. Patel decided against marrying again, he raised his children with the help of his family and sent them to English-language schools in Mumbai. At the age of 36 he enrolled at the Middle Temple Inn in London. Completing a 36-month course in 30 months, Patel finished at the top of his class despite having had no previous college background. Returning to India, Patel settled in Ahmedabad and became one of the city's most successful barristers. Wearing European-style clothes and sporting urbane mannerisms, he became a skilled bridge player. Patel nurtured ambitions to expand his practice and accumulate great wealth and to provide his children with a modern education.
He had made a pact with his brother Vithalbhai to support his entry into politics in the Bombay Presidency, while Patel remained in Ahmedabad to provide for the family. At the urging of his friends, Patel ran in the election for the post of sanitation
New Delhi is an urban district of Delhi which serves as the capital of India and seat of all three branches of the Government of India. The foundation stone of the city was laid by Emperor George V during the Delhi Durbar of 1911, it was designed by Sir Edwin Lutyens and Sir Herbert Baker. The new capital was inaugurated on 13 February 1931, by Viceroy and Governor-General of India Lord Irwin. Although colloquially Delhi and New Delhi are used interchangeably to refer to the National Capital Territory of Delhi, these are two distinct entities, with New Delhi forming a small part of Delhi; the National Capital Region is a much larger entity comprising the entire NCT along with adjoining districts in neighboring states. Calcutta was the capital of India during the British Raj, until December 1911. Calcutta had become the centre of the nationalist movements since the late nineteenth century, which led to the Partition of Bengal by Viceroy of British India, Lord Curzon; this created massive political and religious upsurge including political assassinations of British officials in Calcutta.
The anti-colonial sentiments amongst the public led to complete boycott of British goods, which forced the colonial government to reunite Bengal and shift the capital to New Delhi. Old Delhi had served as the political and financial centre of several empires of ancient India and the Delhi Sultanate, most notably of the Mughal Empire from 1649 to 1857. During the early 1900s, a proposal was made to the British administration to shift the capital of the British Indian Empire, as India was named, from Calcutta on the east coast, to Delhi; the Government of British India felt that it would be logistically easier to administer India from Delhi, in the centre of northern India. The land for building the new city of Delhi was acquired under the Land Acquisition Act 1894. During the Delhi Durbar on 12 December 1911, George V Emperor of India, along with Queen Mary, his consort, made the announcement that the capital of the Raj was to be shifted from Calcutta to Delhi, while laying the foundation stone for the Viceroy's residence in the Coronation Park, Kingsway Camp.
The foundation stone of New Delhi was laid by King George V and Queen Mary at the site of Delhi Durbar of 1911 at Kingsway Camp on 15 December 1911, during their imperial visit. Large parts of New Delhi were planned by Edwin Lutyens, who first visited Delhi in 1912, Herbert Baker, both leading 20th-century British architects; the contract was given to Sobha Singh. The original plan called for its construction in Tughlaqabad, inside the Tughlaqabad fort, but this was given up because of the Delhi-Calcutta trunk line that passed through the fort. Construction began after World War I and was completed by 1931; the city, dubbed "Lutyens' Delhi" was inaugurated in ceremonies beginning on 10 February 1931 by Lord Irwin, the Viceroy. Lutyens designed the central administrative area of the city as a testament to Britain's imperial aspirations. Soon Lutyens started considering other places. Indeed, the Delhi Town Planning Committee, set up to plan the new imperial capital, with George Swinton as chairman, John A. Brodie and Lutyens as members, submitted reports for both North and South sites.
However, it was rejected by the Viceroy when the cost of acquiring the necessary properties was found to be too high. The central axis of New Delhi, which today faces east at India Gate, was meant to be a north-south axis linking the Viceroy's House at one end with Paharganj at the other. Owing to space constraints and the presence of a large number of heritage sites in the North side, the committee settled on the South site. A site atop the Raisina Hill Raisina Village, a Meo village, was chosen for the Rashtrapati Bhawan known as the Viceroy's House; the reason for this choice was that the hill lay directly opposite the Dinapanah citadel, considered the site of Indraprastha, the ancient region of Delhi. Subsequently, the foundation stone was shifted from the site of Delhi Durbar of 1911–1912, where the Coronation Pillar stood, embedded in the walls of the forecourt of the Secretariat; the Rajpath known as King's Way, stretched from the India Gate to the Rashtrapati Bhawan. The Secretariat building, the two blocks of which flank the Rashtrapati Bhawan and houses ministries of the Government of India, the Parliament House, both designed by Baker, are located at the Sansad Marg and run parallel to the Rajpath.
In the south, land up to Safdarjung's Tomb was acquired to create what is today known as Lutyens' Bungalow Zone. Before construction could begin on the rocky ridge of Raisina Hill, a circular railway line around the Council House, called the Imperial Delhi Railway, was built to transport construction material and workers for the next twenty years; the last stumbling block was the Agra-Delhi railway line that cut right through the site earmarked for the hexagonal All-India War Memorial and Kingsway, a problem because the Old Delhi Railway Station served the entire city at that time. The line was shifted to run along the Yamuna river, it began operating in 1924; the New Delhi Railway Station opened in 1926, with a single platform at Ajmeri Gate near Paharganj, was completed in time for the city's inauguration in 1931. As construction of the Viceroy's House, Central Secretariat, Parliament House, All-India War Memorial was winding down, the building of a shopping district and a new plaza, Connaught Place, began in 1929, was completed by 1933.
Named after Prince Arthur, 1st Duke of Connaught, it was designed by Robert Tor Russell, chief architect to the P
An atomic clock is a clock device that uses an electron transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, in global navigation satellite systems such as GPS; the principle of operation of an atomic clock is based on atomic physics. Early atomic clocks were based on masers at room temperature. Since 2004, more accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the national primary time and frequency standards of the United States; the accuracy of an atomic clock depends on two factors. The first factor is temperature of the sample atoms—colder atoms move much more allowing longer probe times.
The second factor is the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision. National standards agencies in many countries maintain a network of atomic clocks which are intercompared and kept synchronized to an accuracy of 10−9 seconds per day; these clocks collectively define the International Atomic Time. For civil time, another time scale is disseminated, Coordinated Universal Time. UTC is derived from TAI, but has added leap seconds from UT1, to account for variations in the rotation of the Earth with respect to the solar time; the idea of using atomic transitions to measure time was suggested by Lord Kelvin in 1879. Magnetic resonance, developed in the 1930s by Isidor Rabi, became the practical method for doing this. In 1945, Rabi first publicly suggested that atomic beam magnetic resonance might be used as the basis of a clock; the first atomic clock was an ammonia absorption line device at 23870.1 MHz built in 1949 at the U.
S. National Bureau of Standards, it served to demonstrate the concept. The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen and Jack Parry in 1955 at the National Physical Laboratory in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time. In 1967, this led the scientific community to redefine the Second in terms of a specific atomic frequency. Equality of the ET second with the SI second has been verified to within 1 part in 1010; the SI second thus inherits the effect of decisions by the original designers of the ephemeris time scale, determining the length of the ET second. Since the beginning of development in the 1950s, atomic clocks have been based on the hyperfine transitions in hydrogen-1, caesium-133, rubidium-87; the first commercial atomic clock was the Atomichron, manufactured by the National Company. More than 50 were sold between 1956 and 1960.
This bulky and expensive instrument was subsequently replaced by much smaller rack-mountable devices, such as the Hewlett-Packard model 5060 caesium frequency standard, released in 1964. In the late 1990s four factors contributed to major advances in clocks: Laser cooling and trapping of atoms So-called high-finesse Fabry–Pérot cavities for narrow laser line widths Precision laser spectroscopy Convenient counting of optical frequencies using optical combs. In August 2004, NIST scientists demonstrated a chip-scale atomic clock. According to the researchers, the clock was believed to be one-hundredth the size of any other, it requires no more than 125 mW. This technology became available commercially in 2011. Ion trap. In April 2015, NASA announced that it planned to deploy a Deep Space Atomic Clock, a miniaturized, ultra-precise mercury-ion atomic clock, into outer space. NASA said. Since 1967, the International System of Units has defined the second as the duration of 9192631770 cycles of radiation corresponding to the transition between two energy levels of the ground state of the caesium-133 atom.
In 1997, the International Committee for Weights and Measures added that the preceding definition refers to a caesium atom at rest at a temperature of absolute zero. This definition makes the caesium oscillator the primary standard for time and frequency measurements, called the caesium standard; the definitions of other physical units, e.g. the volt and the metre, rely on the definition of the second. The actual time-reference of an atomic clock consists of an electronic oscillator operating at microwave frequency; the oscillator is arranged so that its frequency-determining components include an element that can be controlled by a feedback signal. The feedback signal keeps the oscillator tuned in resonance with the frequency of the electronic transition of caesium or rubidium; the core of the atomic clock is a tunable microwave cavity containing a gas. In a hydrogen maser clock the gas emits microwaves on a hyperfine transition, the field in the cavity oscillates, the cavity is tuned for maximum microwave amplitude.
Alternatively, in a caesium or rubidium clock, the beam or gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate. For both types the atoms in the gas
In metallurgy, stainless steel known as inox steel or inox from French inoxydable, is a steel alloy, with highest percentage contents of iron and nickel, with a minimum of 10.5% chromium content by mass and a maximum of 1.2% carbon by mass. Stainless steels are most notable for their corrosion resistance, which increases with increasing chromium content. Additions of molybdenum increase corrosion resistance in reducing acids and against pitting attack in chloride solutions. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure. Stainless steel's resistance to corrosion and staining, low maintenance, familiar luster make it an ideal material for many applications where both the strength of steel and corrosion resistance are required. Stainless steels are rolled into sheets, bars and tubing to be used in: cookware, surgical instruments, major appliances. Stainless steel's corrosion resistance, the ease with which it can be steam cleaned and sterilized, no need for surface coatings has influenced its use in commercial kitchens and food processing plants.
Stainless steels do not suffer uniform corrosion, like carbon steel, when exposed to wet environments. Unprotected carbon steel rusts when exposed to the combination of air and moisture; the resulting iron oxide surface layer is fragile. Since iron oxide occupies a larger volume than the original steel this layer expands and tends to flake and fall away exposing the underlying steel to further attack. In comparison, stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in air and the small amount of dissolved oxygen in water; this passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal. This film is self-repairing if it is scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade; the resistance of this film to corrosion depends upon the chemical composition of the stainless steel, chiefly the chromium content.
Corrosion of stainless steels can occur. It is customary to distinguish between 4 forms of corrosion: uniform, galvanic and SCC. Uniform corrosion takes place in aggressive environments chemical production or use and paper industries, etc; the whole surface of the steel is attacked and the corrosion is expressed as corrosion rate in mm/year Corrosion tables provide guidelines This is the case when stainless steels are exposed to acidic or basic solutions. Whether a stainless steel corrodes depends on the kind and concentration of acid or base, the solution temperature. Uniform corrosion is easy to avoid because of extensive published corrosion data or easy to perform laboratory corrosion testing. However, stainless steels are susceptible to localized corrosion under certain conditions, which need to be recognized and avoided; such localized corrosion is problematic for stainless steels because it is unexpected and difficult to predict. Acidic solutions can be categorized into two general categories, reducing acids such as hydrochloric acid and dilute sulfuric acid, oxidizing acids such as nitric acid and concentrated sulfuric acid.
Increasing chromium and molybdenum contents provide increasing resistance to reducing acids, while increasing chromium and silicon contents provide increasing resistance to oxidizing acids. Sulfuric acid is one of the largest tonnage industrial chemical manufactured. At room temperature Type 304 is only resistant to 3% acid while Type 316 is resistant to 3% acid up to 50 °C and 20% acid at room temperature, thus Type 304 is used in contact with sulfuric acid. Type 904L and Alloy 20 are resistant to sulfuric acid at higher concentrations above room temperature. Concentrated sulfuric acid possesses oxidizing characteristics like nitric acid and thus silicon bearing stainless steels find application. Hydrochloric acid will damage any kind of stainless steel, should be avoided. All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentration and elevated temperature attack will occur and higher alloy stainless steels are required. In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid.
As the molecular weight of organic acids increase their corrosivity decreases. Formic acid is a strong acid. Type 304 can be used with formic acid. Acetic acid is the most commercially important of the organic acids and Type 316 is used for storing and handling acetic acid. Stainless steels Type 304 and 316 are unaffected by any of the weak bases such as ammonium hydroxide in high concentrations and at high temperatures; the same grades of stainless exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will experience some etching and cracking. Increasing chromium and nickel contents provide increasing resistance. All grades resist damage from aldehydes and amines, though in the latter
A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. In 1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced intelligible replication of the human voice; this instrument was further developed by many others. The telephone was the first device in history that enabled people to talk directly with each other across large distances. Telephones became indispensable to businesses and households and are today some of the most used small appliances; the essential elements of a telephone are a microphone to speak into and an earphone which reproduces the voice in a distant location. In addition, most telephones contain a ringer to announce an incoming telephone call, a dial or keypad to enter a telephone number when initiating a call to another telephone.
The receiver and transmitter are built into a handset, held up to the ear and mouth during conversation. The dial may be located either on a base unit to which the handset is connected; the transmitter converts the sound waves to electrical signals which are sent through a telephone network to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones are duplex devices; the first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were replaced by manually operated centrally located switchboards; these exchanges were soon connected together forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission between mobile stations on ships and automobiles in the mid-20th century. Hand-held mobile phones were introduced for personal service starting in 1973.
In decades their analog cellular system evolved into digital networks with greater capability and lower cost. Convergence has given most modern cell phones capabilities far beyond simple voice conversation, they may be able to record spoken messages and receive text messages and display photographs or video, play music or games, surf the Internet, do road navigation or immerse the user in virtual reality. Since 1999, the trend for mobile phones is smartphones that integrate all mobile communication and computing needs. A traditional landline telephone system known as plain old telephone service carries both control and audio signals on the same twisted pair of insulated wires, the telephone line; the control and signaling equipment consists of three components, the ringer, the hookswitch, a dial. The ringer, or beeper, light or other device, alerts the user to incoming calls; the hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call.
A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the 1960s dials used exclusively the rotary technology, replaced by dual-tone multi-frequency signaling with pushbutton telephones. A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interference and crosstalk better than a single wire or an untwisted pair; the strong outgoing speech signal from the microphone does not overpower the weaker incoming speaker signal with sidetone because a hybrid coil and other components compensate the imbalance. The junction box arrests lightning and adjusts the line's resistance to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths; the line voltages are negative compared to earth. Negative voltage attracts positive metal ions toward the wires.
The landline telephone contains a switchhook and an alerting device a ringer, that remains connected to the phone line whenever the phone is "on hook", other components which are connected when the phone is "off hook". The off-hook components include a transmitter, a receiver, other circuits for dialing and amplification. A calling party wishing to speak to another party will pick up the telephone's handset, thereby operating a lever which closes the switchhook, which powers the telephone by connecting the transmitter and related audio components to the line; the off-hook circuitry has a low resistance which causes a direct current, which comes down the line from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, sends a dial tone to indicate readiness. On a modern push-button telephone, the caller presses the number keys to send the telephone number of the called party; the keys control a tone generator circuit. A rotary-dial telephone uses pulse