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81-409: Modern masers can be designed to generate electromagnetic waves at microwave frequencies and radio and infrared frequencies. For this reason, Townes suggested replacing "microwave" with "molecular" as the first word in the acronym "maser". The laser works by the same principle as the maser, but produces higher-frequency coherent radiation at visible wavelengths. The maser was the precursor to

162-450: A rotational transition at a frequency of 96 GHz. Extremely powerful masers, associated with active galactic nuclei , are known as megamasers and are up to a million times more powerful than stellar masers. The meaning of the term maser has changed slightly since its introduction. Initially the acronym was universally given as "microwave amplification by stimulated emission of radiation", which described devices which emitted in

243-441: A specific range of frequencies . The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz (20 kHz), though the high frequency limit usually reduces with age. Other species have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz. In many media, such as air, the speed of sound is approximately independent of frequency, so

324-522: A 150 μm (0.006 in) micrometer-adjustable entry to the chamber. The whole system noise temperature looking at cold sky (2.7  kelvin in the microwave band) was 17 kelvin. This gave such a low noise figure that the Mariner IV space probe could send still pictures from Mars back to the Earth , even though the output power of its radio transmitter was only 15  watts , and hence

405-405: A body B at the temperature ( T − 1)° , would give out the same mechanical effect, whatever be the number T ." Specifically, Thomson expressed the amount of work necessary to produce a unit of heat (the thermal efficiency ) as μ ( t ) ( 1 + E t ) / E {\displaystyle \mu (t)(1+Et)/E} , where t {\displaystyle t}

486-510: A committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the kelvin would refer to water having the isotopic composition specified for Vienna Standard Mean Ocean Water . In 2005, the CIPM began a programme to redefine the kelvin (along with other SI base units ) using a more experimentally rigorous method. In particular,

567-411: A fractional error of Δ f f = 1 2 f T m {\textstyle {\frac {\Delta f}{f}}={\frac {1}{2fT_{\text{m}}}}} where T m {\displaystyle T_{\text{m}}} is the timing interval and f {\displaystyle f} is the measured frequency. This error decreases with frequency, so it

648-562: A frequency particular to the element or molecule used as the masing medium (similar to what occurs in the lasing medium in a laser). By putting such an amplifying medium in a resonant cavity , feedback is created that can produce coherent radiation . In 2012, a research team from the National Physical Laboratory and Imperial College London developed a solid-state maser that operated at room temperature by using optically pumped, pentacene -doped p-Terphenyl as

729-433: A gas cooled to about −273 °C would occupy zero volume. In 1848, William Thomson, who was later ennobled as Lord Kelvin , published a paper On an Absolute Thermometric Scale . The scale proposed in the paper turned out to be unsatisfactory, but the principles and formulas upon which the scale was based were correct. For example, in a footnote, Thomson derived the value of −273 °C for absolute zero by calculating

810-497: A given substance can occur only at a single pressure and only at a single temperature. By the 1940s, the triple point of water had been experimentally measured to be about 0.6% of standard atmospheric pressure and very close to 0.01 °C per the historical definition of Celsius then in use. In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly and allowing

891-416: A known frequency near the unknown frequency is mixed with the unknown frequency in a nonlinear mixing device such as a diode . This creates a heterodyne or "beat" signal at the difference between the two frequencies. If the two signals are close together in frequency the heterodyne is low enough to be measured by a frequency counter. This process only measures the difference between the unknown frequency and

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972-537: A relative standard uncertainty of 3.7 × 10 . Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now 273.1600(1) K . The new definition officially came into force on 20 May 2019, the 144th anniversary of the Metre Convention . The kelvin is often used as a measure of the colour temperature of light sources. Colour temperature

1053-402: A repeating event is accomplished by counting the number of times that event occurs within a specific time period, then dividing the count by the period. For example, if 71 events occur within 15 seconds the frequency is: f = 71 15 s ≈ 4.73 Hz . {\displaystyle f={\frac {71}{15\,{\text{s}}}}\approx 4.73\,{\text{Hz}}.} If

1134-483: A second (60 seconds divided by 120). For cyclical phenomena such as oscillations , waves , or for examples of simple harmonic motion , the term frequency is defined as the number of cycles or repetitions per unit of time. The conventional symbol for frequency is f or ν (the Greek letter nu ) is also used. The period T is the time taken to complete one cycle of an oscillation or rotation. The frequency and

1215-412: A starting point, with Celsius being defined (from the 1740s to the 1940s ) by calibrating a thermometer such that: This definition assumes pure water at a specific pressure chosen to approximate the natural air pressure at sea level. Thus, an increment of 1 °C equals ⁠ 1 / 100 ⁠ of the temperature difference between the melting and boiling points. The same temperature interval

1296-481: Is "the mechanical equivalent of a unit of heat", now referred to as the specific heat capacity of water, approximately 771.8 foot-pounds force per degree Fahrenheit per pound (4,153 J/K/kg). Thomson was initially skeptical of the deviations of Joule's formula from experiment, stating "I think it will be generally admitted that there can be no such inaccuracy in Regnault's part of the data, and there remains only

1377-521: Is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise . The only SI derived unit with a special name derived from the kelvin is the degree Celsius. Like other SI units, the kelvin can also be modified by adding a metric prefix that multiplies it by a power of 10 : According to SI convention,

1458-399: Is based upon the principle that a black body radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about 4000 K appear reddish, whereas those above about 7500 K appear bluish. Colour temperature is important in the fields of image projection and photography, where a colour temperature of approximately 5600 K

1539-495: Is called a radio wave . Likewise, an electromagnetic wave with a frequency higher than 8 × 10  Hz will also be invisible to the human eye; such waves are called ultraviolet (UV) radiation. Even higher-frequency waves are called X-rays , and higher still are gamma rays . All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called electromagnetic radiation . They all travel through vacuum at

1620-764: Is common convention to capitalize Kelvin when referring to Lord Kelvin or the Kelvin scale. The unit symbol K is encoded in Unicode at code point U+212A K KELVIN SIGN . However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004B K LATIN CAPITAL LETTER K instead; that is, a normal capital K . "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126 Ω OHM SIGN , U+212A K KELVIN SIGN , and U+212B Å ANGSTROM SIGN . In all three instances,

1701-405: Is equivalent to one hertz. As a matter of convenience, longer and slower waves, such as ocean surface waves , are more typically described by wave period rather than frequency. Short and fast waves, like audio and radio, are usually described by their frequency. Some commonly used conversions are listed below: For periodic waves in nondispersive media (that is, media in which the wave speed

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1782-470: Is expressed with the unit reciprocal second (s ) or, in the case of radioactivity, with the unit becquerel . It is defined as a rate , f = N /Δ t , involving the number of entities counted or the number of events happened ( N ) during a given time duration (Δ t ); it is a physical quantity of type temporal rate . Kelvin The kelvin (symbol: K ) is the base unit for temperature in

1863-411: Is generally a problem at low frequencies where the number of counts N is small. An old method of measuring the frequency of rotating or vibrating objects is to use a stroboscope . This is an intense repetitively flashing light ( strobe light ) whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down. When

1944-427: Is in allowing more accurate measurements at very low and very high temperatures, as the techniques used depend on the Boltzmann constant. Independence from any particular substance or measurement is also a philosophical advantage. The kelvin now only depends on the Boltzmann constant and universal constants (see 2019 SI unit dependencies diagram), allowing the kelvin to be expressed exactly as: For practical purposes,

2025-458: Is independent of frequency), frequency has an inverse relationship to the wavelength , λ ( lambda ). Even in dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ . {\displaystyle f={\frac {v}{\lambda }}.} In the special case of electromagnetic waves in vacuum , then v = c , where c

2106-410: Is observed from molecules such as water (H 2 O), hydroxyl radicals ( •OH ), methanol (CH 3 OH), formaldehyde (HCHO), silicon monoxide (SiO), and carbodiimide (HNCNH). Water molecules in star -forming regions can undergo a population inversion and emit radiation at about 22.0  GHz , creating the brightest spectral line in the radio universe. Some water masers also emit radiation from

2187-555: Is proportional to μ {\displaystyle \mu } . When Thomson published his paper in 1848, he only considered Regnault's experimental measurements of μ ( t ) {\displaystyle \mu (t)} . That same year, James Prescott Joule suggested to Thomson that the true formula for Carnot's function was μ ( t ) = J E 1 + E t , {\displaystyle \mu (t)=J{\frac {E}{1+Et}},} where J {\displaystyle J}

2268-402: Is red light, 800 THz ( 8 × 10  Hz ) is violet light, and between these (in the range 400–800 THz) are all the other colors of the visible spectrum . An electromagnetic wave with a frequency less than 4 × 10  Hz will be invisible to the human eye; such waves are called infrared (IR) radiation. At even lower frequency, the wave is called a microwave , and at still lower frequencies it

2349-620: Is required to match "daylight" film emulsions. In astronomy , the stellar classification of stars and their place on the Hertzsprung–Russell diagram are based, in part, upon their surface temperature, known as effective temperature . The photosphere of the Sun , for instance, has an effective temperature of 5772 K [1] [2] [3] [4] as adopted by IAU 2015 Resolution B3. Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide

2430-463: Is that higher colour temperature produces an image with enhanced white and blue hues. The reduction in colour temperature produces an image more dominated by reddish, "warmer" colours . For electronics , the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate noise floor , i.e. the noise temperature . The Johnson–Nyquist noise of resistors (which produces an associated kTC noise when combined with capacitors )

2511-400: Is the speed of light in vacuum, and this expression becomes f = c λ . {\displaystyle f={\frac {c}{\lambda }}.} When monochromatic waves travel from one medium to another, their frequency remains the same—only their wavelength and speed change. Measurement of frequency can be done in the following ways: Calculating the frequency of

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2592-400: Is the number of occurrences of a repeating event per unit of time . It is also occasionally referred to as temporal frequency for clarity and to distinguish it from spatial frequency . Ordinary frequency is related to angular frequency (symbol ω , with SI unit radian per second) by a factor of 2 π . The period (symbol T ) is the interval of time between events, so the period is

2673-652: Is the temperature in Celsius, E {\displaystyle E} is the coefficient of thermal expansion, and μ ( t ) {\displaystyle \mu (t)} was "Carnot's function", a substance-independent quantity depending on temperature, motivated by an obsolete version of Carnot's theorem . The scale is derived by finding a change of variables T 1848 = f ( T ) {\displaystyle T_{1848}=f(T)} of temperature T {\displaystyle T} such that d T 1848 / d T {\displaystyle dT_{1848}/dT}

2754-442: The Boltzmann constant to exactly 1.380 649 × 10   joules per kelvin; every 1 K change of thermodynamic temperature corresponds to a thermal energy change of exactly 1.380 649 × 10  J . During the 18th century, multiple temperature scales were developed, notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics , including atomic theory and

2835-655: The International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature ( absolute zero ), taken to be 0 K. By definition, the Celsius scale (symbol °C) and the Kelvin scale have the exact same magnitude; that is, a rise of 1 K is equal to a rise of 1 °C and vice versa, and any temperature in degrees Celsius can be converted to kelvin by adding 273.15. The 19th century British scientist Lord Kelvin first developed and proposed

2916-595: The USSR Academy of Sciences in May 1952, published in October 1954. Independently, Charles Hard Townes , James P. Gordon , and H. J. Zeiger built the first ammonia maser at Columbia University in 1953. This device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 gigahertz . Townes later worked with Arthur L. Schawlow to describe

2997-399: The kinetic theory of gases which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct Newton scale and Réaumur scale ) the melting point of ice served as such

3078-418: The melting point at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a fluctuating value) close to 0 °C. This was justified on the grounds that the triple point was judged to give a more accurately reproducible reference temperature than the melting point. The triple point could be measured with ±0.0001 °C accuracy, while

3159-408: The reciprocal of the frequency: T = 1/ f . Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals ( sound ), radio waves , and light . For example, if a heart beats at a frequency of 120 times per minute (2 hertz), the period—the time interval between beats—is half

3240-475: The 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K. The 13th CGPM also held in Resolution ;4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction ⁠ 1 / 273.16 ⁠ of the thermodynamic temperature of the triple point of water." After the 1983 redefinition of the metre , this left

3321-614: The absolute temperature as T H = J / μ {\displaystyle T_{H}=J/\mu } . One finds the relationship T H = J × Q H × ( t H − t C ) / W {\displaystyle T_{H}=J\times Q_{H}\times (t_{H}-t_{C})/W} . By supposing T H − T C = J × ( t H − t c ) {\displaystyle T_{H}-T_{C}=J\times (t_{H}-t_{c})} , one obtains

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3402-491: The alternating current in household electrical outlets is 60 Hz (between the tones B ♭ and B; that is, a minor third above the European frequency). The frequency of the ' hum ' in an audio recording can show in which of these general regions the recording was made. Aperiodic frequency is the rate of incidence or occurrence of non- cyclic phenomena, including random processes such as radioactive decay . It

3483-425: The amplifier down to a temperature of 4  kelvin . Amplification was achieved by exciting a ruby comb with a 12.0 gigahertz klystron . In the early years, it took days to chill and remove the impurities from the hydrogen lines. Refrigeration was a two-stage process, with a large Linde unit on the ground, and a crosshead compressor within the antenna. The final injection was at 21 MPa (3,000 psi) through

3564-404: The amplifier medium. It produced pulses of maser emission lasting for a few hundred microseconds. In 2018, a research team from Imperial College London and University College London demonstrated continuous-wave maser oscillation using synthetic diamonds containing nitrogen-vacancy defects. Masers serve as high precision frequency references . These "atomic frequency standards" are one of

3645-401: The committee proposed redefining the kelvin such that the Boltzmann constant ( k B ) would take the exact value 1.380 6505 × 10  J/K . The committee hoped the program would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered part of a larger program . A challenge

3726-456: The desire to increase the importance of his invention, and his reputation in the scientific community. When the laser was developed, Townes and Schawlow and their colleagues at Bell Labs pushed the use of the term optical maser , but this was largely abandoned in favor of laser , coined by their rival Gordon Gould. In modern usage, devices that emit in the X-ray through infrared portions of

3807-421: The frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integer multiple of

3888-473: The general principle of an absolute thermodynamic temperature scale for the Carnot engine, Q H / T H = Q C / T C {\displaystyle Q_{H}/T_{H}=Q_{C}/T_{C}} . The definition can be shown to correspond to the thermometric temperature of the ideal gas laws . This definition by itself is not sufficient. Thomson specified that

3969-451: The kelvin is never referred to nor written as a degree . The word "kelvin" is not capitalized when used as a unit. It may be in plural form as appropriate (for example, "it is 283 kelvins outside", as for "it is 50 degrees Fahrenheit" and "10 degrees Celsius"). The unit's symbol K is a capital letter, per the SI convention to capitalize symbols of units derived from the name of a person. It

4050-514: The kelvin, the second, and the kilogram as the only SI units not defined with reference to any other unit. In 2005, noting that the triple point could be influenced by the isotopic ratio of the hydrogen and oxygen making up a water sample and that this was "now one of the major sources of the observed variability between different realizations of the water triple point", the International Committee for Weights and Measures (CIPM),

4131-464: The laser, inspiring theoretical work by Townes and Arthur Leonard Schawlow that led to the invention of the laser in 1960 by Theodore Maiman . When the coherent optical oscillator was first imagined in 1957, it was originally called the "optical maser". This was ultimately changed to laser , for "light amplification by stimulated emission of radiation". Gordon Gould is credited with creating this acronym in 1957. The theoretical principles governing

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4212-496: The many forms of atomic clocks . Masers were also used as low-noise microwave amplifiers in radio telescopes , though these have largely been replaced by amplifiers based on FETs . During the early 1960s, the Jet Propulsion Laboratory developed a maser to provide ultra-low-noise amplification of S-band microwave signals received from deep space probes. This maser used deeply refrigerated helium to chill

4293-462: The maser as a timing standard. More recent masers are practically identical to their original design. Maser oscillations rely on the stimulated emission between two hyperfine energy levels of atomic hydrogen . Here is a brief description of how they work: Maser-like stimulated emission has also been observed in nature from interstellar space , and it is frequently called "superradiant emission" to distinguish it from laboratory masers. Such emission

4374-437: The melting point just to ±0.001 °C. In 1954, with absolute zero having been experimentally determined to be about −273.15 °C per the definition of °C then in use, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) introduced a new internationally standardized Kelvin scale which defined the triple point as exactly 273.15 + 0.01 = 273.16 degrees Kelvin. In 1967/1968, Resolution 3 of

4455-419: The microwave region of the electromagnetic spectrum . The principle and concept of stimulated emission has since been extended to more devices and frequencies. Thus, the original acronym is sometimes modified, as suggested by Charles H. Townes, to " molecular amplification by stimulated emission of radiation." Some have asserted that Townes's efforts to extend the acronym in this way were primarily motivated by

4536-447: The modern Kelvin scale T {\displaystyle T} , the first scale could be expressed as follows: T 1848 = 100 × log ⁡ ( T / 273 K ) log ⁡ ( 373 K / 273 K ) {\displaystyle T_{1848}=100\times {\frac {\log(T/{\text{273 K}})}{\log({\text{373 K}}/{\text{273 K}})}}} The parameters of

4617-420: The negative reciprocal of 0.00366—the coefficient of thermal expansion of an ideal gas per degree Celsius relative to the ice point. This derived value agrees with the currently accepted value of −273.15 °C, allowing for the precision and uncertainty involved in the calculation. The scale was designed on the principle that "a unit of heat descending from a body A at the temperature T ° of this scale, to

4698-545: The number of counts is not very large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error into the count of between zero and one count, so on average half a count. This is called gating error and causes an average error in the calculated frequency of Δ f = 1 2 T m {\textstyle \Delta f={\frac {1}{2T_{\text{m}}}}} , or

4779-774: The operation of a maser were first described by Joseph Weber of the University of Maryland, College Park at the Electron Tube Research Conference in June 1952 in Ottawa , with a summary published in the June 1953 Transactions of the Institute of Radio Engineers Professional Group on Electron Devices, and simultaneously by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics , at an All-Union Conference on Radio-Spectroscopy held by

4860-601: The period are related by the equation f = 1 T . {\displaystyle f={\frac {1}{T}}.} The term temporal frequency is used to emphasise that the frequency is characterised by the number of occurrences of a repeating event per unit time. The SI unit of frequency is the hertz (Hz), named after the German physicist Heinrich Hertz by the International Electrotechnical Commission in 1930. It

4941-560: The principle of the optical maser , or laser , of which Theodore H. Maiman created the first working model in 1960. For their research in the field of stimulated emission, Townes, Basov and Prokhorov were awarded the Nobel Prize in Physics in 1964. The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been induced into an excited energy state, they can amplify radiation at

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5022-431: The redefinition was unnoticed; enough digits were used for the Boltzmann constant to ensure that 273.16 K has enough significant digits to contain the uncertainty of water's triple point and water still normally freezes at 0 °C to a high degree of precision. But before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of 1.380 649 03 (51) × 10  J/K , with

5103-410: The reference frequency. To convert higher frequencies, several stages of heterodyning can be used. Current research is extending this method to infrared and light frequencies ( optical heterodyne detection ). Visible light is an electromagnetic wave , consisting of oscillating electric and magnetic fields traveling through space. The frequency of the wave determines its color: 400 THz ( 4 × 10 Hz)

5184-400: The relationship between work and heat for a perfect thermodynamic engine was simply the constant J {\displaystyle J} . In 1854, Thomson and Joule thus formulated a second absolute scale that was more practical and convenient, agreeing with air thermometers for most purposes. Specifically, "the numerical measure of temperature shall be simply the mechanical equivalent of

5265-560: The rotation rate of a shaft, mechanical vibrations, or sound waves , can be converted to a repetitive electronic signal by transducers and the signal applied to a frequency counter. As of 2018, frequency counters can cover the range up to about 100 GHz. This represents the limit of direct counting methods; frequencies above this must be measured by indirect methods. Above the range of frequency counters, frequencies of electromagnetic signals are often measured indirectly utilizing heterodyning ( frequency conversion ). A reference signal of

5346-414: The same speed (the speed of light), giving them wavelengths inversely proportional to their frequencies. c = f λ , {\displaystyle \displaystyle c=f\lambda ,} where c is the speed of light ( c in vacuum or less in other media), f is the frequency and λ is the wavelength. In dispersive media , such as glass, the speed depends somewhat on frequency, so

5427-488: The scale should have two properties: These two properties would be featured in all future versions of the Kelvin scale, although it was not yet known by that name. In the early decades of the 20th century, the Kelvin scale was often called the "absolute Celsius " scale, indicating Celsius degrees counted from absolute zero rather than the freezing point of water, and using the same symbol for regular Celsius degrees, °C. In 1873, William Thomson's older brother James coined

5508-504: The scale were arbitrarily chosen to coincide with the Celsius scale at 0° and 100 °C or 273 and 373 K (the melting and boiling points of water). On this scale, an increase of approximately 222 degrees corresponds to a doubling of Kelvin temperature, regardless of the starting temperature, and "infinite cold" ( absolute zero ) has a numerical value of negative infinity . Thomson understood that with Joule's proposed formula for μ {\displaystyle \mu } ,

5589-495: The scale. It was often called the "absolute Celsius" scale in the early 20th century. The kelvin was formally added to the International System of Units in 1954, defining 273.16 K to be the triple point of water . The Celsius, Fahrenheit , and Rankine scales were redefined in terms of the Kelvin scale using this definition. The 2019 revision of the SI now defines the kelvin in terms of energy by setting

5670-750: The spectrum are typically called lasers , and devices that emit in the microwave region and below are commonly called masers , regardless of whether they emit microwaves or other frequencies. Gould originally proposed distinct names for devices that emit in each portion of the spectrum, including grasers ( gamma ray lasers), xasers (x-ray lasers), uvasers ( ultraviolet lasers), lasers ( visible lasers), irasers ( infrared lasers), masers (microwave masers), and rasers ( RF masers). Most of these terms never caught on, however, and all have now become (apart from in science fiction) obsolete except for maser and laser . Frequencies Frequency (symbol f ), most often measured in hertz (symbol: Hz),

5751-461: The strobing frequency will also appear stationary. Higher frequencies are usually measured with a frequency counter . This is an electronic instrument which measures the frequency of an applied repetitive electronic signal and displays the result in hertz on a digital display . It uses digital logic to count the number of cycles during a time interval established by a precision quartz time base. Cyclic processes that are not electrical, such as

5832-571: The system ( Q H − Q C {\displaystyle Q_{H}-Q_{C}} ), t H {\displaystyle t_{H}} is the temperature of the hot reservoir in Celsius, and t C {\displaystyle t_{C}} is the temperature of the cold reservoir in Celsius. The Carnot function is defined as μ = W / Q H / ( t H − t C ) {\displaystyle \mu =W/Q_{H}/(t_{H}-t_{C})} , and

5913-428: The term triple point to describe the combination of temperature and pressure at which the solid, liquid, and gas phases of a substance were capable of coexisting in thermodynamic equilibrium . While any two phases could coexist along a range of temperature-pressure combinations (e.g. the boiling point of water can be affected quite dramatically by raising or lowering the pressure), the triple point condition for

5994-411: The thermal unit divided by Carnot's function." To explain this definition, consider a reversible Carnot cycle engine, where Q H {\displaystyle Q_{H}} is the amount of heat energy transferred into the system, Q C {\displaystyle Q_{C}} is the heat leaving the system, W {\displaystyle W} is the work done by

6075-684: The total signal power received was only −169  decibels with respect to a milliwatt  (dBm). The hydrogen maser is used as an atomic frequency standard . Together with other kinds of atomic clocks, these help make up the International Atomic Time standard ("Temps Atomique International" or "TAI" in French). This is the international time scale coordinated by the International Bureau of Weights and Measures . Norman Ramsey and his colleagues first conceived of

6156-466: The uncertainty regarding the density of saturated steam". Thomson referred to the correctness of Joule's formula as " Mayer 's hypothesis", on account of it having been first assumed by Mayer. Thomson arranged numerous experiments in coordination with Joule, eventually concluding by 1854 that Joule's formula was correct and the effect of temperature on the density of saturated steam accounted for all discrepancies with Regnault's data. Therefore, in terms of

6237-419: The wavelength is not quite inversely proportional to frequency. Sound propagates as mechanical vibration waves of pressure and displacement, in air or other substances. In general, frequency components of a sound determine its "color", its timbre . When speaking about the frequency (in singular) of a sound, it means the property that most determines its pitch . The frequencies an ear can hear are limited to

6318-462: The wavelength of the sound waves (distance between repetitions) is approximately inversely proportional to frequency. In Europe , Africa , Australia , southern South America , most of Asia , and Russia , the frequency of the alternating current in household electrical outlets is 50 Hz (close to the tone G), whereas in North America and northern South America, the frequency of

6399-457: Was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, officially replacing the previous name, cycle per second (cps). The SI unit for the period, as for all measurements of time, is the second . A traditional unit of frequency used with rotating mechanical devices, where it is termed rotational frequency , is revolution per minute , abbreviated r/min or rpm. 60 rpm

6480-414: Was later used for the Kelvin scale. From 1787 to 1802, it was determined by Jacques Charles (unpublished), John Dalton , and Joseph Louis Gay-Lussac that, at constant pressure, ideal gases expanded or contracted their volume linearly ( Charles's law ) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0 °C and 100 °C. Extrapolation of this law suggested that

6561-415: Was to avoid degrading the accuracy of measurements close to the triple point. The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition, but was finally adopted at the 26th CGPM in late 2018, with a value of k B  =  1.380 649 × 10  J⋅K . For scientific purposes, the redefinition's main advantage

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