Hot answers tagged

19

There is a historical reason. But it was not a fluke of history, the underlying reason is that energy comes up in non-mechanical (thermal, electric) contexts whereas momentum does not. Derived alternative, newton-meter in SI, did not arise naturally in such contexts, and alternative units, like calories, were used prior to the discovery of the general energy ...


16

This Wikipedia page, says: In 1861, Charles Bright and Latimer Clark proposed the names of ohm, volt, and farad in honour of Georg Ohm, Alessandro Volta and Michael Faraday respectively for the practical units based on the centimetre-gramme-second absolute system. This was supported by Thomson (Lord Kelvin). These names were later scaled for use in the ...


9

August Horstmann first introduced the concept of gram-molecular weight in the sense of today’s mole concept in 1881. In 1865 Loschmidt first estimated the number of molecules in a cubic centimetre of a gas under normal conditions as 1.83 × 10$^{18}$, and in 1889 Than first determined the gram-molecular volume of gases under normal conditions as 22,330 cm$^3$....


7

In 1887 a committee of the British Association was appointed for the purpose of "considering the desirability of introducing uniform nomenclature for the fundamental units of mechanics of co-operating with other bodies engaged in similar work.” The committee issued a series of questions to members, and collected their replies. The result was that in 1888, ...


7

The kilogram is the base unit of mass because electrical engineers in the late 19th century chose a particular set of practical electrical units. Their practical units were a success, and we are still using them today: ohm, volt, and ampere. In 1881 the International Electrotechnical Commission (IEC) created two sets of units: a set of theoretical units, and ...


7

Why is kg the standard unit for mass and not g? Tongue in cheek answer: Because a foolish consistency is the hobgoblin of little minds? More seriously, none of the immediate predecessors of the SI bothered to have all of their base units be consistent with the prefix-free units. Gauss proposed a millimeter-gram-second system in the 1830s. Maxwell and ...


7

Resolution 12 of the 11th Conférence Générale des Poids et Mesures (CGPM) adopted 12 SI prefixes in 1960, including pico- and nano-. Google Ngrams show steep decline in the use of millimicro- after 1964, and micromicro- after 1966. Some other double prefixes, like kilomega- and hectokilo- were also in use, decimilli- was even standardized in Frace until 1961....


6

What is the importance of SI units in Physics? If by "physics" you mean units used by professional physicists and related disciplines, it's not as much as one would think. While motivated by science, SI units exist primarily to benefit commerce, not science. The ordinary, everyday phenomena described well in terms of SI units aren't the phenomena that ...


6

The unit "Siemens" was only introduced in 1971 whereas the company was founded the mid-19th century. Both get their name from the same person, Werner von Siemens.


5

The UK copies are still stored at the National Physical Laboratory. See these links: Metre Kilogram


5

Alright after doing some research I figured out the answers to all my questions. 1)Carnot's definition of energy is as follows: it's the energy needed to lift a cubic meter of water one meters high. By definition a cubic meter of water is 1000 kg, and following from the definition of work according to Carnot is: $W=Fd=mgh$ , where $g=9.8 ms^{-2}$ and $h=1 ...


5

It is a pure coincidence. And the agreement is not so good. Meter was introduced in connection with decimal system. They wanted all units to be based on decimal system, including angular and time units. So it was decided to have 100 decimal degrees in the right angle, and 20 hours in a day. Each hour was divided into 100 decimal minutes, a decimal (time) ...


4

The Système International d'Unités (SI) is not just a collection of units but rather a system of units*. The units all work together, so if you use some quantities in SI units, other quantities that you calculate from them will also be in SI units. You give the examples "distance, time, speed etc." These units are related: if you give distance in meters (m) ...


4

Right after your quote Wikipedia has "In 1267, the medieval scientist Roger Bacon, writing in Latin, defined the division of time between full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates". This use was retained in astronomy but not colloquially, ...


4

The decision that k is lowercase was made in the early days of the cgs system, long before the SI system. The available prefixes were milli up to myria. In 1879 the International Committee for Weights and Measures, CIPM, adopted the proposal to abbreviate only in lowercase, because a mixture with capitals would be sensitive to mistakes and reduce the ease ...


4

Issues regarding ancient measure systems are not easy to manage... Galileo Galilei, in Discorsi, uses braccia; the "old" translation by Crew & Di Salvio uses: "cubits". A braccio in Florence was: 583 mm. In ancient Rome: according to Vitruvius, a cubit was equal to $1 \frac 1 2$ Roman feet or 6 palm widths, which is 443.8 mm (17.47 in). According ...


4

Thomas Harriot, 1560-1621 matched coefficients when generating formulas for sums of powers of positve integers. "Gathering like terms", is, of course, an obvious technique, and was also used in the geometric derivations of these formulas. The link provided goes to the middle of a long article on the history of these formulas; Harriot appears to be the first ...


4

It goes back to the ancient Romans. The Romans, when working with fractions, often divided things into twelfths rather than tenths (since twelve divides nicely into thirds as well as quarters, whereas ten only nicely divides into tenths and fifths which aren't as useful as thirds and quarters). So the Roman foot was divided into twelve smaller units called "...


4

We often forget that even the minute is not an SI unit, only the second and its decimal multiples and fractions are. It is a leftover of the sexagesimal system (base 60), whose use predates the decimals by many centuries, and goes back to the ancient Babylon. So are the angular degrees. Heinrich Hertz was born in 1857, and the International Electrotechnical ...


3

The OED's "electronvolt" entry gives this as the earliest usage in English: 1925 Proc. Royal Soc. A. 109 559 Now one electron-volt = 1·59 .10−12 ergs, and 4·18 .10−17 ergs = 1 gram-calorie. But it defines what its measure is, not what it is.


3

I believe the question is asking about the following tables, one for the period 1800 AD to 2050 AD, the other two for 3000 BC to 3000 AD. Note that in these tables, "EM Bary" refers to the Earth-Moon barycenter, the center of mass of the Earth-Moon system. Table 1. Keplerian elements and their rates, with respect to the mean ecliptic and equinox of J2000, ...


3

SI units are important because: They are common to the people of the entire world, so that people from different countries can communicate with each other conveniently regarding business and science. It makes systematic use of prefixes, making it easy to express very large or very small numbers. It makes calculations very fast.


3

Much of the personal naming occured in 1860-70s for CGS and 1870-1880s for SI, see History of the Metric System. Most of the units named referred to deceased people: newtons, volts, ohms, amperes, pascals, coulombs, watts, etc. Some passed away a while ago, like Pascal and Newton, some relatively recently, like Ampere and Ohm. Farads, joules and webers ...


3

In this context, the letter e stands for the quantum of charge, not for the particle called the electron. That's why eV is notated as a multiplication, and that's why it has a positive value. Stoney introduced the term "electron" in 1891 to refer to hypothetical positive and negative charges that were permanently attached to atoms. The absolute value of the ...


3

Turns are used in mathematics when convenient. For example, some authors write Fourier transform as $$\int e^{-2\pi i xz}f(x)dx,$$ while other authors as $$\int e^{-ixz}f(x)dz.$$ In the first formula $x$ is in "turns" (1 turn=$2\pi$ radians). This is a question of convenience. Using turns, the Taylor expansion look more complicated: $$e^{2\pi x}=\sum_{n=1}^\...


3

The unabridged Oxford English dictionary clarifies the "when" part of the question. Also consult the paper in Nature (1949) https://www.nature.com/articles/163427a0 for the historic discussion (paper is open access). However, the modern reference point is triple point of water, "The Celsius scale is defined using the T.Pt. = 0.01 °C with 1 °C made identical ...


2

HERE is a listing of the SI derived units. As far as I know, none of these were someone naming it for himself. If your name is Henry Gray then you are a double unit.


2

Most of these derive their names from colloquial usage, and their origins can be found in etymology dictionaries, which also usually give pointers on the first scientific usage. For example, temperature derives from Latin "temperatus", due proportion, and was in usage since 1530s, it was first used to denote the degree of heat by Boyle in 1670s. Mass comes ...


2

SI units started to be used. In 1960 (not 1970!), the Système international d'unités was established at the General Conference on Weights and Measures. The hertz was officially recognized as a unit derived from metric system units (namely, the second). It was already in partial use (the International Electrotechnical Commission began using it in 1930). With ...


2

Sometimes when referencing historical documents, it's useful to know how the units of the time relate to units in the modern day, but I'm finding it hard to locate that information by web searches. You can't find that information because it doesn't exist; the values haven't changed. The various redefinitions of the meter did not change what a meter ...


Only top voted, non community-wiki answers of a minimum length are eligible