alloy; this has served as the standard ever since and is stored in a Paris vault. The kilogram became the base unit in 1901.
Also in 1901, a kilogram of distilled pure water at its densest (+3.98° ) under a standard atmosphere of pressure was used to define the liter, a more convenient unit than the very large cubic meter. Because this liter turned out to be different from the cubic decimeter by about 28 millionths, this definition was abandoned in 1964 in favor of the cubic decimeter.
The kilogram is the only base unit not to have been redefined in terms of an unchanging natural phenomenon. Such a definition, said to be in terms of an artefact (the cylinder in Paris), is particularly inconvenient, because, in principle, it can be used only by traveling to Paris and, with permission, comparing one's own candidate standard to the reference one. For this reason, as well as the effort required to protect the standard from absorption or dispersion of gases and vapors, at a meeting of the Royal Society in on February 15, 2005, scientists called for the mass of the standard kilogramme in Paris to be replaced by a standard based on "an invariable property of nature"; but no decision on redefinition can be taken before 2007.
Temperature
The metric unit of temperature originally was the centigrade or Celsius scale. This was determined by divided into 100 equal-length parts the difference between a water-ice mixture at 0 °C and the boiling point of pure distilled water at 100 °C (under a standard atmosphere). This is still the metric unit of temperature in everyday use. With the discovery of , a new temperature scale, the was developed, which relocates the zero point to absolute zero. The freezing point of water, 0 °C, becomes 273.15 K. It is the Kelvin scale that is used as the base SI unit.
Time
The metric unit of time is the second. It was originally defined as 1/86,400th of a mean solar day. The formal definition of the second has been changed several times as more accurate definitions became possible, based first on astronomic observations, then the tuning fork clock, quartz clock, and today the atomic clock
SI prefixes
The SI system of units is a metric system. That is, the units are expressed in powers of 10 (i.e. 1×10 3 ). In order to simplify writing the powers of ten, they are expressed as prefixes and the symbol for the prefix put before the unit. Thus, 7.4×10 3 m is written as 7.4 km.
The following SI prefixes can be used to prefix any of the units to produce a multiple or submultiple of the original unit. This includes the degree Celsius (e.g. "1.2 m°C"); however, to avoid confusion, prefixes are not used with the time-related unit symbols min (minute), h (hour), d (day). They are not recommended for use with the angle-related symbols ° (degree), ' (minute of arc), and " (second of arc), but for astronomical usage, they are sometimes used with seconds of arc.
Answer the questions.
1. What units were used in Britain?
2. What is a system of measurement used in most of the world?
3. When were Imperial units defined in the United Kingdom?
4. Is the most important unit length or mass?
5. The metric unit of time is the first, isn’t it?
Put the verb into active or passive form.
1. Before _____widely around the world, the British systems of English units and later Imperial units were used in Britain, the Commonwealth and the United States. (adopt) 2. These various systems of measurement ___at times ____ foot-pound-second systems after the Imperial units for distance, weight and time. (call) 3. Imperial units ___in many other places, for example, in many Commonwealth countries that ____ metricated, land area is measured in acres and floor space in square feet. (use)/ (consider) 4. Imperial units ____ in countries that were part of the (use).5. The ___ officially in after the . (adopt)
Put in a/an or the where necessary.
1. There are 20 imperial fluid ounces in ___imperial pint, but 16 US fluid ounces in ___US pint. 2. Additionally, ___US fluid ounce is slightly bigger than ___ imperial fluid ounce. 3. ___emergence of __internationally recognized system of units at __ time of increasing international cooperation and trade is highly significant. 4. During __history of __metric system, __number of variations have evolved and their use spread around __ world replacing many traditional measurement systems.5. ___International Committee for Weights and Measures (CIPM) to conduct ___international study of the measurement needs of ___ scientific, technical, and educational communities.
Put in the missing preposition.
1. The is a system ___ measurement used in most ____ the .
2. These are similar ___the British imperial units and are also based ___ the units used ___ the United Kingdom __before . But some ___ the units are different___the British ones. 3. The most important unit is that___ length: one was originally defined to be equal ___1/10,000,000th ___ the distance ___the pole ___ the ___ the meridian ___ .4. The name derives __ the French phrase, Système International d'Unités, or __ English International System __ Units. 5. The system consists of a set __seven base units together ___ a set ___ prefixes ___ which all other units are derived.
Put the verb into the correct form.
1. The original base unit of mass in the metric system ___ the gram. (to be) 2. In 1889, the first General Conference on Weights and Measures (CGPM) ____a replacement prototype, a cylinder of a 90 percent platinum, 10 percent alloy. (sanction)3. This ___as the standard ever since and is stored in a Paris vault. (serve) 4. The kilogram ____the base unit in 1901.( become) 5. The formal definition of the second ____several times as more accurate definitions ___possible, based first on astronomic observations, then the tuning fork clock, quartz clock, and today the atomic clock. (change/ become)
Complete the sentence. Use the comparative or superlative form.
1. The result is that US pints and gallons are (small) imperial pints and gallons. 2. They are (common) system for everyday in the world.3. The US fluid ounce is slightly (big) the imperial fluid ounce. 4. The result is that US pints and gallons are (small) imperial pints and gallons. 5. Prior discussions had often suggested the length of a seconds pendulum in some standard gravity, which would have been only slightly (short), and perhaps (easy) to determine.) This is approximately 10 percent (long) one yard.
Make questions to the underlined words
1.
