Provide decimal multiples and submultiples to make computations easier (based on man’s possessing 10 fingers). Use common prefixes across the various base and derived units, thus making the entire system more universal and more “user-friendly”. Be practical, in the sense that the base units should be close in size to customary units (again, more making adoption by nations and individuals more likely to “user-friendly”), succeed. (adapted from ) More on SI base units The SI system has only seven base units to measure length, mass, time, electric current, temperature, amount and luminous intensity. All other measurable quantities are derived from these seven base units. Because of this minimalist approach, SI is a much simpler system of measurement than any that has preceded it. The below lists the seven SI base units, the quantity they measure, their symbol, and the original (historical) and most recent (scientific) basis for the size of that unit: Quantity Unit Symbol Original Basis Newest Basis Measured (interim) Various (1793) 1/10,000,000 of measurements involving length of a quadrant of wave-lengths of visible (North Pole to emission of light from various Length meter m Equator); elements (first, 1960, Kr-86); Later (1960), length of Now (1983), distance light platinum-iridium alloy travels in a vacuum in prototype meter 1 / 299,792,458 of a second (1793) Unit of “grave” (1901) Mass of (now obsolete), mass Mass kilogram kg platinum-iridium alloy of 1 decimeter3 of prototype kilogram water (ancient) 1/86,400 (1997) Duration of of one day; 9,192,631,770 periods of Later (1956) radiation corresponding to Time second s 1/31,556,925.9747 of transition between two tropical year for hyperfine levels of ground January 1900 state of the Cs-133 atom
Provide decimal multiples and submultiples to make computations easier (based on man’s possessing 10 fingers). Use common prefixes across the various base and derived units, thus making the entire system more universal and more “user-friendly”. Be practical, in the sense that the base units should be close in size to customary units (again, more “user-friendly”), making adoption by nations and individuals more likely to succeed. (adapted from ) More on SI base units The SI system has only seven base units to measure length, mass, time, electric current, temperature, amount and luminous intensity. All other measurable quantities are derived from these seven base units. Because of this minimalist approach, SI is a much simpler system of measurement than any that has preceded it. The below lists the seven SI base units, the quantity they measure, their symbol, and the original (historical) and most recent (scientific) basis for the size of that unit: Quantity Measured Unit Symbol Original Basis Newest Basis Length meter m (1793) 1/10,000,000 of length of a quadrant of (North Pole to Equator); Later (1960), length of platinum-iridium alloy prototype meter (interim) Various measurements involving wave-lengths of visible emission of light from various elements (first, 1960, Kr-86); Now (1983), distance light travels in a vacuum in 1 / 299,792,458 of a second Mass kilogram kg (1793) Unit of “grave” (now obsolete), mass of 1 decimeter3 of water (1901) Mass of platinum-iridium alloy prototype kilogram Time second s (ancient) 1/86,400 of one day; Later (1956) 1/31,556,925.9747 of tropical year for January 1900 (1997) Duration of 9,192,631,770 periods of radiation corresponding to transition between two hyperfine levels of ground state of the Cs-133 atom
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