Measurement of properties

Many properties of matter — mass, length, area, pressure, volume, time and so on — are quantitative, meaning they can be expressed as a number. Making a measurement always means comparing the thing being measured against some fixed, reproducible, unchanging standard quantity; any quantitative measurement is then reported as a number together with the unit in which it was measured (for example, a classroom's length might be reported as '10 m', where 10 is the number and 'm' — metre — is the unit). These arbitrarily-chosen but universally-accepted standards are what we call units. Historically, several different systems of units were used side by side: the CGS system (centimetre for length, gram for mass, second for time), the FPS system (foot, pound, second), and the MKS system (metre, kilogram, second). In 1960, the General Conference of Weights and Measures proposed a single revised metric system to unify all of this, called the International System of Units, or SI units — building on the decimal metric system that had originated in France in the late eighteenth century, valued precisely because it was based on powers of ten. The SI system defines seven independent base physical quantities (length, mass, time, electric current, thermodynamic temperature, amount of substance, and lumi …