Copyright © 2007 Dorling Kindersley
How big is an ant? How high is a mountain? An observation of a property (characteristic) as a number on a scale of units is a measurement. A child’s height may be measured on a measuring tape as 4 ft (1.2 m).
A simple measurement scale is a series of numbered graduations (marks) on a measuring instrument. Modern electronic instruments often have digital readouts that automatically display the reading.
Early people used measurements to build structures such as Stonehenge and the pyramids. Many measurements from this time were based on parts of the body—for example, a cubit was the distance from the elbow to the tip of the outstretched hand.
Units are fixed by international agreement. Scientists define a standard against which scales can be calibrated (set and checked). The standard kilogram is the mass of a platinum-iridium cylinder kept in Sèvres, France. The meter is defined as the distance traveled by light in 1/299,792,458 of a second.
Table 22. MEASUREMENT UNITS