34 MEASUREMENTS 



provide him with information from which he can draw generahza- 

 tions. 



Examples of measurement 



The units used in measurement may be the standard units (meters, 

 kilograms, or seconds) or fractions or muhiples thereof. Generally the 

 fractions or multiples are chosen so that the numbers are of a convenient 

 size. For example, 8 cm is a slightly more convenient expression for the 

 length of my finger than 0.08 meters. Several of the units which have 

 been given names of their own are listed in Table 4-1. 



Table 4-1. Fractions and Multiples of Units 



There are some exceptions in the use of the prefixes, but generally 

 they apply to the fundamental units of length, mass, and time and to 

 derived and electrical units, as in microwatt and kilocalorie. 



Length: Measurements of length are comparisons with standards, 

 which are copies derived from the standard meter. Some measure- 

 ments of length can be made more accurately now than previously. Until 

 1960, the standard meter was the distance between two lines scribed on 

 a bar of platinum-iridium kept in the vault of the International Bureau 

 of Weights and Measures in Paris. The standard meter is now defined 

 as 1,650,763.73 times the wavelength of a specified orange-red line in 

 the light emitted by krypton^. The new standard is a value believed to 

 be constant, and can be reproduced in any well-equipped laboratory. 

 By mutual agreement in 1959 among the countries using the English 

 system, the international yard is defined as exactly 0.9144 meter. The 

 inch (Vse yd) is thus exacdy 25.4 mm. 



A number of fractions or multiples of meters are commonly used in 

 measurement. These units are chosen to permit the use of small whole 



