* American National Standard for Metric Practice. The American National Standard* also calls for the spellings “megohm” and “kilohm”, but the spelling “megaohm” is often encountered. In combining the prefix with the unit name, the prefixes' final vowel is retained, except in the case of “hectare”. We offer, just for fun, a silly quiz mostly based on the SI prefixes. These hoaxes are being tracked by Gérard Michon. See prefixes for binary multiples.Ī number of bogus prefixes have been described, mostly on the Internet. To avoidĬonfusion, a separate set of prefixes has been defined for them. Powers of 2 not 10, for example kilobytes and megabytes. The metric prefixes have been used for units whose magnitudes are based on Move the decimal point _ places to the _.įor your convenience, this little utility is also provided as a small separate Since the prefixes are all decimal multiples or submultiples, it is easy to convert a measurement in a unit with one prefix to one with another: just move the decimal point (or add and subtract powers of ten). * The spelling “deca-” is often used (but not in the United States). Examples include the UnitĪbsorbed dose, specific energy imparted, kerma, absorbed dose indexĮlectric potential, potential difference, electromotive force SI has a class of such units, ones completely defined in terms of base units, but with their own names. Some such units are used so often that it is worthwhile to give them their own names. UnitĬar drivers often use the term “miles per hour.” “Miles per hour” is, in fact, a unit, but one defined simply by a mathematical relationship between the units mile and hour. For details, click on the names of the individual units. The “Year adopted” column in the table shows those years in which the CGPM approved a new definition of the unit. At present, the second is the unit that can be measured to the largest number of significant decimal places. In 2019 the last unit still defined by a physical object, the kilogram, was redefined in terms of the meter and the second. More precise definitions make possible more precise measurements that sometimes reveal previously undiscovered phenomena. The improved definitions are one reason it has been necessary to add prefixes like zepto- and yotta. Trying to use a meter so defined to measure objects very much smaller than the scratch itself is lunacy. Even with a powerful microscope, it is very, very difficult, if not impossible, to determine where within the width of the scratch the meter ends. (Here, by “more precise” we mean the measurement can have a greater number of meaningful decimal places after the decimal point.) Imagine determining the length of the meter from scratches on a bar. A second advantage of the new definitions is that they make possible more precise measurements. From the information in the definition, anyone with the right tools can re-create the unit. Over the years improvements in technology have made it possible to redefine SI’s base units in terms of physical constants like the speed of light. Depending on a prototype can be problematical even if it isn’t lost or destroyed in the 1990's it was feared that, for some unknown reason, the closely-guarded prototype of the kilogram was losing mass. Making certain that the newly-made replacements were a close match to the vanished originals took years. In the 19ᵗʰ century the British lost their prototypes in a fire in the House of Commons. If something happens to a prototype, its system is in trouble. The meter, for example, was the distance between two scratches on a certain bar. In the original metric system the key base units, the meter and kilogram, were defined by actual physical objects, called prototypes.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |