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SI Base Units

Seven units that all other metric units are based on.

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The Great Metric Unit Debate

The Great Metric Unit Debate

Credit: Cassavau
Source: http://commons.wikimedia.org/wiki/File:WikiReloadableMotorCases.jpg
License: CC BY-NC 3.0

Inches or centimeters, pounds or kilograms, liters or quarts? There are two major systems of measurement used in the world today – the metric system and the British (or imperial) system. One originated with defined standards, the other grew out of a somewhat arbitrary set of dimensions. By the end of the seventeenth century, the metric system had been developed and its usage began across the world. Today, the United States is the last major hold-out in terms of complete conversion to the metric system.

Why It Matters

  • The metric system was first established in France in 1799. It was based on units of ten and allowed easy conversion from one unit to another. By 1875, enough countries had adopted this system of measurements that it became necessary to appoint an international committee promote the use of the metric system and establish standards for each type of unit.
  • The United States has not formally adopted the metric system as its primary way of measuring, but this set of measurements is in common use in many industries. Scientific work is routinely reported in metric units. Food labeling frequently includes metric measurements for mass and volume. It is common to see two-liter soft drinks advertised instead of the British equivalents. The nutritional content of foods is routinely reported in grams.
  • Credit: bradleypjohnson
    Source: http://www.flickr.com/photos/bradleypjohnson/5894445434/
    License: CC BY-NC 3.0

    Peanuts being measured in grams [Figure2]

  • Even when the metric system is used, some controversy can occur. Some years ago a major medical journal proposed that all concentration units be reported in terms of mmol/L or mM instead of the commonly used mg/dL (note that both sets of units are expressed in metric terms). Thus, a blood glucose level of 110 mg/dL would become 6.2 mM. Physicians became concerned about the possibility of confusion, so the journal compromised and agreed to publish both sets of numbers as a way to ease the transition to the newer system.
  • Watch the following video about the metric system: https://www.youtube.com/watch?v=SYn5UJ76h1Y

Show What You Know

Use the links below to learn more about the metric system debate. Then answer the following questions.

  1. What is the difference between the metric system and SI units?
  2. What is a major problem in giving medicine, especially to children?
  3. What is one major reason for the U.S. not converting to the metric system?
  4. What is one major reason the metric system is not supported in daily life?

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Image Attributions

  1. [1]^ Credit: Cassavau; Source: http://commons.wikimedia.org/wiki/File:WikiReloadableMotorCases.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: bradleypjohnson; Source: http://www.flickr.com/photos/bradleypjohnson/5894445434/; License: CC BY-NC 3.0

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