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ARTICLES

THE SI SYSTEM

Any physical quantity is the product of a numerical value and a unit. Since 1964, the International System of Units (SI system) has gradually been adopted worldwide, with the exception of Liberia, Myanmar and the United States.

Basic information can be found in standard ISO 31, which is under revision and will be superseded by ISO/IEC 80000: Quantities and Units. Units are divided into four different classes :

  • Base units
  • Supplementary units
  • Derived units
  • Additional units
Base units, supplementary units and derived units are called SI units. The additional units are not SI units, although they are accepted for use with SI units. Base units are any of the established, independent units in which all other units can be expressed.
# length meter m
1 Mass Kilogram Kilogram kg
2 Time second s
3 Current ampere A
4 Temperature Kelvin k

Derived units are formed as a power or product of powers of one or more base units and/or supplementary units according to the physical laws for the relationship between these different units. Additional units: A limited number of units outside the SI system cannot be eliminated for different reasons, and continue to be used along with the SI as additional units.

# Quantity Unit Symbol Expressed in Other SI Units
1 frequency hertz Hz s^-1
2 force newton N kgx mxs^-2
3 Power Watt W J/s
4 Capacitance farad f C/V
length Units Abbreviation Number of Metres approximate U.S. equivalent
1 kilometre km 1,000 0.62 mile
2 centimetre cm 0.01 0.39 inch
3 millimetre mm 0.001 0.039 inch
4 micrometre μm 0.000001 0.000039 inch
Area Units Abbreviation Number of square metres Approximate U.S. equivalent
1 square kilometre sq km, or km² 1,000,000 0.3861 square mile
2 hectare ha 10,000 2.47 acres
3 square centimetre sq cm, or cm² 0.0001 0.155 square inch
4 are a 100 119.60 square yards
Volume Units Abbreviation Number of cubic metres approximate U.S. equivalent
1 cubic metre m3 1 1.307 cubic yards
2 cubic centimetre cu cm, cm3, or cc 0.000001 0.061 cubic inch

Common equivalents and conversion factors for U.S. Customary and SI systems

approximate Common equivalent
1 inch 1.307 cubic yards
1 foot = 0.3 metre
1 yard 0.9 metre
1 mile 1.6 kilometres
1 square foot 0.09 square metre
1 square inch 6.5 square centimetres
1square yard 0.8 square metr
conversions accurate within 10 parts per million
inches × 25.4** = millimetres
feet × 0.3048** = metres
yards × 0.9144** = metre
miles × 1.60934 = kilometres
square inches × 6.4516** = square centimetres
square feet × 0.0929030 = square metres
square yards × 0.836127 = square metres
acres × 0.404686 = hectares
cubic inches × 16.3871 = cubic centimetres
cubic feet × 0.0283168 = cubic metres
cubic feet × 0.0283168 = cubic metres
cubic yards × 0.764555 = cubic metres
quarts (liq) × 0.946353 = litres
gallons × 0.00378541 = cubic metres
ounces (avdp) × 28.3495 = grams

THE METRIC SYSTEM

The metric system is a system of measuring things. It is used worldwide in calculations and research. Here are some examples of how we measure things using the metric system:

  • Meter is the unit of measuring distances and lengths. We use this unit in our daily life, for example, the distance between school and home, length of cloth, etc.
  • You may have noticed that there is a weight mentioned in the bag of your favourite snack. For example, 250 grams of potato chips. Here, gram is the unit of weight.
  • Similarly, on the bottle of your favourite beverage, there’s volume mentioned (for example, 250 milliliters of cola). Here, milliliter is the unit of volume.

The metric system encompasses several variations, each designed for different contexts and applications. Here, we’ll delve into the primary metric systems: the Mètre des Archives, CGS (Centimeter-Gram-Second), MKS (Meter-Kilogram-Second), and the SI (International System of Units), highlighting their differences and unique features.

The term "metric system" generally refers to a system of measurement that is decimal-based, with the International System of Units (SI) being the most widely used and recognized metric system globally. However, there have been other metric systems historically, and variations still exist. Here’s an overview of the key differences:

  1. International System of Units (SI)
    • Base Units: Meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), candela (luminous intensity).
    • Characteristics: Standardized and globally accepted. It is based on seven base units from which other units are derived (e.g., newton for force, joule for energy).
    • Usage: Predominantly used worldwide in science, industry, and everyday life.
  2. Centimetre-Gram-Second (CGS) System
    • Base Units: Centimeter (length), gram (mass), second (time).
    • Characteristics: An older metric system, primarily used in certain scientific fields such as electromagnetism.
    • Usage: Less common today, but still used in some areas of physics.
  3. Metre-Tonne-Second (MTS) System
    • Base Units: Meter (length), tonne (mass), second (time).
    • Characteristics: Similar to SI but uses tonne (1000 kg) instead of kilogram as the base unit of mass.
    • Usage: Rarely used, mostly replaced by SI.
  4. Kilogram-Force Second (MKfS) System
    • Base Units: Meter (length), kilogram-force (force), second (time).
    • Characteristics: Uses kilogram-force (kgf) instead of kilogram for mass.
    • Usage: Primarily used in engineering contexts where force is a significant measurement.
  5. Key Differences
    • Base Units: The most noticeable difference lies in the base units used for length, mass, and sometimes force.
    • Field of Application: Different metric systems have been preferred in various scientific, engineering, and everyday contexts.
    • Standardization: SI has become the standardized system internationally, while others are now considered more specialized or historical.

THE METER

Historical Development

The Origins of the Meter

The concept of the meter was first proposed in the late 18th century during the French Revolution. The aim was to establish a uniform and natural unit of length that could be universally adopted, replacing the diverse units of measurement used across France and Europe.

  1. 1960 Redefinition: The meter was redefined in terms of the wavelength of light. Specifically, it was defined as 1,650,763.73 wavelengths of the orange-red emission line of krypton-86 in a vacuum. This provided a more accurate and reproducible standard.
  2. 1983 Redefinition: The current definition of the meter was adopted by the 17th General Conference on Weights and Measures (CGPM). The meter is now defined as the distance that light travels in a vacuum in 1/299,792,458 of a second. This definition leverages the constant speed of light, which is a fundamental constant of nature, ensuring a highly precise and universally accessible standard.

Current Usage and Importance

The Role of the Meter in the International System of Units (SI)

The meter is the fundamental unit of length in the International System of Units (SI), the modern form of the metric system. It is used worldwide in science, industry, and everyday life, providing a consistent and reliable means of measuring distance and length.

  1. Science and Technology: In scientific research, the precise definition of the meter is crucial for experiments and measurements at microscopic and astronomical scales. The meter is used to calibrate instruments, conduct research, and develop new technologies.
  2. Industry and Commerce: Industries rely on the meter for manufacturing processes, quality control, and trade. Standardization ensures that products and components fit and function correctly, facilitating international trade and cooperation.
  3. Everyday Life: The meter is used in everyday activities such as construction, navigation, and education. It provides a common reference that simplifies communication and understanding across different fields and regions.

Evolution of the Meter

From Physical Artifacts to Fundamental Constants

The evolution of the meter reflects the broader trend in metrology (the science of measurement) toward definitions based on fundamental constants rather than physical artifacts. This shift ensures greater stability, precision, and universality.

  1. Artifacts to Constants: The transition from the platinum-iridium bar to the speed of light represents a move from a tangible but potentially variable standard to an immutable natural constant. This change has made the definition of the meter independent of any specific physical object.
  2. Future Developments: As science and technology continue to advance, further refinements to the definition of the meter may occur. However, any changes will likely build on the current foundation of using fundamental constants, ensuring continuity and precision.

Traditional US Units of Measurement

The United States is one of the few countries that still uses a system of measurement derived from older British units. These traditional units, commonly referred to as the U.S. customary units, are used in various aspects of daily life, from cooking and construction to road signs and sports. Here’s a detailed look at some of the key traditional units of measurement used throughout the United States.

Length

  1. Inch (in)
    • Definition: 1 inch = 1/12 of a foot.
    • Usage: Used for measuring small dimensions such as the width of a book or the size of a nail.
  2. Foot (ft)
    • Definition: 1 foot = 12 inches.
    • Usage: Commonly used in construction, height measurement, and for determining short distances.
  3. Yard (yd)
    • Definition: 1 yard = 3 feet = 36 inches.
    • Usage: Often used in sports (e.g., football fields) and for measuring fabric.
  4. Mile (mi)
    • Definition: 1 mile = 5280 feet = 1760 yards.
    • Usage: Used for measuring long distances such as road lengths and running tracks.

Area

  1. Square Inch (sq in)
    • Definition: Area of a square with sides of 1 inch.
    • Usage: Used for small surfaces like smartphone screens or pieces of paper.
  2. Square Foot (sq ft)
    • Definition: Area of a square with sides of 1 foot.
    • Usage: Commonly used in real estate for measuring the area of rooms and buildings.
  3. Acre
    • Definition: 1 acre = 43,560 square feet.
    • Usage: Primarily used in agriculture and real estate to measure large plots of land.
  4. Square Mile (sq mi)
    • Definition: Area of a square with sides of 1 mile.
    • Usage: Used for measuring large areas such as cities, states, and entire regions.

Volume

  1. Teaspoon (tsp)
    • Definition: A small unit used in cooking, approximately 1/3 of a tablespoon.
    • Usage: Commonly used in recipes for measuring small quantities of ingredients.
  2. Tablespoon (tbsp)
    • Definition: 1 tablespoon = 3 teaspoons.
    • Usage: Used in cooking for slightly larger quantities of ingredients.
  3. Fluid Ounce (fl oz)
    • Definition: 1 fluid ounce = 2 tablespoons.
    • Usage: Used for measuring liquids, often in cooking and beverages.
  4. Square Mile (sq mi)
    • Definition: 1 cup = 8 fluid ounces.
    • Usage: Commonly used in cooking and baking for both dry and liquid ingredients.
  5. Pint (pt)
    • Definition: 1 pint = 2 cups = 16 fluid ounces.
    • Usage: Used for larger quantities of liquids, such as milk or beer.
  6. Quart (qt)
    • Definition: 1 quart = 2 pints = 32 fluid ounces.
    • Usage: Used for even larger liquid quantities, such as containers of milk or juice.
  7. Gallon (gal)
    • Definition: 1 gallon = 4 quarts = 128 fluid ounces.
    • Usage: Used for very large quantities of liquids, like gasoline or large containers of beverages.

Weight

  1. Ounce (oz)
    • Definition: A small unit of weight, often used for postal items or small food items.
    • Usage: Commonly used in cooking, mailing, and for measuring small objects.
  2. Pound (lb)
    • Definition: 1 pound = 16 ounces.
    • Usage: Used for measuring body weight, produce, and packaged goods.
  3. Ton (short ton)
    • Definition: 1 ton = 2000 pounds.
    • Usage: Used for very large weights, such as vehicles or large shipments.

Temperature

  1. Fahrenheit (°F)
    • Definition: A scale where 32°F is the freezing point of water and 212°F is the boiling point.
    • Usage: Used for weather forecasts, cooking temperatures, and body temperature.