Why We Have Different Ways to Measure Weight
Early Babylonian and Egyptian records, as well as biblical texts, indicate that weight was originally measured by the capacities of containers such as gourds or clay vessels. These were filled with plant seeds that were then counted to measure the volumes. With the development of scales as a means of weighing, seeds and stones served as standard counterweights. For instance, the "carat," still used today as a mass unit for gems, is derived from the carob seed.
The Babylonians invented the talent as their basic unit of weight, which was based on their sexagesimal (60-based) numerical system. It was equal to the amount of water that filled an amphora (a kind of vase).
The Roman Origin of the Ounce
The Greeks used the same weight measurement as the Babylonians, but the Romans changed it. Their basic unit of weight was the uncia, from which the English word "ounce" is originally derived. The uncia is a twelfth part of the pes, which is the Roman "foot" (our word inch is also derived from uncia).
For the Arabs, a silver Dirhem was determined by the weight of 45 full-grown barley grains. Ten Dirhems made a Wukryeh of 450 grains, which translates into the Latin "uncia" or twelfth.
History of the Pound
Translated into English as the ounce, King Offa (who lived at the end of the 8th century) initially accepted the silver ounce but then ran short of silver. The Dirhem was halved to 22½ grains for the penny, twenty of which made the ounce as before, and twelve ounces made the pound in silver.
This measure has come down to us as part of the British Imperial System of Weights and Measures, which is also used in the U.S. This system of units was first defined in the British Weights and Measures Act of 1824. In 1855, a fire destroyed the Houses of Parliament in London, where the standard physical prototypes for these weights were kept, forcing a new standards bureau to be established.
Metric Weights
Metric weights have a very different history. The gram and the kilogram were developed by scientists in 18th century France. In 1791, the French parliament imposed the use of the metric system on the country, and it is now used in Continental Europe and almost every other part of the world.
The metric system was not initially popular in revolutionary France, and Emperor Napoleon abolished it. However, when Napoleon lost power, the system was reinstated.
How the Metric System Works
The kilogram was originally defined as the weight of exactly one liter of water. One thousandth of a kilogram is a gram. All multiples and submultiples of the base units are in powers of ten. Fractional units are not halves or quarters (like inches), but tenths. Basing all units on multiples of ten makes conversion from one unit to another particularly easy.
Here are some typical metric conversions to help visualize the scale:
- 1 milliliter is the exact same volume as 1 cubic centimeter.
- The mass of 1 milliliter of water is approximately 1 gram.
- The mass of 1 liter of water is therefore approximately 1 kilogram.
- There are 1,000 liters in a cubic meter, so the mass of 1 cubic meter of water is approximately 1,000 kilograms (or 1 metric ton).
- A US nickel weighs exactly 5 grams, and a penny weighs 2.5 grams.