Two-pan comparison balance

Illustration of three common balances that are useful for the measurement of mass, (a) A two-pan comparison balance for approximate mass measurement suitable for routine work requiring accuracy to 0.1 g (or perhaps 0.01 g). 

The ability to make reliable measurements of mass has always been a fundamental aspect of experimental science. Simple balances are known to have been used as early as 5000 BC. Indeed the word balance is derived from the Latin word bilanx which means two pans , as illustrated in the picture of a crude two-pan balance below. Such a balance is a comparison instrument that compares standard or known weights (really standard masses) placed on the left-hand pan to unknown samples or objects placed on the right-hand pan when the standard and unknown masses are equal their corresponding equal weights (i.e., vertical gravitational forces) restore the indicator needle attached to the rod carrying the pans to the central position on the attached scale. 

The mass of an object remains the same at the North Pole as at the equator, and it can easily be determined, at any place on the earth s surface, by comparison with a standard set of masses. For small objects a chemical balance is used. Since the weights of two bodies of equal mass are the same at any place on the earth s surface, these bodies will balance one another when placed on the two pans of a balance with arms of equal length. 

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