Care and use of analytical balances

With these top pan balances it is not necessary to shield the balance pan from gentle draughts, and weighings can be accomplished very rapidly and with the usual facility of the results being recorded with a printer. 

Although with modern balances it is not necessary to make use of a box of weights in the weighing process, as indicated in Section 3.2 a set of weights is desirable for checking the accuracy of a balance. 

For scientific work the fundamental standard of mass is the international prototype kilogram, which is a mass of platinum-iridium alloy made in 1887 and deposited in the International Bureau of Weights and Measures near Paris. Authentic copies of the standard are kept by the appropriate responsible authorities in the various countries of the world these copies are employed for the comparison of secondary standards, which are used in the calibration of weights for scientific work. The unit of mass that is almost universally employed in laboratory work, however, is the gram, which may be defined as the one-thousandth part of the mass of the international prototype kilogram. 

The National Bureau of Standards at Washington recognises the following classes of precision weights  

Class M For use as reference standards, for work of the highest precision, and where a high degree of constancy over a period of time is required. 

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