Mass metrology-after 4000 years, a change is coming

The basics of mass metrology have their roots in prehistory. Evidence can be seen in a set of typical Bronze Age weights from the Indus Valley in the collection of the Museum of Fine Arts in Boston, MA (Fig. 1). The civilization flourished until about 1900 B.C.E. The weights are well-crafted stone cubes. The smallest unit is about 0.86 g. Besides being well made, the sizes of many of the weights are in a binary series. Perhaps one of the weights designated “16” was taken to be the standard. Other “16” weights could then be manufactured to be the same by comparing them on an equal-arm balance. A “32” weight could be made equal to two “16” weights. Two “8” weights would be made equal to each other, and their sum equal to a “16” weight—again calling on the services of an equal-arm balance. Exactly the same principle was still used millennia later in much of Europe in the form of nested cups (Fig. 2). When all elements are nested together, the mass in this example is one Spanish pound, or sixteen ounces. Such weight sets were still in use two centuries ago. The most famous weights of this type were manufactured in Nuremburg; and prior to 1799, the primary mass standard of France had the form of nested weights. For thousands of years, the game has been to improve balance design, improve the objects used as mass standards, and, much more recently, to understand mass metrology in a wider context of physical measurements.