he ability to make precise measurements of mass is a fundamental aspect of scientific
inquiry. Analytical balances were exquisitely designed to
perform such a function. They served as basic tools for the determination of chemical
composition, an essential process in biomedical research.
Simple balances were used
as early as 5000 B.C. So long is their history that the very word
"balance" comes from the Latin word bilanx which
means "two pans." The modern analytical balance originated
during the mid-18th century, when the Scottish chemist
Joseph Black developed the technique of using a lightweight, rigid
beam supported on a knife-edged fulcrum. The accuracy achieved by
this innovation far surpassed that of any other weighing device.
The operating principle of these instruments was similar to that of a seesaw. They
worked by placing a sample in one pan and then adding known weight to the other pan until
an equilibrium was established. To avoid the need to handle extremely small reference
weights, balances were equipped with a rider mass that could be moved along the beam to
effect small gravitational torque changes. Since the operation of such balances was
affected by air currents and humidity, the working parts were enclosed in a glass case.
Although there have been hundreds of variations on the equal arm balance, the fundamental
design, reflected in these examples, served as the mainstay in laboratories for 200 years,
from 1750 to 1950. Starting in 1948, the design of balances moved in the direction of
single pan devices.
In their most advanced form, balances of this type achieved accuracies of less than
0.001 milligram; that is, they could detect differences of less than 1 part in 10,000,000.
Some of these technical refinements included: pan brakes, magnetic damping of beam
oscillation, built-in weight sets operated by dial knobs, and microscopic or
microprojection reading of the angle of beam inclination.
For further reading:
- McGraw Hill, Encyclopedia of Science and Technology.
McGraw Hill, Inc., New York, 1987.
- Wilson, C.L. and Wilson, D.W., Comprehensive Analytical
Chemistry. Elsevier Publishing Company, New York, 1959, pp. 107-123.
- Smith, H.M., Torchbearers of Chemistry. Academic Press
Inc., New York, 1949.
Currently on exhibit:
The Equal Arm Analytical
Balances exhibit is currently on exhibit in the Claude Pepper Building
(Building 31) connector, between A and B Wings.
The NIH Stetten Museum
collects and exhibits biomedical research instruments and other artifacts
relating the history of the National Institutes of Health (NIH)..
