A "balance" was originally an equal-arm balance, with a pan hanging from the end of each arm. The item to be weighed was placed on one pan, and "balanced" by adding known weights to the other pan, or, for very small weights, along one arm. Such a balance was typically inside a wooden case with glass windows to avoid air currents, and was used in a chemical laboratory to weigh items of up to about 100 grams to a precision of about 0.0001 g. It was often called an "analytical balance". A long pointer at the center of the beam was used to determine "balance", usually defined as the condition in which the pointer would swing equally far to each side of center. The center of the beam had a "knife-edge", such as an agate prism sitting on a hard plate. Careful handling was required to avoid chipping the knife-edge.

The "torsion balance" had the beam supported on a metal ribbon which twisted somewhat when the system was out of balance. It was not usually enclosed, and was not as sensitive.

Other weighing instruments, not "equal arm" were often also called balances. The typical example was the triple-beam balance, which had a pan on which the item being weighed was placed, and three calibrated 'beams' along which weights were slid to bring the system to balance. Such a balance might have one beam calibrated at 100 gram intervals up to 1000 grams, another beam at 10-gram intervals up to 100 g, and the third beam calibrated in grams and tenths of grams.

More recently, the "equal arm" principle has been more or less abandoned even for analytical balances, with known weights added mechanically inside the balance case. They were still called "balances", but with only one pan used, these became known as "single-pan" balances. The 'balance' condition was indicated by a scale projected on a ground-glass window on the front. "Analytical" types usually have a glass case with sliding windows for adding materials. "Top-loaders" are used for higher weights up to a few kilograms.

Still more recently, the concept of 'balance' has changed from a mechanical balance to electrically balancing the weight of the sample. These balances typically provide a digital display of the weight. They look much like the single-pan balances mentioned above.

Microbalances for weighing very small loads once were made using a quartz fiber with the sample hung at the end. The deflection of the fiber was read against a scale calibrated with known weights. They have largely been replaced by electrobalances ("Cahn" was an early model).

Copyright © 2010 Denzel Dyer, all rights reserved.