The Difference between a Scale and a Balance
A scale, on the other hand, is a weighing device where the unknown weight is determined by scaling a different parameter into a weight value. The spring scale is a good illustration of a scale. In a hanging spring scale, the unknown weight stretches a coil spring which has a pointer attached to it. The pointer moves next to a “scale” that has been scribed on to the scales surface and is calibrated such that it reflects the unknown weight that caused the deflection. In this case distance that the spring stretches is scaled to match the unknown weigh.
There are a great many different forms of scales. The strain gauge scale converts the unknown weight into an electronic signal that is amplified, and then scaled, so that it's value, when converted to its digital equivalent, equals the unknown weight as shown on a display. The strain gauge itself consists of fine wires which change their electrical resistance when they are stretched or compressed. These wires are glued to the surface of a metal bridge that is designed to produce the proper deflection when loaded with the unknown weight.
The scales which we know today as laboratory balances and analytical balances are, in fact, scales. These devices produce a force in a magnetic motor which “balances” or equals the unknown weight. It is the current required by the magnetic motor in order to produce this force that is “scaled” in an analog to digital converter and fed into a digital computer to be corrected for temperature variations and for magnetic non-linearity in the force motor. All modern precision scales provide a digital readout of weight.
It would appear that the future will probably see new force transducers that will convert weight into an electrical equivalent. Therefore, future weighing devices will be scales and the balance will have gone the way of the dodo. The equal arm balance continues to exist today in the form of prescription balances (the Torbal DRX-3) and any number of mechanical versions (Ohaus and others).