An Analytical Balance for Recording Rapid Changes in Weight

Abstract

An analytical balance has been modified by adding a solenoid and magnet so that changes in weight can be balanced electrically. The operation is made automatic by reflecting a beam of light from the balance beam into a dual phototube in a bridge circuit and amplifying the output of the bridge to provide current in the solenoid. Recording the voltage drop across a resistor in series with the solenoid gives a linear weight scale which can be made to read directly in milligrams. An electronic damping circuit enables the balance to follow very rapid changes in weight without oscillating and with an accuracy better than 0.5 percent of full scale for ranges of 100 mg and over.