increase of 0 035 percent which corresponds to an equivalent absorbance change of 0 015 a in operation the hysteresis operates as follows assume that the sample solution s is darker than reference solution a tp4 should be less than tp1 however further assume that because of some transient state tp4 is actually greater than tp1 feedback through resistor r7 causes more current to flow through led ds1 causing it to become brighter as the solutions return to their normal state tp1 will start to drop until it becomes lower than tp4 when that happens the output of comparator u1b will drop because of the feedback path through resistor r7 the current through led ds1 will also drop causing it to become dimmer the net result is to stabilize the circuit by minimizing the number of transitions between indicator states source modulation hysteresis provides a significant improvement over normal voltage feedback hysteresis with source modulation hysteresis the hysteresis is constant in absorbance units equivalent to db throughout the dynamic range of the instrument this results in a well behaved system that is not twitchy or overly senstive for clear solutions nor insensitive for dark solutions as is the case using normal voltage feedback source modulation hysteresis yields stable readings even though the tubes may exhibit slight scratches or move.