PURPOSE: To judge whether the separation of ingredients of a specimen is perfect or not by comparing the length of a zone corresponding to a ferrogram with that of another zone corresponding to another ferrogram to be obtained at a different time.

CONSTITUTION: Desired ions are caused to electrically pulsate by an electrophoresis tube 1, a leading liquid tank 2 and a terminal liquid tank 3. Detectors D1 and D2 are potential gradient measuring instruments which are capable of recording ferrograms. The change of output in terms of time of the two detectors D1 and D2 is represented as d1 and d2 respectively. Since D1 is positioned to the left of D2, it shows the condition of separation of ingredients in the initial stage. All stages of ferrograms obtained by detectors D1 and D2 respectively are caused to correspond each other by the potential gradient of each zone and a base portion at the extreme left is a potential gradient in the leading liquid and that at the uppermost stage extreme right is a potential gradient in the terminal liquid. Since the length of zone X between zone A and B of detector D1 differs from that of D2, it can be known that this zone is a zone of mixed ingredients.