19 and 35 does not vary so that this portion of the series connected circuit may be considered substantially constant the measuring electrode 67 of the embodiment shown in fig4 and 5 is formed as a rod electrode located a short distance upstream of measuring slot 60 and extending parallel thereto rod electrode 67 projects through the bottom 45 of measuring chamber 49 into diving bell 56 ground electrode 66 as well as measuring electrode 67 are each connected to terminals 68 and 69 respectively extending outwards of housing 41 both ground electrode 66 and measuring electrode 67 are coated with a thin layer of a water and fat repellent insulating material as in this circuit arrangement the capacity varies in response to variations of the retained liquid level only in proportion to the surface of the measuring electrode 67 covered or wetted by the milk only the thickness of the coating on measuring electrode 67 is critical and variations of the coating thickness result in measuring errors contrary thereto the thickness of the coating layer on ground electrode 66 is not critical and may be selected different from that of the coating layer on measuring electrode 67 slight variations of the thickness of the coating layer on measuring electrode.