Abstract: An apparatus, for example, an intravenous drop-feed apparatus for sensing the rate at which liquid drops fall along a predetermined vertical path and for regulating this drop rate to be substantially equal to a preselected drop rate includes a disposable drip chamber which defines a vertical drop path. The drip chamber is formed with an indentation having an upper wall that projects into the drop path to be impinged by drops falling on its interior surface. A probe is removably mounted on the outside of the chamber in interfitting engagement with the chamber indentation and includes an electrically conductive element located to contact the exterior surface of the upper indentation wall. The element, which is electrically but not thermally isolated from falling drops, exhibits conductivity that varies significantly with its temperature and, accordingly, generates pulses at a rate that corresponds to the periodic rate of the falling drops.