Abstract: A blood coagulation time measuring device includes a stirrer drive arrangnt for rotating a stirring rod placed inside the measuring vessel when the vessel is inserted in a measuring channel of the device. A light detector assembly within the device determines the degree of coagulation formation in the measuring vessel by transmitting a light beam from a light source through the measuring vessel and onto a photo-electric cell. Light balancing circuitry coupled to the light source and the photo-electric cell automatically adjusts the brightness of the light from the light source in correspondence with the turbidity of liquid in the measuring vessel. A control switch arrangement operates to turn the stirrer drive arrangement on or off in response to operation of the light balancing circuitry.