Drawbacks of previous machines are avoided by a new principle. When sol → gel, viscosity increases, impeding passage of coagulating fluid through a constriction, augmenting the amplitude of forced pressure changes in a closed system of air in communication with the assay mixture; the resulting amplification of the movements of mercury in a U-shaped manometer breaks a light beam, stopping a counter which was started at recalcification. The analytical variation is acceptable, and the bias differs very little from that of visual end-point determination.