Spirometer with spring-balanced floating bell jar - has jar connected to one of two coaxial pulleys with other variable radiused pulley connected via spring to fixed point

摘要

The spirometer comprises a bell jar which floats in a liquid filled container with a flexible tube passing up into it from its submerged mouth. Two pulleys are mounted on the same axis and the bell-jar is suspended from a vertical wire passing over one of them and fixed to it. The ends of another wire is attached to a fixed point via a spring after passing over the second pulley. One of the pulleys is in the form of a variable radius cam. When the patient blows into the tube, the apparent weight of the jar is equal and opposite to the force exerted by the spring.