This paper explains work done on the development of an instrument designed to determine the oxygen content of a sample of respiratory gas. The device relies on the fact that oxygen is paramagnetic and the other exhaled gases are not. The sample is passed through very small nonmagnetic tubes arranged in a bridge. At the application of a magnetic field to two opposite arms of the bridge, a pressure transducer placed across the bridge measures a pressure change that is entirely due to the oxygen in the sample.