Air Speed Indicator early diaphragm type, made by C.S.I. Co.

This air speed indicator was made by the Cambridge Scientific Instrument Co. Two long parallel tubes, one measuring air pressure due to aircraft speed from a forward-pointing fixture and the atmospheric pressure, lead to two chambers separated by a diaphragm. Any difference in pressure in the two tubes moves the diaphragm which operates a spring controlled arm and shifts the pointer. Its range is from 20 to 60 miles per hour.