An au tomatic photoelectric instrument and method for t he continuous meas urement of t he ozone in the earth's atmosphere at low altitudes are described. The method is physical rather t han chemical in character and is based upon t he optical absorption characteristics of ozone in the Hartley and Huggins ul t raviolet bands. Th e ins trument makes use of a lowpressure mercury are, which is situated at a di stance of 1,450 feet from the reco rding s tation t hat emp loys a 1P28 photo mult

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... as a d etector . The lig ht beam is modulated, at 510 cycles per se cond, so that the output of t he photomultiplier is fed into a t uned alternatingcurrent ampllfier and amphfied to the recorder level. By means of a Geneva m ec han is m which changes t he glass filters, the radiant energy from t he lamp is separated into band~ primarily at wavelengths 253 .7, 365.5, a nd 405 .0 millimi cro ns. From the ratios of the de-1kctions for t he different spectral regions it is possible to d etermine ozone concentration in t he range from a few te nths of 1 part to many p ar ts per 100 million.