A Photoelectric Nephelometer

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Abstract
A nephelometer is described that does not require a precision optical system or an electronic amplifier. It is sensitive enough to measure the growth of E. coli from a starting concentration of approximately 2×106 bacteria per ml. The readings are reliable from 107 to 109 bacteria per ml. A graph of the photo‐current caused by the scattered light versus the number of E. coli per ml is given. The E. coli culture is a saturated 24‐hr. nutrient broth culture in the lag phase. The bacteria were killed with 1 percent formaldehyde. In this range of bacterial concentrations the scattered light is nearly proportional to the number of scattering centers. The instrument is well adapted to the study of growth curves of bacterial cultures and the lysis phenomena that occur when the organisms are attacked by bacteriophages. A horizontal beam of light passes through the vertical test tube containing the specimen. The light scattered downward is measured with a barrier layer photo‐cell. The horizontal incident light produces a photo‐current of 2.4×10−3 ampere. The light scattered by a specimen whose titer is 5×107 E. coli (growing exponentially) per ml produces a photo‐current of 1.2×10−5 ampere. In this case the scattered light reaching the photo‐cell is 1/200 of the incident light.