Continuous recording of residual chlorine is not a new idea. Rather this has been the aim of scientists and operating personnel since chlorination of potable water was first placed on a continuous basis in 1908. Actually the development of reliable equipment for this purpose could not take place until the nature and characteristics of residual chlorine in water, in small concentrations (0.1 to 10.0 ppm), had been thoroughly studied and certain instruments such as the photoelectric cell and amperometric titrator had been perfected. The perfection and acceptance of the photoelectric cell offered one of the first avenues of approach to the problem. Early work was devoted largely to the measurement and recording of the various intensities of color, which are proportional to the amount of chlorine present, produced by the addition of certain reacting chemicals, such as potassium iodide or orthotolidine. Several instruments utilizing this principle were constructed and placed in operation during the early thirties. Some were notably successful, particularly those making use of orthotolidine. However, practically every scheme using the photoelectric cell method was subject to certain fundamental errors, the most common of which was the accumulation of material on the glass surfaces of the water cell. These accumulations changed the intensity of the color measured by the photoelectric cell and introduced errors that were difficult to overcome. In addition, the equipment was either complicated or required a degree of skill in operation.