CC STUDIO / SCIENCE PHOTO LIBRARY CC STUDIO / SCIENCE PHOTO LIBRARY
Historical colorimeter. This instrument was used to find the concentration of a substance in a liquid. First, a light beam was shone through a test tube containing distilled water. The light transmission reading on the electric dial (centre left) was set to 100%. The light beam was then shone through a test tube containing the sample and another reading was taken. The higher the sample's concentration, the greater the reduction in the intensity of the light passing through it. Colorimeters are still in use today. This photoelectric colorimeter was made by Lumetron.
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