Smyth's revised ozonometer, 1865.
© Science Museum / Science & Society Picture Library
To test ozone levels in the atmosphere, a test paper is held acros an air stream drawn into the ozonometer for a fixed length of time or for a fixed volume of air. The test paper, having been treated with chemicals (such as iodide of starch) before exposure, changes colour according to the amount of ozone in the air and can be compared with a prepared scale. Smyth's revisions to his original instrument incorporated a boxwood canister to support the test paper and the design apparently made the measurements both faster and more accurate.