Model of an Ozonometer, improved form, designed in 1865 by Smyth with cardboard box.

To test ozone levels in the atmosphere, a test paper is held across 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.