Smyth's ozonometer components, 1864.

The glass tap, brass supports for the test papers and a canister of test papers from John Smyth's ozonometer. 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.