This is the earliest commercial sensitometer preceding the work of Hurter and Driffield. The standard light source is a phosphorescent tablet that has been exposed to burning magnesium ribbon. The plate to be measured is placed behind the graduated density plate in the printing frame and exposed for a fixed time to the phosphorescent tablet. The rapidity of the plate can be gauged by seeing which of the graduated squares is visible. This instrument was widely used and several manufacturers labelled their plates with Warnerke numbers. The value measured was the threshold value of the plate. Each Warnerke number represented a 1/3 increase in speed over the previous. The price was 15/-.

Having found a value for the speed of the plate, the Warnerke value, the photographer had to calibrate this in terms of exposure (aperture and time). After finding the Warnerke value of a plate another plate from the same batch was exposed in a camera to find the exposure required by that particular Warnerke value. A calculator could then show the relative exposure for plates with other Warnerke values.