In 1893 Watkins discovered that the time of appearance of the image on a plate (the high lights) indicated the speed at which the developer was working. A multiple of this time gives the overall time to develop the plate. The multiplying factor was called the 'Watkins Factor' and varied for different types of developer. In use an index is set against the Watkins Factor, the appearance time is noted and against this value (marked on the inner scale) will be found the time to develop the plate (marked in minutes on the outer scale). The calculator simply multiplies the appearance time by the Watkins Factor and divides by 60. The Watkins Factor was published by Watkins, the photographer could adjust this depending on the amount of contrast required. B & J were the agents for the USA.