The drop and the metric system: how an unruly unit survived revolutions

This paper presents the peculiar story of the drop, a non-standard unit which outlived the standardizing forces of the late eighteenth and nineteenth centuries, as an instructive lens for understanding how and why non-standard units survive. With the establishment of the metric system in France came a large-scale discussion on the merits of various units, the drop among them. Some partisans of the metric system argued that this unstandardizable unit should be abandoned in favour of decimal weight units. Despite this, the drop remained in use in both lay and scientific contexts. By closely studying medical and chemical practice, we show that the drop was a rich tool for both communication and scientific quantification. The drop's utility went beyond cases where achieving precision was superfluous; drops were also used by practitioners in ways that could not easily be replaced by the scales, accomplishing aims for which standard units were ill-adapted or even damaging to the task at hand. The drop's familiar and flexible nature made it uniquely useful in instructions to be carried out by lay people or factory workers, as well as in the many experimental procedures that required the practitioner's attention to be directed at sensory indications.