text: Pockels used her sliding trough to evaluate the effect of small amounts of impurities on water. These impurities were typically household substances such as oils and soaps. With her apparatus, she measured the surface tension as the amount of force required to remove the disk from the surface of the water. Pockels found that small amounts of impurities could have a large effect on the surface tension of water, compared to pure water.
  In particular, Pockels studied soapy water. Soaps are examples of surfactants, which are molecules which have a hydrophilic portion of the molecule, while the remainder is hydrophobic. These are insoluble in the water and tend to reside on the surface of the water. When placing small amounts of the surfactants in her sliding trough, Pockels found that the effect on surface tension was small. However, once the slider on the surface of the trough passed a certain point, the effect on surface tension was large. When she plotted the surface tension on one axis of a graph compared to the position of the slider on her sliding trough, the graphical representation was a compression isotherm.
  Using her knowledge of chemistry, Pockels realized that the point of the compression isotherm at which the surface tension changes abruptly is the point at which a film of surfactant molecules that is continuous and one molecule thick is formed. This is a monolayer of the surfactant. Pockels further found that this threshold point is the same for a variety of surfactant soaps. Since she knew how much surfactant was in her sliding trough as well as the area covered by the monolayer film, she could calculate the area on the surface of the water occupied by single molecule. She calculated this area to be 20 square angstroms (20 Å). Later, as her investigations became known in the scientific community, this area determined by the sliding trough became known as the Pockels Point.