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Greasy washing-up water is a nuisance to most people. To Agnes Pockels it was a question: why does a dirty water surface behave so differently from a clean one? Without a university place or a laboratory, she built the apparatus to find out. This post covers who she was, what her trough did, a worked example with her own measurements and NCERT's surface tension formulas, and what her way of working teaches.
Who was Agnes Pockels?
The Technical University of Braunschweig, which names its school chemistry lab after her, records that she was born in Venice on 14 February 1862. Her family moved to Braunschweig, where she went to the town's girls' high school. She wanted to study physics, but women were not then admitted, and when they were, she gave up the chance at her father's wish. Her younger brother Friedrich, who became a physics professor, sent her scientific literature. She ran the household and nursed her parents for most of her life.
Her diary, as the university quotes it, gives the timeline: around 1880 or 1881 she discovered the "anomalous" behaviour of the water surface, and in 1882 she invented her sliding trough.
What was her trough?
In 1891 she described it in a letter to the British physicist Lord Rayleigh, who had published on the same subject. He sent a translation to Nature, which printed it on 12 March 1891 under the title Surface Tension. Rayleigh's covering note says she had obtained "valuable results respecting the behaviour of contaminated water surfaces" with "very homely appliances". She explained her position plainly: "For various reasons I am not in a position to publish them in scientific periodicals."
The apparatus was a tin trough 70 cm long, 5 cm wide and 2 cm high, filled to the brim. A strip of tin laid across it touched the water and divided the surface in two. Sliding the strip made one surface larger and the other smaller, and a scale showed by how much. To measure surface tension, she found the weight needed to pull a small disc, 6 mm across, off the surface, using a light balance with a sliding weight.
What did she find?
Her letter reports that a strongly contaminated surface has a surface tension that changes with its area. Squeezing it lowers the tension; stretching it raises the tension to a maximum, after which it stays constant like a clean liquid. Between the lowest and highest values, the separating weight changed in the ratio 52 : 100. She also found that a fresh, almost clean surface could be made by pushing the strip from one end, and that every solid touching the water, even clean glass or metal, contaminates it a little.
The university notes that Irving Langmuir later developed her trough into the Langmuir balance, still used to study surface films. Our post on Irving Langmuir covers his side of the story.
A worked example with her numbers
The NCERT Class 11 chapter Mechanical Properties of Fluids gives the surface tension of water at 20 °C as 0.0727 N/m, and treats surface tension as force per unit length or energy per unit area. It even describes measuring it with a balance, as she did.
Lowest tension. Assume the separating weight is proportional to surface tension (an approximation). Then the most contaminated surface had about 0.52 × 0.0727 ≈ 0.038 N/m.
Force on the partition. With clean water on one side and the most contaminated surface on the other, the net pull on the strip is the difference in tension times the width of the trough: (0.0727 − 0.038) × 0.05 m ≈ 1.7 × 10⁻³ N.
As a weight. Using g = 9.8 m/s², that equals the weight of about 1.7 × 10⁻³ ÷ 9.8 ≈ 1.8 × 10⁻⁴ kg, about 0.18 g.
Energy for a clean surface. The whole surface is 0.70 × 0.05 = 0.035 m². Creating it fresh takes about 0.0727 × 0.035 ≈ 2.5 × 10⁻³ J.
Step 2 explains a problem she described: when the tension difference was large, contaminated water could break through around the edge of the strip.
Common mistakes:
Using F = 2Sl here. The factor 2 is for a film with two surfaces, as in NCERT's wire frame. The trough has one surface.
Leaving lengths in cm. 5 cm is 0.05 m.
Reading 52 : 100 as a tension in N/m. It is a ratio of balance weights.
Our guide to surface tension and viscosity has more practice. If you need to rebuild the idea of surface energy first, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing.
How was she recognised?
The university records that after 1891 she published in Nature and in German journals, including the Annalen der Physik. In 1931 the Kolloid-Gesellschaft gave her its Laura R. Leonard Prize, and in February 1932 the Technical University of Braunschweig made her an honorary Doctor of Engineering. She died in Braunschweig on 21 November 1935.
What can a student take from Pockels's way of working?
Start with what you can reach. A tin trough and a balance were enough to find a new effect.
Measure, then measure again. She ran her experiments for about ten years before writing to Rayleigh.
Report honestly. Her letter points out the trough's leak and calls some results "very uncertain".
Share the work. Writing to the right person put her results in front of the world.
Try it now: repeat step 2 for the separating-weight ratio 80 : 100 and compare the force.
