Pitch Drop Experiment: The World’s Longest Running Lab Experiment

Dec 17, 2020 0 comments

The pitch drop experiment began in 1927 when Professor Thomas Parnell of the University of Queensland in Brisbane, Australia, set out to demonstrate to his students that some substances that appear to be solid are in fact very high viscous fluids. He used tar pitch, or bitumen, a derivative of coal once used to waterproof boats, in an experiment to prove his point. At room temperature, pitch appears to be solid and can even shatter if hit with a hammer, but despite its look and feel, pitch can also flow, albeit extremely slowly.

Pitch drop experiment

Curious visitors inspecting the setup of the pitch drop experiment. Photo: University of Queensland

For his experiment, Parnell melted some pitch into a glass funnel with a sealed stem and allowed it to cool for three years. In 1930 he cut the sealed stem, hung the funnel over a beaker, and waited. It took eight year before the first drop fell into the beaker and another nine years before the second drop hit. Parnell didn’t live to see the third drop fall in 1954, as he passed away in September 1948. By then, the experiment was stored away in a cupboard of the physics department.

The pitch-drop experiment might have fallen into obscurity had it not been for John Mainstone, who joined the University of Queensland physics department in 1961. One day a colleague said, “I’ve got something weird in this cupboard here” and presented Mainstone with the funnel, beaker and pitch, all housed under a bell jar. Mainstone asked the department head to display it for the school’s science and engineering students, but he was told that nobody wanted to see it. Finally, around 1975, Mainstone persuaded the department to publicly display the experiment in a cabinet in the foyer of the department building.

Pitch drop experiment

The Pitch Drop Experiment with John Mainstone in a picture taken in 1990.

Today the experiment is broadcast on a live webcam. Unbelievably, no one had actually witnessed the pitch drop fall for 87 years, until April 2014, when the ninth drop fell. Thanks to round-the-clock monitoring by a webcam, the drop was seen touching down sometime between 9th and 14th of April—the wide time range is due to the fact that the pitch flows extremely slowly, and the exact moment of touchdown is uncertain. Although the webcam was present when the eight drop fell in November 2000, it couldn’t be recorded as the camera malfunctioned at the critical moment. The tenth drop was predicted to fall this year.

Date Event
1927 Hot pitch poured
October 1930 Stem cut
December 1938 1st drop fell
February 1947 2nd drop fell
April 1954 3rd drop fell
May 1962 4th drop fell
August 1970 5th drop fell
April 1979 6th drop fell
July 1988 7th drop fell
November 2000 8th drop fell
April 2014 9th drop fell

Timeline of the pitch drop experiment.

The experiment was not originally carried out under any special controlled atmospheric conditions, meaning that the viscosity could vary throughout the year with fluctuations in temperature. However, sometime after the seventh drop fell in 1988, air conditioning was added to the location where the experiment resided. The temperature stability has lengthened the interval between each drop.

The pitch drop experiment is recorded in the Guinness Book of Records as the world’s longest continuously running laboratory experiment, but it isn’t the only pitch drop experiment in the world. A pitch drop experiment was recently discovered at Aberystwyth University in Wales, that actually predates the famous Queensland experiment by 13 years. But the pitch used is stiffer and has never yielded a single drop.

Lord Kelvin’s pitch flow experiment.

Lord Kelvin’s pitch flow experiment. Photo: Geni/Wikimedia Commons

In 1944, an unknown professor started another one at Trinity College Dublin in Ireland. This physics experiment sat on a shelf in a lecture hall at Trinity College unmonitored for decades as it dripped a number of times from the funnel to the receiving jar below, also gathering layers of dust. In April 2013, physicists at Trinity College noticed that another drip was forming. They moved the experiment to a table and set up a webcam to record the falling drop. The pitch dripped on 11 July 2013, marking the first time that a pitch drop was successfully recorded on camera.

In the Hunterian Museum at the University of Glasgow, are two different experiments that demonstrate the incredible viscosity of pitch, installed by Lord Kelvin in the 19th century. Kelvin placed some bullets on top of a dish of pitch, and corks at the bottom: over time, the bullets sank and the corks floated. Lord Kelvin also showed that the pitch flows like glaciers, with a mahogany ramp that allowed it to slide slowly downward and form similar shapes and patterns to rivers of ice in the Alps.

old church windows

Glass is viscous, but old church windows are not thicker at the bottom because the glass had sagged. Photo: Neil Lang/Shutterstock.com

There is another material that exhibit liquid like properties despite being seemingly solid. That material is glass.

Although glass has all the mechanical properties of a solid, its molecular structure is jumbled, like that of a liquid. Laboratory experiments have confirmed that even at room temperature, glass has a tendency to flow, but the viscosity of glass is several magnitudes higher than pitch. The popular myth that ancient windows are thicker at the bottom because the glass has sagged with the centuries is untrue.

To see glass flow would take an almost unimaginably long time, explains Dr Kostya Trachenko of Queen Mary University of London, who has been carrying out his own pitch drop experiments.

“If you wait longer than the age of the universe, you'll see this as a liquid.” Dr Trachenko said, tapping the glass cabinet that houses his students' pitch funnels. “It would flow. And that would be the end of it.”


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