Fluidity and the Pitch Drop Experiment

It’s probably not the most important scientific research project to come out of Queensland, but it may well be the most famous.  In 2005 the University of Queensland Physics Department’s ‘pitch drop experiment’ won the Ig Nobel Prize.  According to the 2002 Guinness Book of Records, it is the oldest continuously running scientific experiment in the world.  It has its own YouTube site.

What is the difference between a solid and a liquid?  For most materials, the answer is simple – water is a liquid, ice is a solid – but for some materials, the answer is less straightforward.  Which category does glass fit into, for instance?  It is often thought that it flows very slowly, so that gravity gradually distorts the shape of stained glass windows so that they are discernibly thicker at the bottom.  This seems to be disputed: hand-blown medieval glass panels are distorted, but it may be that cathedral builders very sensibly placed the heaviest, thickest parts of the glass on the lower side of a panel as this would be the more stable arrangement.

In 1927, Professor Thomas Parnell, the first professor of physics at the University of Queensland, set up a demonstration for his students of the way that something apparently solid, pitch, is capable of behaving like a liquid, though a very viscous liquid.  He heated a sample of pitch until it was liquid and poured it into a large glass funnel with a sealed stem.  He waited three years for it to cool and consolidate back to its solid state.  Then, in 1930, he cut the glass stem of the funnel and the pitch began very slowly to flow.


Since then, a single drop of pitch has dripped from the funnel eight times:

The stem was cut (1930)
1st drop fell (December 1938)
2nd drop fell (February 1947)
3rd drop fell (3 April 1954)
4th drop fell (May 1962)
5th drop fell (August 1970)
6th drop fell (April 1979)
7th drop fell (July 1988)
8th drop fell (28 November 2000)

On current form, the ninth drop is due to fall any year now.

In some ways, it is wrong to call this an experiment, for Parnell was not testing a new theory, but demonstrating something already well understood at the time, and something we all encounter when our shoes stick to the bitumen on a hot day. He could not control the ambient conditions, either.  The room temperature varied from day to night, from summer to winter, and at some stage, the entire apparatus had to be moved from the University’s old site near the City Botanical Gardens to the new St Lucia site.

Tom Parnell died in 1948, but his experiment lives on without him, sitting in the foyer of the Parnell Building, part of the School of Mathematics and Physics at the University of Queensland.  Nobody has ever seen the drop fall – by 2000, a webcam was installed in anticipation but missed the drop because of technical problems.  Another drop is due to fall any day soon, and it is now under constant surveillance.  It is estimated that there is enough pitch still in the funnel for the experiment to continue for another hundred years.

In 2005, Dr John Mainstone went to Harvard to accept an Ig Nobel prize, awarded to himself and (posthumously) to Tom Parnell.  John Mainstone died in August 2013, aged 78, without ever seeing ‘his’ drop fall, but the month before, on 11 July, a falling pitch drop was filmed for the first time at Trinity College, Dublin. This mere youngster of an experiment began in 1944, based on the Queensland original.

The Ig Nobel awards, according to the citation, are given to ‘honor achievements that first make people laugh, and then make them think. The prizes are intended to celebrate the unusual, honor the imaginative — and spur people’s interest in science, medicine, and technology.’  Hopefully, the pitch-drop experiment does just that.

Login here to join the Pitch Drop watch. The university promises that ‘If you’re logged in when the Pitch Drop falls, your name will make the official record of the world’s longest running lab experiment.’ Now that’s something to add to your CV.

Or follow @RealPitchDrop for the #9thwatch #pitchdrop

Further reading:
R. Edgeworth, B.J. Dalton and T. Parnell, ‘The Pitch Drop Experiment’, in European Journal of Physics, 1984, pp. 198-200

One response to “Fluidity and the Pitch Drop Experiment

  1. Reblogged this on Historians are Past Caring and commented:

    Forget the unforgettable drop. The Pitch Drop that I wrote about some time ago has finally dropped! http://www.smh.com.au/national/pitch-err-this-worlds-longestrunning-experiment-finally-drops-20140423-373s6.html

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