Experiment reaches delicate pitch
The famous "pitch drop" physics experiment that has intrigued generations of students at the University of Queensland is approaching a moment of high tension: the drop is soon to drip.
University of Queensland visiting scholar John Mainstone expects that sometime before the end of summer, the drop of pitch that's been forming at the tip of the funnel since the latest drip during World Expo 88 will fall into the glass beaker below.
"When it does, it's possible the event will be viewable live on the Internet," he said.
Professor Mainstone was head of the University of Queensland Physics Department until December 1996 and is now a visiting scholar and lecturer. He has been honorary professor in the Faculty of Sciences at the University of Southern Queensland since 1997.
The pitch experiment was set up by the University of Queensland's first physics professor, Thomas Parnell, in 1927. He wanted to show his students that materials like pitch, a concentrated derivative of tar which is quite solid at room temperature, can nevertheless exhibit fluid properties.
Professor Parnell heated a sample of pitch and poured it into a glass funnel with the stem sealed. Three years later, he broke the seal. The rate of drip has been exceedingly slow because the pitch's (seasonally-averaged) coefficient of viscosity turned out to be at least 10 billion times that of water.
Since 1930, seven drops of pitch have formed and fallen. "People who look at the pitch drop think that it would feel like chewing gum, but it's not soft and sticky at all," Professor Mainstone said. "In fact, it's so hard that if you hit it with a hammer, it would shatter like glass."
In seven decades, no-one has actually seen a drop fall - "hardly surprising, because the fall is only about 5cm and that takes only a very small fraction of a second".
Now, another drop is "heavily pregnant". When it delivers, the details of the process are unlikely to remain secret for an eighth time.
"At the moment the drop is holding by a solid neck. The next phase is that the neck ceases to be solid and consists of just four or five fibres. We don't know quite what happens then," Professor Mainstone said.
"The Physics Department is considering putting the experiment live on the Internet and nearer the time we might run a videotape continuously."
The period between drops is lengthening as the "head" of pitch in the funnel decreases and, more recently, because the airconditioning of the lecture theatres on either side of the experiment in the Parnell Building's foyer has reduced the average temperature. The latest interval is more than 10 years, compared with an average of between eight and nine years.
"The experiment is a demonstration, not a controlled experiment meant to produce repeatable and verifiable results, so variation in temperature conditions is of no real concern," Professor Mainstone said.
"There is sufficient pitch, at the present rate of dropping, for the experiment to go on for another 100 years or more."
It is already one of the longest-running physics experiments in the world. New Scientist, citing The European Journal of Physics, mentions an air-powered clock at the University of Otago, New Zealand, which is fuelled by diurnal changes in temperature and has run without winding since 1864; and an electric bell which has been ringing almost continuously since 1840 in the University of Oxford's Clarendon Laboratory (fortunately for the lab's inhabitants, the bell is "practically inaudible").
It emerged in 1984 that St Andrew's University in Scotland also has a "pitch drop" experiment started in 1927. Professor Mainstone, who has been researching the history of science and technology in Queensland, said there is no evidence of any correspondence between Professor Parnell and physicists in Scotland at that time.
"It is likely that both experiments were suggested as a result of an interest in the properties of matter, including molecular and eddy viscosity, over the latter part of the 19th Century and the early part of the 20th Century," he said.
"However, it has to be said that in 1927 the scientific world was buzzing with excitement at the implications of the revolutionary new quantum mechanics, and not this decidedly classical physics demonstration!"
While the Brisbane experiment has produced a series of drops, the pitch at St Andrew's flows out in an unbroken dribble.
"I think the reason for the difference is that no two samples of pitch are quite the same - it's likely the more volatile hydrocarbons are sufficiently different to produce contrasting outcomes," Professor Mainstone said.
For more information, contact Professor John Mainstone (telephone 07 3365 2405, email mainstone@physics.uq.edu.au).
