The Pitch Drop Experiment
The first Professor of Physics at the University of Queensland, Professor
Thomas Parnell, began an experiment in 1927 to illustrate that everyday materials
can exhibit quite surprising properties. The experiment demonstrates
the fluidity and high viscosity of pitch, a derivative of tar once used for
waterproofing boats. At room temperature pitch feels solid - even brittle
- and can easily be shattered with a blow from a hammer (see the video
clip below). It's quite amazing then, to see that pitch at room temperature
is actually fluid!
In 1927 Professor Parnell heated a sample of pitch and poured it into a glass
funnel with a sealed stem. Three years were allowed for the pitch to
settle, and in 1930 the sealed stem was cut. From that date on the pitch
has slowly dripped out of the funnel - so slowly that now, 77 years later,
the ninth drop is only just forming.
The experiment was set up as a demonstration and is not kept under special
environmental conditions (it is actually kept in a display cabinet in the
foyer of the Department), so the rate of flow of the pitch varies with seasonal
changes in temperature. Nonetheless, it is possible to make an estimate
of the viscosity of this sample of pitch (R.
Edgeworth, B.J. Dalton and T. Parnell, Eur. J. Phys (1984) 198-200).
It turns out to be about 100 billion times more viscous than water!
In the 77 years that the pitch has been dripping no-one has ever seen the
drop fall. If you're interested in trying your luck, or at least just
having a look at the experiment, you can view it live.
Fortunately you can also see students
of The University of Queensland milling around outside the cabinet, so it
is more exciting than watching grass grow! The audio over the live video is
an interview with Professor John Mainstone, who maintains the experiment.
There is also a Video clip
showing what pitch is like at room temperature and what happens when you hit
it with a hammer.
Pitch, before and after being hit with a hammer
To view the Pitch Drop Experiment you'll need the Windows Media Player:
Some other links:
If you're interested in more old stuff, have a browse through the
Physics
Museum
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