A pitch drop experiment is a long-term experiment which measures the flow of a piece of pitch over many years. 'Pitch' is the name for any of a number of highly viscous liquids which appear solid, most commonly bitumen. At room temperature, tar pitch flows at a very low rate, taking several years to form a single drop.
The best-known version of the experiment was started in 1927 by Professor Thomas Parnell of the University of Queensland in Brisbane, Australia, to demonstrate to students that some substances which appear solid are actually highly viscous fluids. Parnell poured a heated sample of pitch into a sealed funnel and allowed it to settle for three years. In 1930, the seal at the neck of the funnel was cut, allowing the pitch to start flowing. A glass dome covers the funnel and it is placed on display outside a lecture theatre. Large droplets form and fall over a period of about a decade.
This experiment is recorded in Guinness World Records as the 'world's longest continuously running laboratory experiment', and it is expected there is enough pitch in the funnel to allow it to continue for at least another hundred years. This experiment is predated by two other (still-active) scientific devices; the Oxford Electric Bell (1840) and the Beverly Clock (1864), but each of these has experienced brief interruptions since 1937.
The experiment was not originally carried out under any special controlled atmospheric conditions, meaning the viscosity could vary throughout the year with fluctuations in temperature. Some time after the seventh drop fell (1988), air conditioning was added to the location where the experiment takes place. The lower average temperature has lengthened each drop's stretch before it separates from the rest of the pitch in the funnel, and correspondingly the typical interval between drops has increased from eight years to 12–13 years.
In October 2005, John Mainstone and the late Thomas Parnell were awarded the Ig Nobel Prize in physics, a parody of the Nobel Prize, for the pitch drop experiment. Mainstone subsequently commented:
I am sure that Thomas Parnell would have been flattered to know that Mark Henderson considers him worthy to become a recipient of an Ig Nobel prize. Professor Parnell's award citation would of course have to applaud the new record he had thereby established for the longest lead-time between performance of a seminal scientific experiment and the conferral of such an award, be it a Nobel or an Ig Nobel prize.
The experiment is monitored by a webcam but technical problems prevented the November 2000 drop from being recorded. The pitch drop experiment is on public display on Level 2 of Parnell building in the School of Mathematics and Physics at the St Lucia campus of the University of Queensland. Hundreds of thousands of Internet users check the live stream each year.
The ninth drop touched the eighth drop on 17 April 2014; however, it was still attached to the funnel. On 24 April, Professor White decided to replace the beaker holding the previous eight drops before the ninth drop fused to them (which would have permanently affected the ability of further drops to form). While the bell jar was being lifted, the wooden base wobbled and the ninth drop snapped away from the funnel.
Timeline for the University of Queensland experiment:
|1927||Hot pitch poured|
|October 1930||Stem cut|
|December 1938||1st drop fell||8.1||98|
|February 1947||2nd drop fell||8.2||99|
|April 1954||3rd drop fell||7.2||86|
|May 1962||4th drop fell||8.1||97|
|August 1970||5th drop fell||8.3||99|
|April 1979||6th drop fell||8.7||104|
|July 1988||7th drop fell||9.2||111|
|November 2000||8th drop fell[A]||12.3||148|
|April 2014||9th drop fell[B]||13.4||156|
The pitch drop experiment at Trinity College Dublin in Ireland was started in October 1944 by an unknown colleague of the Nobel Prize winner Ernest Walton while he was in the physics department of Trinity College. This experiment, like the one at Queensland University, was set up to demonstrate the high viscosity of pitch. 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, about a decade after the previous pitch drop, physicists at Trinity College noticed that another drip was forming. They moved the experiment to a table to monitor and record the falling drip with a webcam, allowing all present to watch. The pitch dripped around 17:00 IST on 11 July 2013, marking the first time that a pitch drop was successfully recorded on camera.
Based on the results from this experiment, the Trinity College physicists estimated that the viscosity of the pitch is about two million times that of honey, or about 20 billion times the viscosity of water.
In 2014, media reported that a pitch drop experiment had been recently rediscovered at Aberystwyth University in Wales. Dating from 1914, it predates the Queensland experiment by 13 years. But as the pitch is more viscous (or the average temperature lower) this experiment has not yet produced its first drop and is not expected to for over 1,000 years.
In the Hunterian Museum at the University of Glasgow are two pitch-based demonstrations by Lord Kelvin from 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.
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