Tar pitch looks like a solid black rock that you can shatter with a hammer. But it is actually a highly viscous liquid. In 1927, Professor Thomas Parnell poured pitch into a sealed funnel. Three years later, he cut the stem. Since then, the pitch has been slowly dripping. It takes about a decade for a single drop to fall, proving that some apparent solids are just extremely slow-moving fluids.
A Demonstration Set in Motion
In 1927, Professor Thomas Parnell, the first professor of physics at the University of Queensland in Brisbane, Australia, devised a demonstration to show his students that appearances can be deceiving in fluid mechanics. He selected pitch, a dark, resinous hydrocarbon derivative historically used to caulk wooden ships and seal roofs. At ambient temperatures, pitch feels rigid to the touch, holds a sharp edge, and can be shattered into shards with a strike from a hammer. Despite these brittle, rock-like qualities, pitch is not a true crystalline solid; it is an amorphous substance that behaves as a fluid of extraordinary viscosity.
To illustrate this property, Parnell heated a sample of pitch and poured it into a glass funnel with a sealed stem. He allowed the pitch to settle and cool undisturbed for three years so that internal stresses could relax and the material could fully consolidate at room temperature. In October 1930, Parnell cut the stem of the funnel, opening a path for gravity to act on the column of pitch. The experiment was placed under a glass display dome and left on exhibit in the university's physics department, initiating what would become the longest continuously running laboratory experiment in history.
Decades Between Drops
The movement of pitch is so gradual that human eyes cannot detect motion in real time. It took more than eight years for the first drop to accumulate enough mass to break away, finally falling in December 1938. Parnell lived to see only one more drop detach, in February 1947, before his death in 1948. A third drop fell in April 1954. Following Parnell's tenure, stewardship of the experiment passed through other hands until Professor John Mainstone became its primary custodian in 1961, looking after the apparatus for over five decades.
Under Mainstone's watch, drops continued to fall at intervals of roughly seven to nine years throughout the mid-twentieth century. The fourth drop fell in May 1962, the fifth in August 1970, the sixth in April 1979, and the seventh in July 1988. Remarkably, for the first seven decades of the experiment, no human ever witnessed a drop detach in person. Because the detachment occurs over a matter of seconds after years of silent elongation, observers were repeatedly absent at the decisive moment. In 1988, Mainstone stepped away for a brief refreshment break and returned to find the seventh drop already sitting in the beaker below.
Quantifying an Enormous Viscosity
The scientific value of the pitch drop demonstration lies in calculating the substance's viscosity—its internal resistance to deformation and flow. Water has a very low dynamic viscosity, flowing freely under the gentlest gradient, while substances like honey or motor oil show noticeable resistance. Pitch represents an extreme end of the fluid spectrum. Based on the rate of descent and the geometry of the funnel, researchers at the University of Queensland calculated that the pitch used in the experiment has a viscosity approximately 230 billion times that of water.
This colossal viscosity explains why the funnel yields only a tiny fraction of an ounce of material per decade. Because pitch is an amorphous solid-like liquid, its molecules are tangled and lack the organized lattice structure of a crystal, yet they possess sufficient thermal energy to slide past one another at an imperceptible crawl. The experiment provides a tangible, macro-scale benchmark for understanding materials that sit along the boundary between classic liquids and solids, offering physical proof that states of matter are defined by their response to stress over time rather than superficial hardness.
Temperature and the Lengthening Cycle
Viscosity is not static; it varies significantly with ambient temperature. For much of its history, the Queensland pitch drop experiment sat in an un-air-conditioned hallway, exposed to Brisbane's subtropical climate. Summer heat waves lowered the pitch's viscosity, accelerating flow, while cooler winter months caused it to stiffen. These seasonal fluctuations meant that drops typically stretched more rapidly in the southern hemisphere summer, influencing the exact timing of their release.
In the late 1980s, air conditioning was installed in the building housing the apparatus. The resulting decrease in average ambient temperature noticeably increased the effective viscosity of the pitch. Consequently, the interval between drops lengthened. The eighth drop took over twelve years to fall, finally landing in November 2000. Technology also failed to capture that moment: a webcam monitoring the apparatus suffered an equipment glitch, leaving the detachment unrecorded. The ninth drop took until April 2014 to touch down, colliding with the pile of previous drops before the stem was separated during maintenance to replace the beaker.
Parallel Trials and Filmed Drops
While the Queensland setup is the most famous, it is not the only pitch drop experiment in existence. A similar demonstration was set up in 1944 at Trinity College Dublin by physics Nobel laureate Ernest Walton and his colleagues. That experiment sat unmonitored for long periods on a shelf, quietly dripping over decades. In April 2013, researchers at Trinity College noticed a drop forming a thin neck and set up continuous video surveillance. On July 11, 2013, the Dublin team successfully recorded the first high-definition video of a pitch drop breaking free, capturing an event that had eluded human observers for generations.
Another early apparatus was established in 1914 at Aberystwyth University in Wales, pre-dating Parnell's experiment. However, the University of Queensland apparatus holds the official Guinness World Record for the world's longest continuously running laboratory experiment due to its unbroken lineage of custodial documentation, active observation, and continuous display.
A Monument to Long-Term Science
In 2005, the Ig Nobel Prize in Physics was awarded jointly to Thomas Parnell (posthumously) and John Mainstone for their dedication to maintaining the Brisbane experiment. Though initially created as an educational curiosity, the apparatus has grown into a celebrated cultural and scientific touchstone. It now streams live continuously on the internet, drawing viewers from across the globe who check in on the tenth drop as it slowly takes shape in the neck of the funnel.
Beyond its quirky fame, the experiment offers a rare lesson in patience and institutional continuity. Scientific research often prioritizes rapid results and short funding cycles, but understanding slow planetary and material processes requires decades, if not centuries, of steady observation. With enough pitch remaining in the funnel to continue dripping for more than another century, Parnell's simple glass apparatus stands as an enduring monument to curiosity that outlasts individual human careers.
Key takeaways
•The pitch drop experiment was initiated in 1927 at the University of Queensland to prove that brittle, solid-looking pitch is actually a fluid with extreme viscosity.
•Measurements show the pitch is roughly 230 billion times more viscous than water, causing single drops to take anywhere from eight to more than twelve years to fall.
•No human observed any of the first eight drops fall in real time; the first drop detachment ever captured on video occurred at a sister experiment at Trinity College Dublin in 2013.
•The apparatus holds the Guinness World Record for the longest continuously running laboratory experiment and contains enough pitch to flow for well over another century.