On less violently, meaning that there can

On the 10th
July 1908, the Dutch physicist Heike Kamerlingh Onnes succeeded in liquifying
helium to produce a temperature of 4.2K; a discovery which sowed the seeds for
one of the most remarkable results in modern physics to date 1. Now that liquid
helium could be made, Onnes decided to use his new-found technique to answer a
question that had baffled physicists at the time – what happens to the
resistance of a metal if cooled to absolute-zero?

It was a well-established
fact that as a material is cooled down that its resistance falls; simply on the
basis that the atoms inside vibrate less violently, meaning that there can be less
scattering and subsequently less resistance. Following previous research by Dewar,
Lord Kelvin and Matthiessen, Onnes wanted to investigate the low conductivity
of a range of different metals, seeing just how small the resistance of a material
can become if purified (to reduce further resistance via scattering) and if the
temperature is reduced to the greatest extent possible. In order to achieve
this purity, Onnes decided to work with the liquid metal mercury encased in glass
capillaries, which could be repeatedly distilled to remove any possible impurities
2. These would be frozen using the liquified helium. Onnes’s results showed
that as the samples of mercury were cooled below its boiling point that its
resistance unexpectedly plummeted to zero, a result that bewildered. To try to
explain this abrupt change in R, Onnes repeated the experiment multiple times
to no avail, until an important anomalous result was found. It dawned upon him,
that during a repeat experiment, the temperature of the He was being controlled
by a junior assistant in the laboratory (a ‘blue boy’ – referring to the uniforms
worn at the time). It came to light that this assistant had, in a moment of
aloofness, allowed the helium to boil, raising the temperature of the mercury
until the resistance reappeared, much to their astonishment. It was here that
Onnes was able to see that this was no systematic artefact, but he had inadvertently
discovered mercury’s critical temperature
and the existence of Superconductivity
1. 

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