Getting Technical
CHARLES NICOLSON
Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to
good use by applying speciality chemicals in mining and industrial processes where water is a major factor.
This created an enduring interest in water technology, a passion that expanded to the HVAC industry
in 1984 when he joined BHT Water Treatment. Since then, water technology in HVAC water circuits has
continued to be an abiding interest.
COMPARING X-RAYS AND MRIS
By Charles Nicolson
The feature article in the January 2020 issue of RACA Journal
introduced the lower regions of very cold and even colder – from a now
generally accepted cryogenic upper limit of -50°C down to absolute
zero temperature of -273.15°C.
T
HISTORY OF SUPERCONDUCTIVITY
The discovery of superconductivity was made in 1911 by
Kamerlingh Onnes using solid mercury as the conductor,
supercooled by liquid helium down to close to -270°C.
Subsequently, Onnes attempted to make electromagnets
with various types of superconducting material windings but
found that even relatively low magnetic fields destroyed the
superconductivity within the materials he was investigating.
www.hvacronline.co.za
A section of a hand
X-ray plate showing
the bones and
knuckle joints.
he article went on to describe the first widely-used
cryogenic product – dry ice or solid carbon dioxide – which
conveniently sublimes or changes directly from solid to vapour
phase at -78.15°C at normal atmospheric pressure. The fact that
dry ice is still in common use since the 1920s is a good example
of how substances which have to be made and stored for use
at cryogenic temperatures continue to provide useful benefits.
Many other useful products have been created in controlled
cryogenic conditions including those which become electrically
superconducting below specific temperatures.
The electrical resistance of ordinary metallic conductors
decreases gradually as temperatures are lowered down to levels
only a few degrees above absolute zero. However, certain other
conducting materials which can acquire an ability known as
‘superconductivity’ suddenly lose all of their electrical resistance
when reducing temperatures reach specific levels. Electric
currents in superconducting circuits can persist indefinitely
with no power source. Promising future applications of
superconductivity include magnetic refrigeration at potentially
higher coefficient of performance (COP) levels and far more
efficient refrigeration with added benefits of greatly reduced
carbon emissions.
Further progress of research during the first half of the 20th
century into this new and complex field of superconducting
tended to be drawn out by practical difficulties of running
experiments at temperatures below -200°C. In addition, two
world wars relegated most of the purely scientific research
programmes to back burners although some specific
metallurgical research concerning machining of metals at
cryogenic temperatures was done in the USA. During the 1950s
the physicist G B Yntema determined that superconducting wire
made from niobium-based alloys could be used for windings
which generated stable magnetic fields. Further developments
incorporating niobium/tin combinations showed that these alloys
could carry currents up to 100 000 amperes per square centimetre
(A/cm 2 ) in superconducting conditions controlled within a 5°C
approach to absolute zero.
Despite being brittle and difficult to fabricate, niobium-tin
coils have proved to be practical for generating magnetic fields
RACA Journal I March 2020
57