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When different substances are cooled to very low temperatures, their properties change. In 1908 Heike Kamerlingh Onnes used an ingenious apparatus to cool helium to liquid form. Fluid helium was carefully studied and also became an important aid for the cooling of different substances and charting their properties at low temperatures. In 1911 Heike Kamerlingh Onnes discovered that the electrical resistance of mercury completely disappeared at temperatures a few degrees above absolute zero. The phenomenon became known as superconductivity.