Liquid helium of liquid water. Helium was first liquefied on July 10, 1908, by the Dutch physicist Heike Kamerlingh Onnes at the University of Leiden in the Netherlands. At that time, helium-3 was unknown because the mass spectrometer had not yet been invented. In more recent decades, liquid helium has been used as a cryogenic refrigerant, and liquid helium is produced commercially for use in superconducting magnets such as those used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), Magnetoencephalography (MEG), and experiments in physics, such as low temperature Mössbauer spectroscopy. The temperature required to produce liquid helium is low because
Leiden scale Leiden scale The Leiden scale (°L) was used to calibrate low-temperature indirect measurements in the early twentieth century, by providing conventional values (in kelvins, then termed "degrees Kelvin") of helium vapour pressure. It was used below -183°C, the starting point of the International Temperature scale in the 1930s (Awbery 1934). The Leiden scale probably goes back to around 1894, when Heike Kamerlingh Onnes' cryogenic laboratory was established in Leiden, Netherlands. It has been reported that the scale is the kelvin scale shifted so that the boiling points of hydrogen and oxygen become zero and 70 respectively, but this is unlikely