Kamerlingh Onnes and the discovery of superconductivity

Many papers begin with the statement that Kamerlingh Onnes discovered superconductivity in 1911; one wonders what urged him to do the experiment that led to this discovery. Superconductivity was definitely not foreseen at that moment, and for many years the theory could not even predict it. Hence what drove Kamerlingh Onnes to measure the electrical resistivity at a temperature range where one could not expect to find anything radically new? What were the prevailing theories at that time? This is discussed after a general description of the man, his laboratory, and the state of physics in the preceding years. Conclusion: Although the driving factor in the low temperature resistance work was most likely the need for a reliable and reproducible thermometer, the speculations about what would happen in these regions were based on Kelvin’s theory, at least in the initial phase of the work. This theory predicted exactly the opposite of what was found eventually.