The understanding of superconductivity has progressed in four stages. During the first period following the discovery of disappearance of electrical resistivity of mercury by Kamerlingh Onnes, attempts were made to understand superconductivity based only on zero resistance. The second period was initiated by the discovery of the Meissner effect. Based on new knowledge of the magnetic property of zero resistivity, phenomological theories were developed and new aspects of superconductivity were uncovered, culminating in the establishment of the BCS theory. The rediscovery of a second type of superconductivity together with flux quantization initiated the third period where fluxons play an active role. The fourth period, which is still in progress, was initiated by the discovery of oxides with dramatically high critical temperatures. All these activities were initiated by Kamerlingh Onnes' brilliant insight leading to the discovery of superconductivity.
A modified two-fluid model is applied to study flow and heat transfer of superfluid helium in a micro channel with a diameter as small as that of a superleak in a fountain effect pump. Variable properties of superfluid helium and energy dissipations due to the two-fluid mutual friction and the friction at the channel wall are taken into consideration. It is found that the normalfluid component flow is not trivial even in a channel of a diameter of micro meters, and that there exists an optimum diameter which attains the maximum mass flow rate. The flow of superfluid helium through a channel with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature and pressure, as well as the internal-convection mechanism is found to be the causes of the unique flows.