It is well known how to measure the level of liquid helium surface by using a superconducting wire. However, there are two difficulties in this method. One is due to the electrical resistivity change which depends on self-heating of the wire. Another is due to the change of the plateau region in current-voltage curve of this wire when the helium bath temperature has changed or the evaporation rate of helium gas has changed.
These difficulties have been surmounted by using the new method of two steps pulse current measurement, which makes use of the properties of return curve in current-voltage hysteresis loop.
