Patent ID: 8266958

Claim:
A liquid level sensor device for liquefied hydrogen comprising a superconductive compound layer arranged in succession in the longitudinal direction over the entire or a part of surface of a linear conductor made of metal, wherein the superconductive compound layer contains a binary compound of magnesium and boron where Mg:B=X:Y has a molar ratio of Mg and B falling within a range of X=1 and Y=1.2 to 1.7 or 2.5 to 2.8 or within a range of Y=2 and X=0.4 to 0.7 or 1.4 to 1.8, has a superconducting transition temperature of 21 to less than 30° K, and is configured such that a portion of the compound of magnesium and boron dipped in liquid hydrogen is in a superconductive state at the time of measuring a liquid hydrogen level.