Complex sentence: When the current flows through the tantalum wire it is superconducting, but when a current flows through the niobium a magnetic field is produced, this destroys the superconductivity which makes the current slow down or stop.
Split into simpler sentences: When the current flows through the tantalum wire it is superconducting, but when a current flows through the niobium a magnetic field is produced. This destroys the superconductivity which makes the current slow down or stop.