Final answer:
Thulium is a brittle metal and the niobium wire remains superconducting at 4.2 K when carrying a current of 300 A.
Step-by-step explanation:
Thulium (element symbol Tm, atomic number 69) is a lustrous, silver-grey metal that is part of the lanthanide series on the periodic table. Like most rare earth elements, thulium can be quite brittle, especially when in its pure form. Answering the experimental question about niobium (Nb) wire: niobium becomes superconducting below its critical temperature, which is approximately 9.25 Kelvin (K). Since the niobium wire is immersed in liquid helium at a temperature of 4.2 K, which is below its critical temperature, the wire should remain superconducting, even when carrying a high current like 300 A.