The magnetic field strength in the center of three kinds of superconducting solenoids wound with 3Nb-Zr alloy wire was measured through the course of a gradual variation of the external magnetic field. The manner of a discontinuous change of a magnetic field, viz. the flux jump which gives a clue to clarify the behavior of penetration and trapping of magnetic filaments through the solenoid was studied. In general, the step height of flux jumps becomes smaller as the external magnetic field becomes stronger
