A calculation on magnetic instability of superconductor has been made on the basis of a thermal differential equation. Taking magnetic and thermal quasi-equilibrium conditions in superconductor in an increasing magnetic field, the author derived the upper limit of the equilibrium,
Bfj which shows the same value of stabilization limit already derived by Swartz and Bean with a method of infinite incremental permeability.
By means of a numerical method on the thermal differential equation, the increase of temperature of superconductor was calculated in function of the increase of an applied field,
Ba. This calculation gave
Bfj with the parameters of
dBa/
dt, cooling conditions and copper ratio in superconductor. The calculated value of
Bfj for no cooling condition agrees with experimentally measured flux jump level.
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