An attempt is made at direct experimental verification of the theory of thermomagnetic instability in composite superconductors under conditions of external magnetic field or transport current variations. The development of thermomagnetic instability in the form of a magnetic flux jump is experimentally studied in a bulk low-temperature composite niobium-tin superconductor. The liquid-helium-cooled sample representing a compressed tape helix (helicoid) is placed in an external magnetic field orthogonal to the turn plane and varying with a constant rate. For the first time, both the magnetic induction inside the sample and its temperature are simultaneously detected in experiments. The sample overhear preceding the magnetic flux jump is measured to be 0.23 + 0.02 K. This value is found to be independent of the rate of the external magnetic field variation and the value of the jump itself and coincides, within the experimental accuracy, with the temperature parameter of electric field buildup involved in the general exponential I-V characteristic of the composite superconductor, which depends on temperature and magnetic induction.
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