The core

The core of the model is usually a permanent magnet made with samarium-cobalt or Alnico. The Alnico provides a greater intensity of

An early one-component magnetic balance made at NASA LARC

magnetization than samarium-cobalt, but is characterized by a more rapid decrease in the intensity of magnetization and a greater sensitivity to shocks.

An alternative is a core consisting of a solenoid made with a super­conducting material (NbTi) wrapped around a core of superconducting material (NIOMAX CN 61/25). Superconducting materials provide virtually zero electrical resistance at extremely low temperatures, in the order of a few degrees K. For this reason, the coil is placed in a cylindrical Dewar vessel containing liquid helium.

Permanent magnets have the advantage of being relatively inexpensive and provide an intensity of magnetization that is practically constant. That means significant simplifications in the calibration procedure of the forces acting on the model as a function of electrical current flowing in the electromagnets. The disadvantage of this type of core is the limited strength of the magnetization vector: this requires, for the same forces and magnetic moments, larger electromagnets and a greater electric power.

The drawbacks of the core made with a superconducting solenoid are:

■ the complexity of building the core and the model;

■ the complexity of the procedure used;

■ operational life of about 70 minutes before the complete evaporation of liquid helium;

■ decline of the current in the solenoid during operation (difficulty in calibration).

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