The heating effect in magnetic materials can be eliminated by:

In Eddy Current Testing, the heating effect in magnetic materials occurs due to eddy currents generated within the material when it is subjected to an alternating magnetic field. When a magnetic material is magnetically saturated, the domains within the material are aligned, and the material's permeability decreases significantly. This reduction in permeability minimizes the generation of eddy currents because the material no longer effectively responds to the alternating magnetic field. As a result, the energy lost to heating due to eddy currents is significantly reduced, thereby eliminating the heating effect.

This understanding is critical because controlling the heating effect is essential to obtain accurate test results and prevent damage to the test specimen. The other options do not effectively eliminate the heating effect in the same way. Increasing the displacement rate may lead to less time for eddy currents to develop but doesn't address the root cause of heating. Decreasing the displacement rate would actually allow more time for eddy currents to induce heating, and testing in an air-conditioned room may help in maintaining a stable environment but does not directly affect the generation of eddy currents or their associated heating effects in magnetic materials.