The nullifier core was a component of a Romulan warp engine. A slight misalignment in one of the nullifier cores of a Romulan warbird could cause the ship's artificial quantum singularity power source to show through the cloak as an intermittent polarized magnetic distortion whenever the ship was moving.

In 2369, the engineer aboard the IRW Khazara, a sympathizer with the Romulan underground movement, misaligned a nullifier core at Deanna Troi's behest, to allow the USS Enterprise-D that they were nearby to detect the cloaked vessel. (TNG: "Face of the Enemy")

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