A hand tool with which Scott reaches into the Enterprise's matter-antimatter flow to stop a runaway engine reaction in That Which Survives.
The magnetic probe is an engineering tool of the original Enterprise, a hand-held rod used to work directly on the magnetic flow that contains the ship's matter-antimatter reaction. Using it means reaching into the access tube beside the reaction chamber, where a slip would be fatal to the engineer and the ship alike.
It appears in That Which Survives. After the image of Losira sabotages the engines by fusing the emergency bypass control, the Enterprise accelerates out of control and will explode within minutes. Scott crawls into the service tube with the probe to reverse the flow by hand.
Spock stands ready to jettison the section with Scott inside it if time runs out. Scott completes the job with seconds left, and the ship returns to the Kalandan outpost to recover Kirk's landing party.