A magnetic probe was an engineering instrument that could be used to shut off the fuel flow in a starship's matter-antimatter reaction chamber. The polarity of the magnetic probe could be reversed by repositioning the cap on the end of the handle.
Spock suggested that Montgomery Scott use a magnetic probe, in 2268, to cut through the magnetic flow in the reaction chamber and shut off the fuel supplying the matter-antimatter integrator. While performing the task, Scott expressed concern with the probe not precisely matching the magnetic flow, which would result in the destruction of the USS Enterprise. As Scott attempted to disrupt the flow, the magnetic force indicator began to jump, forcing him to ease off.
Spock, coordinating with the ship's computer revealed that the ship, which had recently been through a molecular transporter, was experiencing an outphase condition of .0009. Based on this information, Spock was able to determine that a reverse field was needed to achieve closure, thus requiring Scott to reverse the polarity of the magnetic probe. In attempting to reverse the cap, and with time running out, Scott found it to be jammed. He insisted that Spock initiate the pod jettison system to save the rest of the ship. Spock ignored the request, and in the last seconds, Scott was able to unjam the probe and finish the task, successfully putting the ship back into the corrected phase. (TOS: "That Which Survives")