Magnetohydrodynamic vernier-pulse

From Memory Alpha, the free Star Trek reference.

A magnetohydrodynamic vernier-pulse was a technological component of the Nomad space probe. (TOS: "The Changeling")

Judging from the name and the probe's mission, this device was presumably utilized for making precise measurements of magnetohydrodynamic activity, the study of electromagnetism through fluids, in determining the existence of life.

It is also possible that it describes the propulsion system of Nomad. Magnetohydrodynamic propulsion creates its propulsive effects from the Lorentz force resulting from plasma flowing through an accelerator. Interestingly, the TNG Technical Manual describes the 24th century impulse engine as basically being a magnetohydrodynamic drive with an added driver coil for reducing the apparent mass.