Magnetic flux
From Memory Alpha, the free Star Trek reference.
Magnetic flux is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field.
In 2366, when non-corporeal Koinonians drained antimatter from the antimatter pods to use it as energy to create their replica of Marla Aster, Geordi La Forge noticed the drainage because of an unusual magnetic flux reading from the pods. (TNG: "The Bonding")
The Ux-Mal criminals stranded by their people on a moon of Mab-Bu VI were vulnerable to a magnetic flux density that was also present in electromagnetic storms on the moon's surface. (TNG: "Power Play")
