For 10 points, name this vector field whose field lines technically do not begin or end, but are usually drawn leaving and entering north and south poles.
The tension forceor more precisely the tension force densitydue to this quantity is proportional to this quantity dot grad applied to this quantity. This quantity may be computed using the Jefimenko equation that contains a cross product. The first adiabatic invariant is exploited in a device that traps plasma in a region where this quantity is locally minimal; that device is its namesake "mirror." It is generated by undulators and wigglers, which are used to deflect and beam ultrarelativistic charged particles in synchrotrons. The strength of this divergenceless field due to steady currents may be computed with the Biot-Savart law. For 10 points, name this vector field whose field lines technically do not begin or end, but are usually drawn leaving and entering north and south poles.
The tension forceor more precisely the tension force densitydue to this quantity is proportional to this quantity dot grad applied to this quantity.
magnetic field [or B field; or magnetic flux density; accept H field; accept magnetic field strength; prompt on magnetism or word forms]