Field is attempting to understand this in as general a way as possible so as to throw light on the magnetic fields that are so ubiquitous in astronomy. The length and position angle of each line indicate the degree of polarization, P, and the plane of vibration of the electric vector, respectively. As can be seen in the figures, a large-scale magnetic field permeates the interstellar medium of our Galaxy and of the galaxy Messier 51. As far as we can tell, the same is true of all galaxies like our own. This fact presents a puzzle.
Interstellar gas contains enough ion-electron pairs that it can easily carry electric current.
As in a coil of wire, this current creates a magnetic field. Any resistance present causes the current, hence the magnetic field, to decay. But by Faraday's law, the decaying magnetic field induces a voltage that compensates for the resistance, thus slowing the decay of the current and the field. For an electric circuit in the laboratory, this postpones the decay of the field for a few milliseconds. But because the induction effect is proportional to the size of the system, and the resistance is inversely proportional to the size, the time constant for the decay of the magnetic field of a galaxy is vastly greater, some 10 billion times the age of the universe.