Helicity force line topology

This equation shows a close relation between the wave and particle aspects of the helicity. On the left side, the wave helicity is the semisum of the electric and magnetic helicities that characterizes the topology of the force lines as a function of the linking number of the pairs of electric lines and of the magnetic lines. On... 

One of these topological constants of the motion is the electromagnetic helicity, defined as the semisum of the magnetic and electric helicities, which is equal to the linking number of the force lines... 

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