{HEAD ~ Didactic device} to aid in understanding and {TAIL ~ teaching} about electromagnetic fields and their effects 
{ HEAD ~ Didactic device } to aid in understanding and { TAIL ~ teaching } about electromagnetic fields and their effects
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH