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