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