▁and ▁oscill ates ▁at ▁one ▁of ▁its ▁reson ant ▁frequencies . ▁Param et ric ▁exc itation ▁diff ers ▁from ▁forcing ▁since ▁the ▁action ▁appears ▁as ▁a ▁time ▁varying ▁modification ▁on ▁a ▁system ▁parameter . ▁ ▁History ▁Param et ric ▁oscill ations ▁were ▁first ▁noticed ▁in ▁mechan ics . ▁Michael ▁Far ad ay ▁( 1 8 3 1 ) ▁was ▁the ▁first ▁to ▁notice ▁oscill ations ▁of ▁one ▁frequency ▁being ▁excited ▁by ▁forces ▁of ▁double ▁the ▁frequency , ▁in ▁the ▁cr isp ations ▁( ru ff led ▁surface ▁waves ) ▁observed ▁in ▁a ▁wine ▁glass ▁excited ▁to ▁" sing ". ▁Franz ▁Mel de ▁( 1 8 6 0 ) ▁generated ▁paramet ric ▁oscill ations ▁in ▁a ▁string ▁by ▁employ ing ▁a ▁tun ing ▁fork ▁to ▁period ically ▁vary ▁the ▁t ension ▁at ▁twice ▁the ▁reson ance ▁frequency ▁of ▁the ▁string . ▁Param et ric ▁oscill ation ▁was ▁first ▁treated ▁as ▁a ▁general ▁phenomen on ▁by ▁Ray le igh ▁( 1 8 8 3 , 1 8 8 7 ). ▁ ▁One ▁of ▁the ▁first ▁to ▁apply ▁the ▁concept ▁to ▁electric ▁circ uits ▁was ▁George ▁Francis ▁Fitz G er ald , ▁who ▁in ▁ 1 8 9 2 ▁tried ▁to ▁ex cite ▁oscill ations ▁in ▁an ▁L C ▁circuit ▁by ▁p ump ing ▁it ▁with ▁a ▁varying ▁induct ance ▁provided ▁by ▁a ▁dynam o . ▁ ▁Param et ric ▁ampl ifiers ▁( param ps ) ▁were ▁first ▁used ▁in ▁ 1 9 1 3 - 1 9 1 5 ▁for ▁radio ▁tele phony ▁from ▁Berlin