RPTR INSTRUCTOR TEACHING STUDENT FORCED LNDG PRACTICE ALLOWED ACFT TO HIT PWR LINES.

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I WAS INSTRUCTING A STUDENT PLT (PRE-SOLO) IN EMER LNDGS. HE SELECTED A FIELD WHICH WOULD NOT HAVE WORKED OUT IN A REAL SIT SO I TOOK OVER THE CTLS AND HEADED FOR A 'GOOD' FIELD. WE WERE SET UP FOR A GOOD APCH WHICH WOULD HAVE RESULTED IN A GOOD LNDG TO A RECENTLY CUT CORN FIELD. I GAVE THE CTLS OVER TO THE STUDENT AT ABOUT 20-30 FT FOR THE GAR. SHORTLY AFTER WE CONTACTED 2 PWR LINES; I TOOK OVER THE ACFT AND MADE A SUCCESSFUL REAL EMER LNDG IN THE CORN FIELD. WE ENCOUNTERED SOME SPARKS AS WE APPARENTLY BROKE 1 OF THE LINES AND RESTED ON THE FIELD WITH 1 WIRE STILL WRAPPED AROUND THE MAIN GEAR. AFTER ASCERTAINING THAT THE AIRPLANE WAS NOT 'HOT' WE SHUT THE ENG DOWN AND EXITED. WE HAD PULLED THE REMAINING LINE ABOUT 300 FT. ACFT DAMAGE WAS MINIMAL WITH NO INJURIES. I DO NOT DO MUCH INSTRUCTING IN MY PIPER J-5A AND HAVE SPENT LITTLE TIME FLYING FROM THE BACK SEAT. I FEEL THAT BEING IN THE BACK SEAT CONTRIBUTED TO MY NOT SEEING THE PWR LINE. THOSE LINES ARE DEFINITELY HARD TO SEE AT TIMES. WE WERE VERY LUCKY THAT THIS REMAINED A LESSON AND NOT A DOUBLE FUNERAL.