a speaker magnet about 4 to 6 Oz will errase tapes but could permently magnitize them rendering them useless.
Answer:
Now you're into the field of electronics and if anyone ever had the nerve to take my advice on anything in that field they'd surely not be happy with the results. Thanx for the information though - I now know more than I did before and probably more than I'll ever put to any profitable use.