chumbak ki khoj kisne ki vidyut chumbak ki khoj kisne ki thi chumbak ki khoj kab hui chumbak ka avishkar kisne kiya tha chammach ki khoj kisne ki story of magnet in hindi magnet ki kahani chumbak ke prakar magnet cha shodh koni lavla magnet ki khoj kab hui magnet kaise banta hai hindi mai magnet kaise banai jati hai