Tsaban 18 hta, hpaji ninghkring ni gaw, wan n-gun hte magnetism hpe tsep kawp n seng ai hkum hkrang masa lahkawng ngu sawn la ma ai. 1820 ning hta Denmark mungdan na physicist Johann Gerhard Oersted gaw wan n-gun a magnetic effect hpe mu tam ngut ai hpang, British physicist Michael Faraday gaw 1831 ning hta wan n-gun induction hpe mu tam wa ai. shinggyim masha ni gaw dai hku nna ndai hpaji ningnan ni a majaw wan n-gun prat de shang wa sai. 1830 ning daram hta, American physicist Joseph Henry gaw, wan n-gun hpe jai lang nna, circuit hpe hkang ai lam hpe hkaja nga ai shaloi, relay hpe hpan shapraw wa ai. Shawng nnan na relay ni gaw electromagnetic relay ni rai nna, dai gaw kaga high-voltage, high-current circuit langai a hpaw ai hte pat ai hpe hkang lu na matu, n-gun jaw ai hte n-gun yawm ai shaloi, electromagnet ni hta magnetic n-gun byin pru wa ai hte mat mat ai mabyin hpe jai lang ai. Shi pru wa ai majaw, tsan tsan kaw nna hkang lu ai hte circuit ni hpe makawp maga lu ai. Relay gaw shinggyim hpaji masa labau hta hpaji ningnan kaba langai mi rai nga ai; shi gaw wan jak hpaji a npawt nhpang sha n-ga, wan jak hpaji hte Microelectronics hpaji masa a matu ahkyak ai npawt nhpang mung rai nga ai.
Relay ngu ai gaw, masat da ai shang ai n-gun hpe jaw nna, aten law law hkam la ai shaloi, wan n-gun pru ai circuit hta hkang ai n-gun hpe tau hkrau masat da ai lahkam galai shai shangun ai wan n-gun hpe hkang ai jak langai mi re. Input quantity gaw madang langai ngai de hkrat sum mat nna aten galu hkra hkam la ai shaloi, shawng na masa de bai nhtang wa ai. Shi a madung tara hpe tsaban 19 hta electromagnetism hta byin pru wa ai rawt jat lam ni kaw nna hkan tam mai ai.
