Electromagnetism relay is one kind uses switch of electro-magnet pilot circuit, its construct and working principle is shown like plan institute. Dominate circuit by power supply of electro-magnet, weak report and telegraph key composition. Comprise working circuit by the contact part of power supply of electromotor, strong report and electromagnetism relay. Close control circuit telegraph key, when there is control electric current to pass in electro-magnet coil, electro-magnet attracts armature, make working circuit contact closes, electromotor is started. Disconnect the telegraph key that controls circuit, electro-magnet loses magnetism, bedspring pulls armature, working circuit is cut off in contact place, electromotor quits the job. The signal circuit that uses electromagnetism relay to be able to use low voltage, weak electricity will dominate the working circuit of tall voltage, strong electric current. The sort of electromagnetism relay is various, but its fundamental is to use what electro-magnet dominates circuit to be connected. Relay has important applying in respect of automata, remote control. CNC Milling