Reed switch is a glass tube containing two iron reeds sealed inside it.
If the iron is unmagnetized, there is a gap between the reeds and no current can flow. This is called a Normally Open Reed (NO)
If a magnet is brought near the reed switch, there is magnetic induction and the two reeds touch each other. Thus, the current can flow and the circuit is complete.
If an extra non-magnetic contact is added to the reed switch, it can be used as a NC (Normally closed) reed switch.
The current can flow until a magnet is brought near. When a magnet is brought near, one of the reed which is in contact with the non-magnetic contact attracts to the other reed and flow of current stops.
