A Form C reed switch is essentially a single pole double throw reed switch. It is hermetically sealed with 3 leads:
- A common lead
- A normally open lead
- A normally closed lead
When operated the common contact will swing from the normally closed contact to the normally open contact. This is caused by the magnetic field produced by a coil or the magnetic field from a magnet. When the magnetic field is removed the common contact will revert back and come to rest on the normally closed contact.