What is a Form C reed contact?

A Form C reed switch is essentially a single pole double throw reed switch.  It is hermetically sealed with 3 leads:

1. A common lead
2. A normally open lead
3. 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.