It is normal open, when the sensor is far way from the magnetic, contact will be closed.

Magnetic is the part on left side of the photo
Sensor is the part on right side of the photo

Example to apply : Connect two part on the door / window, when someone open the dorr ziren will sound.

Size of magnetic and sensor are the same as follows.
2.5" x 0.5" x 0.5" ( Height )