Installing a magnetic contact doesn't have to be a hassle! The US-565 contact set is comprised of a stubby, 3/8" sensor with wire leads and a long 3/8" magnet. This configuration is excellent for doors with limited space in the frame. The magnet and sensor press into 3/8" holes and Rhodium metal contacts and rare earth magnets provide reliable monitoring at a gap of up to .75 inches. This type of contact switch is for normally closed circuits; this means that the when the magnets are lined up, the switch is closed. This type of setup is ideal for monitoring doors, windows, and sliding doors. The magnetic contact is wired to most hardwired security systems to notify you when the contact is broken - when a door is opened, window is opened, etc.