A hinge has two leaves held together with a hinge pin and each leaf is provided with a recess opening out of the back face thereof. A magnet is completely contained within the recess on the one leaf, while a magnetic reed switch is completely contained within the recess of the other leaf. When the hinge is closed the magnet actuates the magnetic reed switch, but not when the hinge is open, so that it is possible to maintain surveillance over a door from a remote location. Both the magnet and the switch are completely concealed when the door is either opened or closed so that it is impossible to detect their presence or to tamper with them. No special preparation of the jamb or door is required to install the hinge.