Don't think of it as a sensor. In fact, the answer is so simple, you could even make a circut of it with only TWO components!
P.S. Considering your ID is ultimatekiller, I wonder if this is a joke :lol: because it kills 1/2 of us, the fact that you don't know the answer to that question.
BUT if you are so honest, that you don't know the answer, take the hint above.
How to detect when a wire loop is broken by a burglar.
The wire loop shorts to ground the base of a transistor or input to a logic gate. When the loop is broken then a pull-up resistor turns-on the transistor or logic gate. Simple.