That was my mistake,
I have seen programs on T.V. when an alarm company will install an alarm with Two positions, Window closed and Window partially open. I thought that it would be great to be able to install an alarm which would allow me to have windows open and secure. Yet, I cannot find a D.I.Y. system, that allows the kind of flexibillity I am looking for, and I cannot afford to have company install one. Maybe X-10 can sell the sensor magnets individualy.