Abstract: A motion detector and unique switch forming a part thereof in which the switch includes a glass housing with a spherically shaped, concave surface in which a pool of mercury rests and a pair of separated annular contacts extending in parallel planes above the mercury when the parallel planes are horizontal and electrically connected together by the mercury when the planes are tilted to a predetermined angle to the horizontal. The switch is fixedly mounted in a base which also contains electronic components for transmitting to a central receiver and the case can be pivotably attached to a glass pane or the like for movement about an axis, which does not coincide with its center of gravity, by a base having adhesive on a planer surface with a threaded stud extending through the case to engage a wing nut.