Any acceptable case material

An electromechanical or electronic (includes solid state semiconductor items) device designed to convert an electrical signal into a modified square wave output signal. For items excited by an alternating current, the output signal is the same frequency as, and bears a definite phase relationship to, the excitation voltage. For those electronic items designed to operate on direct current only, the output frequency of the item is dependent upon the inherent properties of the item, or the internal or external circuit constants, or the applied driving coil voltage impressed on the input signal. For an electromechanical item whose coil is designed for direct current only, see vibrator, interrupter and vibrator, self-rectifying.