Disclosed is a linear motor device provided with a movable member, a stator having coils, and a control system for powering selected coils among the coils in accordance with a position of the movable member. For this purpose, the invention teaches a display element 1 based on one or more color-producing surfaces 2 which can be oriented parallel to the viewing direction 3 of an observer, for example, so that the observer views only the narrow edges of the surfaces 2. The present invention also provides helper vectors comprising a BAV sequence essential for encapsidation which is used in a helper virus for propagating recombinant adenovirus.