This question applies to aircraft being tested, presumably within a certification program.
I've seen what looks like either a ball or "shuttlecock" at the end of a tether attached to the top/rear extremity of an aircraft's vertical fin. An example can be seen toward the end of this video.
What is its purpose? Is is part of or does it contain any instrumentation, or is it there to provide direct visual indication of something - such as trailing airflow patterns?