Spare roll of special black spark paper used on Air Tracks for spark recording.
It is placed under the white paper to leave a black dot on the underside of the white paper. As a spark passes through both papers to earth, a black dot is erupted to the underside of this white paper, thus forming a set of black dots that relate to velocity.
Can be used for many runs before it becomes exhausted. 20mm wide x 10m long.