Disk Speed Indicator Mk XV for use with Norden Sight
The Disk Speed Indicator Mk XV was used by WWII US Army Air Force and Navy bombardiers to verify and calibrate the disk speed of the Norden Bomb Sight during flight before initiating the bomb run.
The disk speed is the parameter which determined the duration of flight between sighting of the target and automatic release of the aircraft's ordnance. With the bomb sight switched on, the bombardier would affix the top axle to the small female receptacle on the bomb sight, engage the disk clutch on the bomb sight, and observe the needle on the indicator. The needle will rotate clockwise and stop at the bomb sights true disk speed. This value is compared to the disk speed that has been determined by tables of altitude, speed, and ordnance terminal velocity. If the speed on the indicator matches, the bomb sight is correctly set up. If the speed is off, the bombardier adjusts the bomb sight's disk speed until the indicator's reading matches that on the table.
The indicator itself is ~2.25 inches diameter and ~1 inch deep. Its case is ~4.5 inches and 3.5 inches. The indicator itself is in very good cosmetic condition but its operational condition cannot be verified without test equipment. The top of the case is marred from adhesive tape.