Worm 2 rotates about fixed axis A-A and meshes with worm wheel 3 which rotates about fixed axis B-B. Drum dial 1 is rigidly mounted on the worm shaft and drum dial 4 on the wheel shaft. Whole revolutions of worm 2 are indicated on dial 4. Fractional revolutions of worm 2 are read off on dial 1 by means of vernier a.
$2835$WG,M$