12 Compasses Great For Watch Repair & Detecting A Magnetic Field In Watches. This a new set of 12 miniature compasses. Great for detecting the presence of a magnetic field in watches, instruments and other components. The faces glow in the dark. These are extremely sensitive. Each measures approximately 1 9/16" (39 mm) in diameterWe work hard to update our descriptions and pictures of our merchandise. As part of a process of ongoing innovation by our suppliers, newer versions of the item shown may be shipped before we are able to complete an update. As a result there may be a slight variation in color and design.