U.S. Naval Observatory, Washington. Motor-driven impersonal micrometer of 6-inch [15 cm.] transit circle (titre original)

U.S. Naval Observatory, Washington. Armature and field magnets of two-phase synchronous motor which drives micrometer (titre original)

U.S. Naval Observatory, Washington. Auxiliary apparatus, consisting of motor-generator with adjustable speed control and phase shifter, which supplies current for the synchronous motor (titre original)