PURPOSE: To suppress a voltage drop and a noise on the surface of a brush, and to obtain a measurement signal having preferable S/N by forming a slide face for the measurement at a part of a collector ring, and bringing the brush for the measurement into slide contact therewith.

CONSTITUTION: A main slide face 1a and a slide face 2 for a measurement are provided on a collector ring 1. A main brush 4 is brought into slide contact with the main slide face 1a. A brush 3 for a measurement is brought into slide contact with the face 2. Thus, it can be used also as a slip ring for a measurement having large S/N without considerably increasing the size and shape of the ring 1.