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beam compass[′bēm ‚käm·pəs]
A compass for drawing large circles, consisting of a beam with sliding sockets carrying steel or pencil points.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
An instrument used to draw large circles or arcs of circles for full-sized working drawings; has a long horizontal bar on which two movable heads slide to and fro, one of which carries a pencil, and the other a sharp-pointed pin or tracer, the distance between them determining the radius of the circle.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.