Rotagram, class set of 20

The Rotagram is a simple, and highly effective, dynamic protractor design which asks students to measure angle by rotating its movable arm, after locating its vertex over the target angle. 

Angle is a difficult and complex topic for many students. As a result students frequently enter secondary school without a full understanding of angle. A common problem for students is that they often develop two different sets of concepts for angle, one static and the other dynamic. Static angles are found in the corners of objects, as two arms joined at a vertex with a relationship between the arms that is sometimes referred to as "openness". Dynamic angles are formed by turns along a path or by rotations about a point. To measure angle students need to bring these two concepts together. They do this by learning to rotate one arm of an angle until it coincides with the other arm. 

The Rotagram is a physical model for angle measurement. Students learn to locate its vertex on the vertex of a target angle. Students measure angle by moving one arm of the Rotagram so that it coincides with the other arm of the angle being measured.  The three sets of angle lessons for Rotagram written by its designer Geoff Giles can be downloaded here at no charge. These lessons take students through a series of activities with the physical Rotagram that will eventually develop in them the confidence to measure angles in their heads as a mental operation. T