| 摘要 |
The protractor has two concentric scales (1a,1b) and two coincident radii of reference (OA,OB). The graduations (2a,2b) of the two scales show that the outer scale increases in an anticlockwise sense and the inner scale increases in a clockwise direction. The centre about which both scales of the protractor rotate is marked by a target (O). The distinction between the two scales may be made more clear by the use of different materials or colours. Using increasing and decreasing radial distances adjacent to the scales and proportionate to them may also be used to help distinction.
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