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Using Tisdell’s Geometry Tool as a Platonic protractor

Protractors are a daunting object for many students to deal with, because of their complexity and level of detail. Ultimately though they are important instruments for students to learn to operate. In Plato’s protractor we use the TGT as a model which can strip a complex protractor down into its basic elements so that it is simple enough to be operated mentally by students. The TGT involves all the basic elements of geometry, point line and arc, and only those elements, arranged in an actionable form. The structure of the TGT presents students with the two arms, vertex and opening of angle arranged so they intercept the arc of the circle with the angles arm as the radius of the circle. In Plato’s protractor students are encouraged to operate this mental model as underlying model for the Mathomat protractor. 

Plato was critical of marked instruments such as protractors, recommending the use of compass and straight edge instead. We believe that reducing a protractor to its basic elements, as the TGT does, is something that Plato would have agreed with. We have named this activity series Plato’s protractor in his honor.


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