This may be a simple model but for it to work well it's important that it is WELL MADE!

Use it, as shown, for measuring diameters, or create different scales that allow you to read either the circumference or the area (without calculation!).

All wood is 10mm square.
The main bar is about 28-30cm long, the two jaws are 7cm and the pieces that make up the slide are 4cm and 5cm.
The sides of the slide are card squares, 50mm x 50mm. The triangles that secure the fixed jaw are 4cm x 4cm.
The paper scale should be glued down carefully and the glue left to dry thoroughly before building the slide.
DO NOT let the cards project beyond the edges of the jaws.

The slide has to be built in position, around the bar, so that a snug sliding fit is obtained. This means very, VERY, careful glueing!
(A tiny bit of glue in the wrong place and you'll have a non-sliding slide!)

Because of this the project is best built by older primary school children, though still very appropriate for younger secondary pupils, who will be better able to cope with the creation of more sophisticated scales.