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Question in title. I'd like to draw a perfect circle on the whiteboard using "expo" dry erase markers. Is there a store I could buy such a compass at? Thank you

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I bought the product pictured below from Amazon, around four years ago, for about $10. That exact product is no longer available, but if you're willing to search Amazon for "whiteboard compass", then you'll see a number of similar products.

Whiteboard compass next to foot-long ruler

It's worked satisfactorily for me, although I don't put it through heavy use (frankly, just one day a year in a college algebra class).

I'd previously tried the "jury rigged on a string" method and that did not work well enough for me. First, there was nothing holding the angle of the marker fixed, so it would wiggle around the circle and not match up properly at the end. Secondly, the little bit of winding in the string at the center would also throw off the consistent radius. Third, the pen would tend to slip out of the string.

The compass I got is large (bordering on unwieldy), but I like the fact that it illustrates/resembles a "traditional" compass, so students have a visual sense of what we mean by that term normally. Granted the limited number of times I use it, I'm very happy with that little bit of tradeoff.

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This looks like it should work with any (regularly-sized) dry erase marker, since just the drawing tip fits through the holes: Compass set for a whiteboard.

[Note: I don't want to link directly to the ebay post since those change over time, and I'm not suggesting this seller. It is currently the first item that Google returns when I search "suction cup draw circle whiteboard".]

Alternatively, what about just getting a regular suction cup that will stick to the whiteboard, tying one end of a string to the metal hook (if it has one), and tying the other end of the string around your dry erase marker?

A suction cup with metal hook on end

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    $\begingroup$ +1 for the second suggestion. In fact, if you're demonstrating constructions, ditch the suction cup and just anchor the center of the circle with your thumb. I used to use a "traditional" foot-long compass with a suction cup end, and it would just mess up whatever was written under the center of the circle. (FWIW, I switched to the document camera and use the same compasses the students do now.) $\endgroup$ Oct 10 at 12:14

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