Circle Drawing Tools, Instruments and Devices
This is a carpenter’s compass. A carpenter’s compass has a locking mechanism that keeps the distance from the center to the outside constant.
This is a short bar compass I made in the metal shop. I had many different lengths of bar compasses in my classroom. You can make a bar compass by buying trammel points.
This is an inexpensive drafting bar compass that I use when I am designing a project or making a plan. It also locks in position making it easy to use. 
This is a circle protractor that I bought at Staples. Circle protractors can be used to draw circles, measure angles, or divide a circle into different parts.
These students are using a circle protractor to design a curve on a road. In this situation, the circle protractor is being used as a tool, not an instrument.