1) Make sure that each student has the SAME protractor. (To avoid having many sizes and types, I purchase a classroom set in the fall when they are on sale.) If each student's protractor is the same, you can teach using the overhead or an Elmo, and everyone can follow along without someone raising their hand to declare that their protractor doesn't look like that! (Since the protractor is clear it works perfectly on the overhead. No special overhead protractor is necessary.)
2) Show how the protractor represents 1/2 of a circle. When two are placed together with the holes aligned, they actually form a circle.
3) Talk about the two scales on the protractor, how they are different, and where they are located. It's important that the students realize that when measuring to start at zero degrees and not at the bottom of the tool. They need to understand that the bottom is actually a ruler.
I use a couple of word abbreviations to help my students remember which scale to use.
4) When the base ray of an angle is pointing to the right, I tell the students to remember RB which stands for Right Below. This means they will use the bottom scale to measure.
5) When the base ray of an angle is pointing to the left, I tell the students to remember LT which are the beginning and ending letters of LefT. This means they will use the top scale to measure the angle.
6) Of course the protractor has to be on the correct side. It's amazing how many students try to measure when the protractor is backwards. All the information is in reverse!
7) Make sure the students line up the hole with the vertex point of the angle, aligning the line on the protractor that extends from the hole, with the base ray. Even if they choose the correct scale, if the protractor is misaligned, the answer will be wrong.
8) Realize that the tools the students use are massed produced, and to expect students to measure to the nearest degree is impossible. To purchase accurate tools such as engineer uses would cost more than any of us are willing to spend!
protractors really do.....They always vote "yes" to donate tractors to farmers!
If you would like supplementary materials for angles, check out these two products: Angles: Hands On Activities or Geometry Vocabulary Crossword Puzzle.