How to Use a Protractor

Video Transcript

"Hi, I'm Steve Jones and I'm going to explain how to use a protractor. First of all, what is a protractor? Well, this is a diagram of it, it's simply a semi-circular piece of plastic, with some markings on it, and the markings are significant because this point is very important. So remember this point and this line is extremely important. If you get this bit right, the rest of it is very easy. What we do, to use it we want to measure an angle. So here are two lines. The line here, we'll call line A, or line OA, and this line we're going to call the line OB. Alright? Now the most important thing to measure the angle. This point is where the two lines join. If you do this bit wrong you get in trouble. This point here, and this point here must fit one on top of the other. So this is now moved here to this point, so that we've got the protractor here and the line here, this line is that line and this point is exactly that point. So we put this point precisely where the two lines join and this line precisely along the first line. Then we can read off the angle, because around here we'll find that we've got markings, ten, twenty, thirty, forty, fifty, sixty, seventy, eighty, ninety, and then around to one-hundred and eighty around here. So here I'll count ten, twenty, and I'll see it will say here ten, it will say there twenty and here it will say thirty. So this is between twenty and thirty and you can judge the number of degrees. So this angle will be say, twenty-seven degrees. So if we are very careful we can accurately measure angles using this protractor. But let's repeat once more, it's very important that this point, this index point goes precisely on where the two lines join and this line, which goes also through the index point is precisely in line with one of the lines, the other line, the distance the angle could be measured off on the protractor. So that is how we can use a protractor to measure an angle between two lines."