I've been making scones this morning which brings up the issue of making measurements. The two photographs below show the same reading. It raises an issue with needle meters. You have to get your head right over the needle to get the true reading. My head was slightly to one side in the first picture. That's called PARALLAX ERROR.
I was reading the top scale which is in grams. The scales divisions are every 20 grams, so I say that the PRECISION = +/- 20 grams. In other words, I could be 20 grams either way. The reading is 40grams but there is some uncertainty because it's hard to be totally certain of the needle's position over the scale. Using technical terms, I say here that the UNCERTAINTY = PRECISION. So my reading is 40 +/- 20 grams and that my PERCENTAGE UNCERTAINTY = 20/40 X 100 = 50%. That's very high.
Advanced note: many physicists say that you can tell when the needle is half way between scale divisions so they say UNCERTAINTY = 1/2 x PRECISION. I've found it easier to teach classes the top version but this second statement is more accurate.
Look at the scale divisions on the left hand scale, which is in millilitres (ml). Scale divisions = +/-50ml, but they are very wide apart so we can claim a greater precision. The bubbles on the top of the milk don't help so I'd estimate my uncertainty as +/-20ml. Reading = 150 +/- 20ml which gives a percentage uncertainty = 20/150 x 100 = 13%.
