1. Absolute uncertainty = ½ resolution
When you make a measurement, there is always uncertainty in your result.
A person who says they are 1.94m tall is unlikely to be exactly that height. They could be a certain amount taller or smaller owing to difficulties in reading the scale, etc.
There are two ways of denoting uncertainty:
Absolute uncertainty – as above, where the uncertainty is given in the same units as the quantity measured. For any analogue scale (such as that of a ruler):
For a set of results/data/measurements (e.g. in a table):
Percentage uncertainty – where the uncertainty is given as a % of the measured quantity.
Absolute uncertainty = ½ resolution
Absolute uncertainty = ½ range
%Uncertainty = (Absolute uncertainty/Measured quantity)x100

2. Uncertainty (cont’d.) There are three causes of uncertainty:
Random uncertainty - Caused by random and unpredictable changes in the experimental conditions. Leads to the results being too big or too small. Repeated measurements can help reduce random uncertainty.
Systematic uncertainty - Caused by faulty equipment or technique. Leads to the results being too big or too small. A graph which is a straight line but does not go through the origin as expected is likely to be due to systematic uncertainty.
Reading uncertainty – Caused by how difficult it is to read a scale can be read. For an analogue scale, the uncertainty is half of the smallest division. For a digital scale, the uncertainty is equal to the resolution of the scale.
Question: Find the reading uncertainty in the devices.
= ½ smallest division (analogue) / = resolution (digital)
Analogue voltmeter: = ½ (1V) = ±0.5V
Digital voltmeter: = resolution = ±0.1V