The correlation coefficient is used to find how strong a relationship is between data. We have, as a general rule the following:
1) r=1 indicates a strong positive relationship
2) r= -1 indicates a strong negative relationship
3)r=0 indicates no relationship at all.
In this case we have:
Experiment 1: r=0, therefore, there is no relationship
Experiment 2: r=0.28, here we have a weak linear relationship (since it's closer to 0)
Experiment 3: r=0.82, there is a strong positive relationship
Experiment 4: r=0.73, here we have a somewhat strong positive relationship