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Within a circuit: potential difference is the relative voltage measured between two arbitrary nodes electric potential is the absolute voltage measured on one arbitrary node relative to the ground node Relative density or density index is the ratio of the difference between the void ratios of a cohesionless soil in its loosest state and existing natural state to the difference between its void ratio in the loosest and densest states.
Commonly abbreviated as RR, relative risk/ratio is measure of absolute risk in one population as a proportion of absolute risk in another.
For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.
Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.
These use radioactive minerals in rocks as geological clocks.
These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.