A plot is constructed of Pb-206/Pb-204 versus Pb-207/Pb-204.
If the solar system formed from a common pool of matter, which was uniformly distributed in terms of Pb isotope ratios, then the initial plots for all objects from that pool of matter would fall on a single point.
Over time, the amounts of Pb-206 and Pb-207 will change in some samples, as these isotopes are decay end-products of uranium decay (U-238 decays to Pb-206, and U-235 decays to Pb-207).
This causes the data points to separate from each other.
After the second half-life has elapsed, yet another 50% of the remaining parent isotope will decay into daughter isotopes, and so on.
For all practical purposes, the original isotope is considered extinct after 6 half-life intervals. A small portion of a meteorite is vaporized in the device forming ions.
And from the slope of the line we can compute the amount of time which has passed since the pool of matter became separated into individual objects.
There are some things that affect these measurements.Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months.When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings.The most direct means for calculating the Earth's age is a Pb/Pb isochron age, derived from samples of the Earth and meteorites.This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).