Boltwood studied this concept of "radioactive series," and found that lead was always present in uranium and thorium ores. Principles and applications of geochemistry: On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams. The Age of the Earth. United States Geological Survey. They also discovered that compounds containing U and Th emitted more radiation than either element by itself.
U to Pb 4.
4.55 ± 0.05 Gyr
This was a dramatic increase in the estimate of Earth's age for the time. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. Some isotopes are stable and others are radioactive. This causes induced fission of U, as opposed to the spontaneous fission of U. The carbon dating limit lies around 58, to 62, years.