 The actual measurement of the ratios is done with a mass spectrometer. Like a regular spectrometer breaks light into its constituent parts, a mass spectrometer breaks a mixture of elements with different masses into separate streams for counting. The first step is to take a small liquid sample, a nanogram or so, and convert it into an ionized gas in a vacuum. This step is called thermal ionization. The sample is placed onto a filament which is then heated to a very high temperature. This simultaneously boils the liquid and strips electrons from the sample, making them positively charged. The best filament is rhenium, one of the rarest metals on earth, because it holds onto its own electrons at high temperatures. In this schematic, the electrons are attracted to the positive plate. The remaining positively charged ions are attracted to the negative plate that has a small opening at its center. The ions then flow into a chamber containing a strong magnetic field. The ions will enter into circular curves according to their unique charge to mass ratios. This creates multiple streams with each stream containing the same ions. The mass spectrometer can run for hours capturing these streams and counting the numbers and producing the ratios. And as we have seen, these ratios give us the age of the rock. In our illustration, the counts for uranium-235 and lead-207 came out at 15-1 lead over uranium. This produces an age calculation of 2.84 billion years. The counts for uranium-238 over lead-206 gave us a ratio of 55 to 99 lead over uranium. This produces an age calculation of 2.85 billion years. They are in agreement to within 0.35%. When they agree like this, you know you have a good measurement. This natural cross-check gives geologists an extremely accurate dating method.