The masses of ${\mathrm{Pb}}^{208}$ and ${\mathrm{Bi}}^{209}$ have been determined by measuring the time of flight of their ions in a magnetic field. The mass values are: ${\mathrm{Pb}}^{208}$=208.0416±0.0015, ${\mathrm{Bi}}^{209}$=209.0466±0.0015, ${\mathrm{Bi}}^{209}$-${\mathrm{Pb}}^{208}$=1.0050±0.0015.

• Received 8 November 1951

DOI:https://doi.org/10.1103/PhysRev.85.630