The unsigned magnetic flux linking earth's surface is evaluated numerically from a model to two days of MAGSAT data and found to be 16.054 GWb. Comparison with values calculated from earlier geomagnetic field models dating back to 1930 reveals a smooth, monotonic and recently accelerating decrease in earth's pole-strength at a fifty year average rate of 8.3 MWb/year or 0.052% per year. Hide's magnetic technique for finding the radius of earth's electrically conducting core is tested in two ways. Main field models for 1960 and 1965 are first extrapolated downward through the nearly insulting mantle and then separately compared to equivalent extrapolated models of MAGSAT data. For each of these two earlier epochs, the unsigned flux is found to equal the MAGSAT value at a unique radius which is within 2% of the seismic core radius. Main field models at the earlier epochs 1950, 1940, and 1930, are then derived from the 1960 main field, secular variation and secular acceleration coefficients of the GSFC 12/66 model. Truncation at degree N = 8 and comparison with an equivalent MAGSAT model again yields magnetic values of the core radius within 2% of the seismic value. These results support the customary geophysical approximations that the mantle is nearly a perfect insulator and the core a perfect conductor on the decade time scale.