Evidence of decay in the earths main magnetic field and the fact that Sir Horace Lamb’s "model" of the earth’s magnetic field indicated that it should decay, have aroused renewed interest in his famous work. However Lambs original treatise are difficult to follow because they are cast in outmoded terminology and draw upon additional are dificult to follow because they are cast in papers that are not well referenced. Because of the great importance of the phenomenon this paper begins with first principles and derives the relevant equations in the terminology of our day and in a more convenient coordinate system. In light of seismic knowledge the earth is assumed to contain a conducting cores of known radius and also assumed to have electric currents. Solutions for the internal and external magnetic fields are derived and show as did Lamb’s solutions that the currents and magnetic fields decay exponentially with time. After evaluating the time constant from existing data the conductivity of the core material is evaluated (assuming a uniform core). A solution is also obtained for the distribution of the current density in the core. Then the total current and Joule heating are solved. The time constant is 1970 year; total current is six billion amperes; and the Joule heating is two hundred million calories per second.