A method has recently been proposed for finding the radiusrcof the electrically-conducting fluid core of a planet of outer radiusrsfrom observations of the magnetic field B in the accessible region near or well above the surface of the planet (Hide, 1978). The method is based on the supposition that when the magnetic field is produced by hydromagnetic dynamo action in the core, implying that the magnetic Reynolds numberRthere is large, (a) fluctuations in B can occur everywhere on the comparatively short advective time-scale τAassociated with fluid motions in the core and so can fluctuations in the quantityN, defined for any closed surfaceSas the total number of intersection of magnetic lines of force withS, provided thatSlies well outside the core, but (b) at the surface of the core, where lines of magnetic force emerge from their region of origin, concomitant fluctuations inNare negligibly small, of the order of τA/τOwhere τO(= RτA) is the Ohmic decay time of the core.