No abstract

The behaviour of two disk dynamos coupled to one another is examined in relation to the earth's magnetic field. It is found that reversals of electric current and magnetic field occur, unlike the case of a single disk dynamo.On assuming a simple engine that drives a disk dynamo, the coupling between electro-magnetism, motion and heat is discussed. A possible range of temperature fluctuation associated with a dynamo action is also estimated very roughly.

The main object of the paper is to discuss the possibility of a body of homogeneous fluid acting as a self-exciting dynamo. The discussion is for the most part confined to the solution of Maxwell’s equations for a sphere of electrically conducting fluid in which there are specified velocities. Solutions are obtained by expanding the velocity and the fields in spherical harmonics to give a set of simultaneous linear differential equations which are solved by numerical methods. Solutions exist when harmonics up to degree four are included. The convergence of the solutions when more harmonics are included is discussed, but convergence has not been proved. The simultaneous solution of Maxwell’s equations and the hydrodynamic equations has not been attempted, but a velocity system has been chosen that seems reasonable from a dynamical point of view. A parameter in the velocity system has been adjusted to satisfy the conservation of angular momentum in a rough way. Orders of magnitude are derived for a number of quantities connected with the dynamo theory of terrestrial magnetism. It is concluded that the dynamo theory does provide a self-consistent account of the origin of the earth’s magnetic field and raises no insuperable difficulties in other directions.

The time for which it is necessary to leave a bore hole in order that the disturbance of temperature due to drilling may have disappeared is discussed.During the drilling of a bore hole the temperature is disturbed by the heat generated by the tool and by the circulation of the drilling fluid. Of these two causes the latter has the larger effect and results in the lower part of the hole being cooled and the upper part heated. If temperatures measured in the hole are to be used for determining the geothermal gradient it is necessary that the well be left long enough for these disturbances to have subsided. This has long been recognized, but little attention seems to have been given to the estimation of the minimum time required.As the hole approaches a point on its axis the temperature will start to fall, then, as the bottom passes the point, the fall will reach its maximum value. Thereafter it will be warmed by the drilling fluid ascending from the bottom of the bore and