GEOMAGNETIC INVESTIGATIONS 



Bj Niels Arley 



Although we have been navigating by compass for many centuries, we 

 are still ignorant of the cause of the Earth's magnetism. We are equally 

 ignorant of why both the sun and certain stars are magnetic, a fact which 

 can be demonstrated by analyzing their emission spectra. All we know 

 is that terrestrial magnetism is active both on the Earth's surface, at alti- 

 tudes within the reach of aircraft, balloons, and rockets, and in bore 

 holes and mine shafts, and consequently that it is not merely a local or 

 surface phenomenon. 



The magnetic force of the Earth may be described as that of a bar 

 magnet, a dipole magnetism, as it is called; and it can be shown, as was 

 first shown by Gauss at the beginning of the nineteenth century, that 

 most of that which causes the magnetism of the Earth is located inside 

 the Earth itself. But whether the interior of the Earth is uniformly magne- 

 tized hke a bar magnet or whether that which induces the magnetic force 

 is concentrated in the core we do not know, any more than we know the 

 causes. Over a large field outside the seat of the causes, and independently 

 of their physical nature, we find that the magnetic force varies in the 

 manner characteristic of dipole magnetic forces ; that is to say, it diminishes 

 in the ratio of one divided by the third power of the distance from the 

 Earth's centre. This has recently been confirmed in the United States by 

 measurements made up to an altitude af 120 kilometres by means of 

 magnetometers sent up by rocket. If we wish to study the various theories 

 about the cause of terrestrial magnetism we must make detailed and exact 

 measurements of the magnetic force below the Earth's surface. Such 

 measurements have been made in mine shafts, but as these are only a 

 kilometre or two deep and are invariably surrounded by rocks which are 

 more or less magnetic, the measurements are not very reliable. In the sea 

 we can go down to greater depths and will encounter fewer disturbances, 

 provided of course that we do not go too close to the bottom. 



Measurements of this kind acquired special interest during the planning 

 of the Galathea Expedition when, in 1947, Professor P. M. S. Blackett 

 put forward an epoch-making new theory which, if correct, would explain 

 at one and the same time the magnetism of the Earth, the sun, and the 

 stars. According to his theory, any body which rotated would be- 

 come magnetic. Pending confirmation of this or one of the earlier 



