238 GEOMAGNETIC INVESTIGATIONS 



theories the cause of the Earth's magnetism still remains a mystery, 

 and for this reason it is an important scientific object to measure 

 the change in magnetic force below the Earth's surface. If the force 

 is of the dipole character referred to, it will increase at the rate 

 of about one-half per thousand per kilometre the deeper we go. But ac- 

 cording to Professor Blackett's theory the change will be slightly differ- 

 ent, the vertical part of the magnetic force probably increasing in the 

 same way as a dipole force, the horizontal probably diminishing by about 

 one per thousand per kilometre. In either case it is a question of a minute 

 effect which only highly sensitive magnetometers will be capable of register- 

 ing. As already shown, it is not so important to measure the change above 

 the Earth because, independently of the physical nature of the causes, 

 this follows the same law. Moreover, we already have a means of studying 

 the magnetic force of the Earth at distances several times as great as the 

 Earth's radius in cosmic radiation. This consists of electrically charged 

 particles (the nuclei of hydrogen and other atoms), which when approach- 

 ing the Earth from outer space are deflected by the Earth's magnetism. 



The measuring of magnetic force at great depths was pioneer work for 

 which Denmark was particularly well equipped in view of her long ex- 

 perience in the construction of precision magnetometers, and our plans 

 based on that experience were the result of team-work by a number of 

 scientific institutions. 



Magnetometers which register and record changes in magnetic force 

 have existed for many years and can work to an accuracy of about a 

 gamma, which is a hundred-thousandth of the unit of magnetic induction 

 known as a gauss. Thus in principle the instruments needed for our work 

 were available; what we had to do was to adapt them to our special 

 lequirements. Our main problem was to find a design which would 

 occupy little space and which would operate both when stationary and 

 when in motion, since it would be suspended from a cable which in turn 

 would be suspended from a ship that might be rolling in a swell. 



Our three instruments were mounted on gimbals like ships' compasses. 

 The first measures the vertical component, Z, of the magnetic intensity. 

 The principle, as indicated in the diagram, is that the bulk of Z is 

 compensated by the magnetic force from a fixed, vertical bar magnet at 

 a point where a horizontal registering magnetic needle is pivoted on a 

 horizontal axis. If Z changes — for example, when the instrument is 

 lowered through the sea — the registering magnetic needle will be slightly 

 deflected from its horizontal position. This will be registered in the usual 

 optical manner by the reflection of a ray of light from a mirror mounted 



