GEOMAGNETIC INVESTIGATIONS 



239 



on the needle, and will then be recorded on a strip of film rotated by 

 clockwork. In order to convert the deflection to the intensity of the mea- 

 sured magnetic force, the instrument is previously adjusted by means of 

 known magnetic intensities. 



light source 



photographic 

 recording paper 



recording needle 

 with mirror 



compensating compass 

 magnet 



recording needle 

 with mirror 



compensating 



compass 



needle 



Principle of the ^-needle instrument. 



* tight source 

 Principle of the H-needle instrument. 



The two other instruments are for measuring the horizontal part, H, of 

 the Earth's magnetism. The principle of one of these is shown in the 

 diagram, and resembles that of the Z instrument. Most of H is compen- 

 sated by a horizontal magnet at a point where a vertical registering mag- 

 netic needle is pivoted on a horizontal axis. If H changes - — • for example, 

 when the instrument is lowered through the sea — the vertical registering 

 needle will be defected from the vertical in the manner described above. 

 If the needle swung the whole time in the vertical plane of the magnetic 

 force, the magnetic North-South direction (the magnetic meridian), it 

 would be possible to read the intensity of H on the film strip, provided 

 that the instrument had been previously adjusted by means of known 

 magnetic intensities. The difficulty is to ensure a recording in the meridian. 

 For this reason the compensating magnet is pivoted so that is also func- 

 tions as a compass. The whole instrument is, furthermore, slowly rotated 

 by clockwork, the compass needle shading the ray of light when the 

 meridian is passed, so that the position of this can be read on the strip 

 of film. 



In the other H instrument, shown in the second diagram, a small coil 

 is rapidly rotated (in our case 15 — 25 times a second) round a vertical 

 axis. The horizontal component, H, of the Earth's magnetism will then 

 induce an alternating current in the coil, with a frequency n equal to the 



