240 



GEOMAGNETIC INVESTIGATIONS 



rotation frequency of the coil ("frequency" meaning the number of rota- 

 tions per second). The intensity of this faint alternating current is pro- 

 portional to the product of the frequency n and H. Here again, however, 

 it is not H itself which we measure but H minus the compensating force 



electric motor ^ 



amplifier 



rotating 

 coil 



etectrom agn e tic 

 recorder 



compensating 

 ^ compass needle 



Principle of the H-coil instrument. 



from a suitably mounted compass needle, this being essential in order to 

 obtain a sufficient degree of accuracy. If this faint current is then am- 

 plified by a special amplifier, the current can be recorded as in a baro- 

 graph. If the instrument is previously adjusted by means of known mag- 

 netic intensities, we shall be able to read the intensity of H on the graph. 

 The essential point of this instrument is that it is possible to design the am- 

 plifier so that the gi\'en amplification is proportional to the current frequency 

 n, equal to the rotation frequency of the coil. In this way we ensure that 

 the registered current is dependent only on the magnetic force H which 

 is to be measured, and not on the rotation frequency n. This is essential, 

 as n cannot be kept constant with the accuracy required in measurements 

 of this kind. 



We knew that the problem of the instruments could be solved in 

 principle, and that it was largely a matter of experimental physics whether 

 the measurements could be made sufficiently accurate for the small varia- 

 tions of, probably, V- — i per thousand per kilometres of depth. But we 

 encountered a serious difficulty when we came to consider the containers, 

 as the instruments could function only in ordinary air and under normal 

 pressure. The containers, which for structural reasons are spherical, must 

 satisfy certain conditions. In the first place they must be made from non- 



