GEOMAGNETIC INVESTIGATIONS 



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magnetic material, and in the second from material strong enough to 

 withstand the enormous pressures to which they would be subjected. For 

 our spherical containers the maximum stresses, corresponding to a water 

 depth of 10,000 metres and thus a pressure of 1,000 atmospheres, are 



Workshop drawing of single and double sphere. 



about 25 kilograms per square millimetre. In other words, the limits of 

 elastic proportionality of the material (the point at which the pressure 

 of the water and the compression of the material cease to be proportional) 

 must be of at least that order. Thirdly, the spheres must be formed from 

 two hemispheres which may be easily opened and closed and which will 

 be fully watertight. 



We tested a number of materials for these properties with little success, 

 as the strength of most known alloys depends on the very fact that they 

 contain a small quantity of iron. Stainless steel seemed to be the most 

 suitable material, being only slightly magnetic; but unfortunately its 

 magnetism increases with increasing pressure, and so we were afraid to 

 use it. A further objection was that it would hardly have been strong 

 enough; the total weight of instruments and container had to be restricted 



