MAGNETIC METHODS 103 



tached to the armature. The variometer is cahbrated by producing a 

 known change of field with a Helmholtz coil arrangement. 



Micromagnetometer 

 The micromagnetometer\ originally was used to determine the mag- 

 netic meridian. Its operation depends on the induction produced by the 

 earth's field in bars made of a magnetic alloy of high permeability. The 

 bars are mounted end to end on a horizontal platform with a small air 

 gap between them. A fine wire, which is free to vibrate, is stretched ver- 

 tically between the two bars and energized with a 100 cycle alternating 

 current. For most positions of the bars, the fine wire will vibrate under 

 the action of the magnetic field produced by the induced magnetization of 

 the magnetic bars. If, however, the bars are accurately perpendicular to 

 the earth's field, the induction is zero and the wire will cease to vibrate. 

 It is reported that the instrument will detect directional changes of less 

 than 0.01 degrees. 



The Magnetron as a Prospecting Instrument 



A magnetron is a diode or thermionic tube having a straight axial cathode sur- 

 rounded by a cyHndrical anode, t Its proposed use as a magnetic prospecting instru- 

 ment derives from the fact that in the presence of a magnetic field the electrons do 

 not travel radially from the cathode to the anode. Instead, they spiral around the 

 cathode in circular paths, and after a critical magnetic field intensity is reached, the 

 electrons will return to the cathode without reaching the anode. At this field strength, 

 the plate current will drop abruptly. 



The procedure in operating the instrument consists in decreasing the plate voltage 

 on the diode until a voltage is reached at which the current falls oflF rapidly. The plate 

 voltage at which this occurs is related to the critical field strength H by the relation 



^_ 672VE 

 r 



where r is the radius of the anode. In practice, a compensation procedure is used, 

 wherein the field to be measured is nullified by a known field produced by a Helmholtz 

 coil arrangement. 



The magnetron is affected only by the component of the earth's field which is 

 parallel to its axis. Hence, the instrument theoretically may be used to measure any 

 component of the earth's field by suitable orientation. The sensitivity of the tube may 

 be increased through regeneration by passing the plate current through an additional 

 solenoid. Interesting experimental results have been obtained by using magnetic alloy 

 field pieces to increase the effective magnetic field. 



Distribution of Earth Magnetism 



The importance of and the need for accurate information on the distri- 

 bution of the earth's magnetism became clear from the work of Gauss. 

 He set up one of the first magnetic observatories in Gottingen (Germany) 

 in 1832, and there developed suitable instruments for measuring variations 



t F. Rieber, "A New Micromagnetometer," A.I.M.E. Geophysical Prospecting, 1929, pp. 409-415. 

 t A. W. Hull, Phys. Rev. 22 (Second Series), 1923, pp. 279-292 



M. Rossiger, Zeits. fur Physik, 43, 1927, pp. 480-488 



yi-Rosslg^T, Zeits. fUr Instrumentenkunde, 49, 1929, pp. 105-113. 



