Chap. 8] MAGNETIC METHOD 353 



graduatod scale on the back of the case. The helical spring is virtually with- 

 out torsion when the magnetic system is vertical (at right angles to the mag- 

 netic meridian). Then the pointer is moved until the magnetic system is in 

 a horizontal position. If t is the torsional coefficient of the spring, and ^ 

 is the angle through which the pointer was turned to bring, the magnetic 

 system into the horizontal position, MZo = T\p, where M is the magnetic 

 moment of the needle and Zo the "normal" magnetic vertical intensity for 

 which the instrument is adjusted. At another station with the anomaly 

 AZ, a deflection (p occurs, so that MAZ cos (p = np or, with sufficient 

 approximation for small angles, AZ = <p- t/M, where t/M is the scale 

 value of the instrument. The compass case (see Fig. 8-33) is suspended 

 from a fairly long bar terminating in a gimbal suspension on the top. 

 The compass for orienting the case into the magnetic prime vertical is 

 permanently mounted on the upper ring of the Cardan suspension. 



8. Prospecting magnetometers based on other principles include the magne- 

 trons, the slow-speed cyclotrons, and the magnetic torsion balance. Others, 

 such as the iron-induction magnetometers, compass variometers, and 

 earth inductors, will be treated under the heading of instruments intended 

 for regional magnetic surveys. The earth-inductor gradiometer is dis- 

 cussed in connection with the earth inductors. 



The magnetron^ is a special type of diode with a straight axial cathode 

 and a cylindrical anode surrounding it. When a magnetic field is present, 

 the electrons will not travel radially from the cathode to the anode but 

 will be curved into circular paths. After a critical field is reached, the 

 electrons return to the filament without reaching the anode (see Fig. 

 8-34a). The critical field strength appears as an abrupt drop in plate 

 current (see Fig. 8-346). In the measurement of a weak field, as that of 

 the earth, a solenoid is placed around the tube coaxially with the filament, 

 and the current through it is so adjusted that the measurements take place 

 on the critical part of the curve. The critical field strength is Ho = 

 6.72 \/EAj where E is the plate voltage and r the radius of the anode. 

 The magnetron may be used not only for the measurement of the total 

 intensity by adjusting its axis parallel to the lines of force of the earth's 

 field but also for any other component, as only the field in the direction of 

 the axis is effective. The sensitivity of the tube can be increased by 

 regeneration (passing the plate current through an additional solenoid). 

 In practice, the field required to compensate the effect of the unknown 

 field is measured. Rossiger^^ designed an instrument intended for the 



3« Heiland, Physics, 3(1), 18-22 (July, 1932). 



" See A. W. Hull, Phys. Rev., 3, 279-292 (Sept., 1923). M. Rossiger, Zeit. Phys., 

 43(7), 480-488 (1927). Idem, Zeit. Instr., 49(3), 105-113 (1929). 

 8* Zeit. Instr., loc. cit. 



