MAGNETIC METHODS 



233 



reached earlier in the excitation cycle. During the period that the ambient 

 field opposes the excitation flux, the total flux is decreased and saturation 

 is reached at a later period in the excitation cycle. The difference in time 

 between the maximum fields in each strip creates a phase lag and distortion 

 of the wave shape. This unbalance creates a small resultant peak voltage, 

 as shown by the wave shapes at B, The magnitude of these induced pulses 

 is fairly proportional to the impressed ambient field. 



SENSING EUEMENT 



Fig. 115. — Schematic diagram of airborne magnetometer. 



The pulses are applied to a differential amplifier which is equipped with 

 a filter network for eliminating undesirable harmonics and background 

 noise. The output from the differential amplifier is composed of unidirec- 

 tional pulses as indicated at C. These pulses are applied to the input of an 

 infinite impedance detector in the integrator unit. In the integrator unit the 

 series of short unidirectional pulses are converted into a direct current volt- 

 age which is proportional to the energy content of the individual pulses. 

 Thus a relatively pure direct current voltage is derived from the inte- 

 grator unit and is in turn applied to the input of a cathode follower unit. 

 This unit activates the DC graphic recorder which furnishes the data desired 

 as a continuous record of the magnetic field intensity acting on the detector 

 unit. 



Around and coaxial with the core is a secondary winding, which in 

 effect serves as a Helmholtz coil arrangement. Any desired portion of the 

 ambient field acting on the core may be neutralized, and the flux regulated 

 to allow operation at the desired magnetic saturation of the core. This 



