How do two coils look inside walls ?

• Coils driven in series

  • Current provided differentially using two output channels
  • Both are pushing that magnetic field into the wall
  • Same current in identical coils; midpoint stays near zero

• Both coils detect response of course

  • The midpoint is the average
  • One input channel monitors it

• One closer to the metal

  • That coil's voltage is larger
  • Software measures that