Two magnetic displacement sensors monitor the relative motions of the tubes and transmit
signals to an electronics module for digital processing. One of those sensor coils may be seen in
the previous photograph. Both the force coil and the sensor coil are nothing more than permanent
magnets surrounded by movable copper wire coils. The main difference between the force coil and
the sensor coil is that the force coil is powered by an AC electric current to impart a vibratory
force to the tubes, whereas the sensor coils are both unpowered so they can detect tube motion by
generating AC voltage signals to be sensed by the electronics module. The force coil is shown in the
left-hand photograph, while one of the two sensor coils appears in the right-hand photograph