Small perturbations in {HEAD ~ the drive voltage} are applied at {TAIL ~ a level} to cause a measurable variation in the sensor output current 
Small perturbations in { HEAD ~ the drive voltage } are applied at { TAIL ~ a level } to cause a measurable variation in the sensor output current
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH