is passed through the coil 56 in response to a change in the inductance of the coil resulting from a variation in the thickness of the web w a control system 116 puts out a suitable signal which is a measure of thickness and which may be used for example to control the calendar air shower valves advantages inherent in inductive measuring techniques in general are their immunity to noise and their ability to operate in harsh environments however problems arise when applying such techniques to a moving paper web factors such as vibration low web strength high web speeds adhesive coatings embedded slugs and shives lateral positioning errors and wide temperature excursions all contribute to runability problems such as sheet breaks holes or streaking and measurement errors our system provides a solution to such problems when our system is running air under pressure is supplied to the space provided by recess 94 to move the pad or reference platen 92 upwardly into engagement with the under side of the web w to move the upper surface of the web into engagement with the rollers 44 and 46 this engagement of the upper surface of the web is limited to line contact with the rollers 44 and 46 the restriction of contact between the web and the non rolling side of the sensor system to the two lines of closest approach between the rollers 44 and 46 on.