frequency component which is indicative of the envelope of amplitude modulated signal a this complex signal is applied to low pass filter 46 39 which passes only the low frequency component of the complex signal the resultant low frequency signal b is then applied to indicator 18 39 indicator 18 39 comprises an ammeter which provides a visual indication of the fluctuations in amplitude modulated signal a and therefore provides an indication of the moisture gradient of web 26 while sensing circuit 16 39 is suitable for static or slow moving webs it has serious disadvantages when used to measure rapidly moving webs with rapidly moving webs the resolution of moisture detecting circuit 10 39 is inadequate for most applications particularly the meter ml will indicate the average moisture content of both the glue lines and the linear 30 thereby monitoring bulk moisture rather than a moisture gradient additionally the signals will be affected by glue line width flute spacing the amount of glue liner moisture and other parameters which complicate the monitoring procedure making difficult the accurate detection of the wetness of the discrete glue lines the above identified limitation may be overcome by employing a peak detector circuit 50 which utilizes a peak reading ac meter to avoid the averaging effects of a fast moving web one such moisture detecting circuit 10 34 is illustrated in fig3 an oscillator 12 34 generates a radio frequency signal which is applied across test gap 14 34 the structure of oscillator circuit 12 34 and test gap 14 34 is identical to.