housing halves 36 and 37 are held together by means of threaded fasteners 40 the sensor head 18 is compressed between the two housing halves 36 and 37 by virtue of the threaded fasteners 40 a central wire 42 projects from the end of rotary contactor 32 and is affixed to the plurality of positive plates which make up sensor head 18 see fig3 and 4 these positive plates are designated by the numeral 44 and are electrically connected to positive conductor 22 via wire 42 and rotary contactor 32 a wire 46 connects a plurality of negative plates 48 comprising sensor head 18 to rotary contactor 34 the negative plates 48 are electrically connected to negative conductor 23 via rotary contactor 34 a plurality of dielectric insulator rings 50 are interleaved between positive and negative plates to electrically insulate respective adjacent plates and to provide a known dielectric material between the plates the outer diameter of insulator plates 50 is slightly smaller than the respective outer diameter of positive plates 44 and negative plates 48 fig5 shows a cross section view of an alternative embodiment of the sensor designated sensor 56 sensor 56 is rotatably mounted between legs 14a and 14b by rotary contactors 72 and 74 which are equivalent to the earlier described rotary contactors 32 and 34 fig6 shows a cross section view taken along the lines 6 6 of fig5 wherein sensor 56 is.