generally indicated as 29 as is best illustrated in fig8 probe assembly 29 comprises a cylindrical rod 31 having a depression 32 proximate one end thereof for receiving fingers 28 of slide 22 the combination of fingers 28 and depression 32 linking slide 22 to rod 31 probe assembly 39 is disposed in a hollow conduit 33 formed in housing 14 rod 31 is adapted to slide in conduit 33 and includes a shoulder 34 against which is abutted extension rod 36 and interior cylinder 37 which sits within hollow conduit 33 abutting the end 31a of extension rod 36 and joined thereto by welding soldering or the like is one end 38a of coil spring 38 the other end 38b of coil spring 38 is joined to one face 39a of a probe support conductive cylinder 39 probe 20 is firmly seated in the probe support conductive cylinder 39 rod 31 extension rod 36 spring 38 conductive cylinder 39 and probe 20 are all of electrically conductive material suitable material being stainless steel copper nickel brass beryllium copper and the like electrical connection between the dc voltage supply 17 and conductive ferrule 13 and the probe assembly 29 is made through first resilient conductor 41 and second resilient conductive 42 which are firmly conductively bonded to flexible leads 18 as by well known means such as soldering or the like as shown in fig3 a and fig3 b the first resilient conductor 41 lies above upper face 23 of slide 22 and second conductor 42 lies below lower major face 24 of slide 22 this configuration is shown in fig4 and 6.