INCKEMENTAL SlIKATll THICKNESS MEASl' KEM ENTS 



305 



to the {)rc)l)0, aiul the otlicr arm to an auxiliary standard of nominal 

 \-ahu' 1.20 fiiiV. The prohc was placed on the surface of a cable moving 

 w ith a speetl of appre)ximately oO feet per minute. The line, along which 

 the probe was sHding over the sheath, was marked for subse(iuent measur- 

 ing purposes. At discrete intervals, the angular position of the probe with 

 respect to the cable circumference was advanced by an angh^ of 00°. 

 The unbalance signals were traced on the recorder chart w ith a standard 

 sensitivity of O.OOo mmF per division. After completion of the cable run, 

 the sheath was stripped and washed (to remove flooding compound) and 

 micrometer measurements weic taken along the probe route at points 

 six inches apart. Subsecjuently these micrometer measurements were 

 plotted in scales equivalent to the recorder chart. 



A t\'pical example of a measurement performed on a cable section ap- 

 proximately 250 feet long (a total of 500 measured points) is shown 

 on Fig. 8. The upper curve represents a photograph of the recorder 

 tracing. The lower cur^'e was obtained by connecting point-to-point 

 actual thickness readings and plotting them on the non-linear vertical 

 scale following the capacitance versus thickness function (similar to that 

 as shown on Fig. 6), to make both charts graphically equivalent. 



From comparison of these graphs a few observations can be made. In 

 fact, these curves represent fundamentally different methods of deriva- 

 tion. The recorder indications are continuous average readings based on 

 an area having a definite width and a length of a few corrugation spaces 



';?, 0.075 



-5 5 1C 15 



CAPACITANCE X 0.005 /i/iF 



Fig. 7 — Capacitance versus thickness measurements of caMe sheathing 

 sample. 



