Continuous Incremental Thickness Meas- 

 urements of Non- Conductive Cable Sheath 



By B. M. WOJCIECHOWSKI 



(Manuscript received August 5, 1953) 



A method has been reccnthj developed for measuring thickness variations 

 of a non-conductive cable sheathing, extruded over a grounded metal jacket. 

 The method translates direct capacitance increments, sensed by probes sliding 

 on the surface of the sheath, into thickness increments. The accuracy of the 

 system based on this method is sufficiently high that the electrical error, 

 which is of the order of a few thousandths of a ^i^iF, can be disregarded. 

 E.xperimental data indicate that accuracy of the new system for absolute 

 thickness measurements of homogeneous samples in stationary conditions is 

 of the order of 0.002". 



The error caused by translating capacitance to thickness depeiuJs on 

 manufacturing elements and process tolerances, and can be evaluated on a 

 statistical basis. Thus incremental measurements of the cable sheath thickness 

 on the production line yield accuracies of the order of 0.003" . 



Application of this method to absolute sheath thickness measurements 

 involves assumptions directly related to calibration and man ufacturing process 

 control. These aspects are rather extraneous to a measuring .system per se, 

 and, therefore, are not within the scope of this paper. 



1 INTRODUCTION 



1.1 The New Cable 



A tj'pe of telephone cable has been developed in which lead is replaced 

 with a polj'ethylene sheath extruded over a metal jacket. Since descrip- 

 tion of various aspects of this development can be found in the technical 

 literature/'"'^'* only some details of the cable construction and pro- 

 duction that are pertinent to the understanding of the new measuring 

 system, will be briefly outlined here. 



The cable core, Fig. 1, is covered with a thin layer of a highly con- 

 ductive metal, such as aliuniiuim,^ or two layers of different metals, such 

 as aluminum and steel,^' ^ sealed longitudinally. To achieve the desired 



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