PRESSURE COEFFICIENTS OF CAPACITAXCE 493 



Also, due to non-uniformity of the thickness of the specimens, it was 

 found that the capacitance-pressure coefficient was larger at low pressures 

 than at high pressures. This was apparent!}' due to the initial squeezing 

 out of voids between the electrodes and samples and the difficult}' was 

 removed by applying silver paint electrodes to the sample discs. 



A Western Electric capacitance bridge was used to make capacitance 

 measurements with this cell. The temperature was kept at 25°C and 

 the humidity of the room was maintained at 50 per cent. The frequency 

 used was 10 kc. 



It is believed that the fact that the steatite is much more rigid than 

 the specimens makes this experimental arrangement closely approximate 

 the assumptions made in deriving (8) (i.e. lateral strains are neghgible). 

 It is important, however, that the specimens be cut to fit the steatite 

 ring ^'er^^ closely. 



Experiments which correspond to the cylindrical capacitor under 

 biaxial stress have been performed as follows.* The specimens in this 

 case were lengths of cable which consisted of a copper wire central 

 conductor surrounded by the polyethylene-5 per cent butyl rubber 

 compound, h/a was 3.87 in most of the measurements but some data 

 were obtained for h/a = 4.68 (actual dimensions 0.620"/'0.1G0" and 

 0.750"/ 0.160")- Twenty foot lengths of cable were placed in a long tank 

 provided with a seal at one end. The end of the cable inside the tank 

 was closed such that the center conductor was isolated. The tank was 

 then filled with water which served as the outer conductor, tap water 

 having sufficiently high conductivity. Pressure was applied by the 

 water. 



A Leeds and Northrup capacitance bridge Avas used and the measure- 

 ments reported were made at 10 kc. 



IV. RESULTS 



It is experimentally observed in all the cases considered that plots of 

 C versus P are nearly linear for polyethylene-5 % butyl rubber. This 

 may be shown to be in agreement with the foregoing theories as follows. 

 Equation (8) may be written 



Jc(0) Jo L o€ J3L1 — o'J 



* The investigation of radial compression on cylindrical (cable) specimens was 

 carried out bj- A. W. Lebert and O. D. Grismore of Bell Telephone Laboratories. 



