PRESSURE COEFFICIENTS OF CAPACITANCE 



491 



Equation (8) for the parallel plate arrangement becomes 



IdC 

 C dP 



' (£ - l)(£ + 2) 



3s 



+ 1 



]' 



(26) 



while (13) is unaltered. The difference between the two cases arises 

 from the fact that in (13) the area was allowed to vary while in the former 

 case it was not. The deviation of 



CdP 

 from (26) when a 9^ 0.5 is given by the factor 



The capacitance-pressure coefficient for the cylindrical configura- 

 tion, (25), becomes 



C dP 



2-| 

 1 -- 



{£ - l)(£:+^_^ b2 



Sf 



2 In 



a 



(27) 



The deviation of 



C dP 



from the value given in (27) when o- 9^ 0.5 is thus given by the factor 



7,2 n 



(1 + t) -. 



a^ 



^, + (1 - 2<r) 



III. APPARATUS 



The experimental arrangement employed to investigate the validity 

 of (8) is shown in Fig. 1.* Pressure was applied by means of a Baldwin 

 tensile testing machine. The cell makes use of a "sandwich" arrangement 

 wherein two disc samples (2" in diameter, 0.050" thick) of dielectric are 

 pressed between three brass electrodes, the outer electrodes being 

 grounded. The capacitance thus formed is well shielded and stray capaci- 



This cell was designed by C. A. Bieling. 



