124 BELL SYSTEM TECHNICAL JOURNAL 



"mike" read to the ten-thousandth of an inch and could be estimated to 

 one-fifth of this. Initially, by means of the amplitude control, the micro- 

 ammeter deflection had been adjusted to the desired scale value at the reso- 

 nance point. The traverse was observed between the two -v/2 microam- 

 meter deflections and was repeated in the opposite direction. When 

 successive round trips showed consistency the value of d' was noted. The 

 dielectric sample, after thickness measurement, was then introduced, cen- 

 tered by cut and try and the Q traverses repeated. This gave d and, 

 after noting At, the change in plunger setting for resonance, the measure- 

 ment was complete. 



During the measurement the generator had to be protected from drafts 

 and, usually, it was necessary to traverse rapidly, the power line voltage not 

 being stable. Settings could usually be reproduced to 1 per cent, with ade- 

 quate care. A sample observation on a good dielectric is the following: 



July 28, 1941 Polystyrene plate, all dimensions in cms. 



/ = 1.28 d' = .0084 X = 22.42 

 At = 1.79 d = .010 P.F. = .00028, e = 2.49 



The dielectric samples were machined on a precision lathe, dimensions 



being held to .001 inch. The nominal dimensions were O.D. .640 inch, I.D. 



.174 inch. A favorable thickness, from the standpoint of ease of measure- 



X I 



in em's. Cleanliness in handling was carefully observed. 



ment, is 



lOe 



After a lapse of several days the interior bearing surfaces of the resonance 

 cavity would have to be cleaned with fine French crocus cloth. The plun- 

 ger bearing surfaces also had to be smoothed up, fine scratches being polished 

 off. Dirt was immediately noticeable when the plunger contacted it, and 

 when microscopic bits of silver were rolled up under the plunger springs 

 cleaning was necessary. Othenvise no particular treatment or smoothing 

 up of the contacting surfaces was required. 



A table of dielectric power factors and constants is a very desirable piece 

 of information. Unfortunately, experience tends to the conclusion that 

 such a table does not exist. The organic plastics in particular, are rather 

 variable from sample to sample and a table of values is merely a table for 

 particular specimens. Where a great number of samples are available 

 "best", "worst" and "most common" values can be established. The 

 accompanying list of observed values must be interpreted in the light of the 

 above statements. 



As a large number of measurements of certain special materials had to 

 be made, dielectrics in general were rather neglected and the tabulated 

 values are more or less incidental. It was noted that for the low loss, sub- 



