250 BELL SYSTEM TECHNICAL JOURNAL 



variable and the second is fixed in frequency. The first oscillator is a 

 very stable tuned-circuit type, the proper adjustment of which is 

 maintained through the use of an improved type of synchronizing 

 automatic-tuning-control system. The second oscillator is crystal 

 controlled. Separation of the carrier and sideband is accomplished in 

 the receiver by means of band-pass crystal filters which provide 

 extremely high selectivity. Unusually high stability and selectivity 

 characterize the performance of the receiver. 



Dielectric Constant and Dielectric Loss of Plastics as Related to their 

 Composition.^ W. A. Yager. Data are presented for the frequency 

 variation of the dielectric constant and dielectric loss factor of various 

 plastics over a broad frequency band extending from 1 kc. to 35 mc. 

 The extremely low loss of polystyrene compared to that of polar plastics 

 confirms the theory that a hydrocarbon is inherently more satisfactory 

 from a dielectric point of view. Of the several possible mechanisms of 

 dielectric loss which might account for the high-frequency dielectric 

 absorption observed in polar plastics, the rotation of polar units in 

 the chain and of polar side groups appears most probable. The fact 

 that the loss factor maxima of phenol fibers, phenol fabrics, and phenol 

 or urea formaldehyde molding compounds containing cellulosic fillers 

 occur at essentially the same frequency is viewed as evidence that this 

 dielectric absorption is an intrinsic property of cellulose. Substitution 

 of mineral fillers for cellulose reduces the high-frequency loss to that 

 residing in the polar resin binder. Furthermore, the dielectric loss of 

 mineral-filled molding compounds is less moisture-sensitive. The 

 large increase in dielectric loss at low frequency always found in ma- 

 terials of relatively high free-ion conductivity manifests itself in 

 Duprene, and the humidified phenolic plastics containing cellulose 

 fillers or laminations. 



^Electrochemical Society Preprint No. 74-24, October 12-15, 1938. 



