210 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



choose the driving unit and horn parameters so that one of the response 

 peaks will occur at a predetermined frequency. 



Eard Rubber. H. Peters.' Bibliography. Ind. & Engg. Chem., v. 42, pp. 

 2007-2019, Oct. 1950. 



Abstract — The chemical engineer who picks hard rubber for various 

 special uses usually does so on the basis of its availability, its relatively 

 reasonable cost, its good physical and chemical properties, its ease of machin- 

 ing and fabrication, and its excellent resistance to a great variety of chem- 

 icals. This annual review of 1949, like the previous one in 1948 (32), deals 

 with the above in addition to some fundamental studies on natural and 

 synthetic hard rubbers. Heretofore, these basic investigations were usually 

 centered around hard rubber prepared from natural rubber; now there is a 

 decided interest in hard rubber prepared from the synthetics. While ad- 

 mittedly this interest is at a very low ebb, the trend is definitely upward 

 and probably will continue that way for years to come. 



Holes and Electrons.* W. Shockley.^ Physics Today, v. 3, pp. 16-24, Oct. 

 1950. 



Abstract — Some new experiments in transistor electronics are described 

 here in which concepts suggested by theory have been verified directly by 

 experiment. 



Metallized Paper Capacitors."^ J. R. Weeks.^ I.R.E., Proc, v. 38, pp. 

 1015-1018, Sept., 1950. 



Abstract — Metallized paper capacitors are being introduced into tele- 

 phone apparatus wherever size is of prime importance. It is shown that low- 

 voltage metallized paper capacitors with about half the volume of a foil- 

 paper capacitor of conventional design have about the same characteristics as 

 the latter. Performance data are discussed which indicate that such capaci- 

 tors will give long service when used within their voltage rating and when 

 well-protected against moisture. It is pointed out that this type of capacitor 

 should be used within its voltage rating if sparking with its attendant 

 circuit noise is to be avoided. When sparking does occur due to abnormal 

 voltage conditions no permanent damage results. 



* A reprint of this article may be obtained on request to the editor of the B. S.T.J. 

 1 B.T.L. 



