532 BELL SYSTEM TECHNICAL JOURNAL 



and skill in every step. Ten years ago this technique had not reached 

 even the laboratory stage. Today, in the commercial production of 

 crystal filters, it has become commonplace to deal with capacitances 

 expressed in tenths of a micromicrofarad, transmission losses in hun- 

 dredths of a decibel, crystal dimensions in thousandths of a millimeter 

 and frequency measurements in thousandths of a per cent. The 

 attainment of such precision at moderate shop cost is the primary 

 engineering problem in the manufacture of the higher frequency types 

 of carrier telephone facilities. 



Bibliography 



1. "Electrical Wave Filters Employing Quartz Crystals as Elements," W. P. Mason, 



Bell System Technical Journal, 1934, pages 405-452. 



2. "A Carrier Telephone System for Toll Cables," C. W. Green, E. I. Green, Electrical 



Engineering, 1938, pages 227-236. 



3. "A Twelve-Channel Carrier Telephone System for Open-Wire Lines," B. W. 



Kendall, H. A. Affel, Bell System Technical Journal, 1939, pages 119-142. 



4. "Crystal Channel Filters for the Cable Carrier System," C. E. Lane, Electrical 



Engineering, 1938, pages 245-249. 



