768 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



db below the free-space field, but are hundreds of db (in one case 700 db) 

 stronger than the value predicted by the classical theory based on a smooth 

 spherical earth with a standard atmosphere. Antenna gains and beam widths 

 are maintained to a first approximation and no long delayed echoes have 

 been found. 



Clark, A. B/ 

 Development of Telephony in the United States, A.I.E.E., Trans., 

 Commun. and Electronics Sect., 71, pp. 348-364, Nov., 1952 (Mono- 

 graph 2045). 



The telephone was invented twenty-four years after the founding of the 

 American Society of Civil Engineers and Architects, which society com- 

 memorated its Centennial in September, 1952. The telephone was eight years 

 old when the American Institute of Electrical Engineers was founded. Since 

 Bell's invention, the telephone business has grown tremendously and this 

 growth has been greatly dependent on developments in science and engineer- 

 ing. This paper traces, and endeavors to give the significance of, the major 

 developments. Brief mention is made of some developments still in the making 

 and of some ideas as to the future potentialities of the business. 



CONWELL, E. M.^ 



Mobility in High Electric Fields, Phys. Rev., 88, pp. 1379-1380, 

 Dec. 15, 1952. 



An extension of conductivity theory to high fields, subject to the usual sim- 

 plifying assumptions, is carried out for the cases in which the change of energy 

 of an electron in a collision can be neglected. This yields a relationship between 

 mobility and relaxation time which is valid over a wide range of fields. 



Davis, K. H.,^ R. Biddulph^ and S. Balashek^ 

 Automatic Recognition of Spoken Digits, J. Acoust. Soc. Am., 24, 

 pp. 637-642, Nov., 1952. 



The recognizer discussed will automatically recognize telephone-quality 

 digits spoken at normal speech rates by a single individual, with an accuracy 

 varying l^etween 97 and 99 per cent. After some preliminary analysis of the 

 speech of any individual, the circuit can be adjusted to deUver a similar ac- 

 curacy on the speech of that individual. The circuit is not, however, in its 

 present configuration, capable of performing equally well on the speech of a 

 series of talkers without recourse to such adjustment. Circuitry involves 

 division of the speech spectrum into two frequency bands, one below and the 

 other alK)ve 900 cps. Axis-crossing counts are then individually made of both 

 band energies to determine the frequency of the maximum syllabic rate 

 energy within each band. Simultaneous two-dimensional frequency portrayal 

 is found to possess recognition significance. Standards are then determined, 

 one for each digit of the ten-digit series, and are built into the recognizer as a 

 form of elemental memory. By means of a series of calculations performed 



' Bell Telephone Laboratories, Inc. 



