764 DELL SYSTEM TECHNICAL JOURNAL 



This paper gives a brief description of this device, together with a 

 discussion of its possibilities and limitations. A number of echo sup- 

 pressors have been operated on commercial telephone circuits for a 

 considerable period so that their practicability has been demonstrated. 



Recent Commercial Development in Short Wave Transmitters and Re- 

 ceivers.^ S. E. Anderson, L. M. Clement, and G. C. DeCoutouly. 

 This paper describes the transmitter and receiver recently developed 

 for use by the United States Coast Guard. This apparatus is for 

 operation on wave lengths between 100 and 200 meters. In describing 

 the development of the transmitter a short summary of the various 

 circuit considerations is included. The actual transmitter finally 

 developed is also described together with its operating characteristics. 



In considering the radio receiver the various problems to be met in 

 the design of a radio receiver of this character are dealt with at sonic 

 length. The frequency characteristics of the radio receiver, as de- 

 veloped, are shown, and the method of determining them is described 

 in detail. 



The transmitter and receiver performed \cry satisfactorily under 

 conditions more severe than will be met in actual service. 



The Distribution of Initial Velocities Among Thermionic Electrons? 

 L. H. Germer. The method used was to measure the number of 

 electrons from a straight tungsten filament which were able to arri\e 

 at a co-axial cylindrical electrode against various retarding potentials. 

 In order to eliminate certain disturbing factors, particularly photo- 

 electric etTects, this electrode was made in the form of a very fine grid 

 and those electrons passing between the grid wires were collected upon 

 an outside electrode and there measured. A rather complicated inter- 

 mittent heating current arrangement allowed emission from the fila- 

 ment only when its surface was at uniform potential, and insured that 

 the retarding potential had exactly the desired value. A current 

 regulator kept the heating current constant to 1/30 per cent. 



Electrons from Tungsten. Measurements of the variation of electron 

 current with voltage were made at eight different temperatures ranging 

 from 1440°K to 2475°K. Correction was made for the contact poten- 

 tial (lifTerence between filament and grid. At each temperature it was 

 found that, except in the range of voltage where the current was 

 limited by the space charge phenomenon, the current varied with 

 voltage in just the manner calculated upon the assumption that the 

 electrons leave the filament with velocity components distributed 

 according to Maxwell's law for an electron atmosphere in temperature 



• I'riH . of I. U. L:., Vol. 13, No. 4, page 41.^, .August, 1925. 



' I'liysical Review, Vol. 25, No. 6, page 795, June, 1925. 



