ABSTRACTS OF RECEXT TECHNICAL PAPERS 387 



main- times larger than ihe negative current. The electron current 

 is in general reduced by the positive rays, but at higher filament 

 temperatures the reduction of space charge by the positive causes a 

 considerable increase of the current over a limited voltage range. 

 By immersing the tube in liquid air the positive ray efTects are almost 

 eliminated, indicating that the alkali metal vapor is the source of the 

 rays, which are probably produced by contact of metal atoms with 

 the hot filament. 



A New Directional Receiving System} H. T. Friis. Reduction 

 of static interference, or to state it more correctly, reduction of the 

 ratio of static to signal, has been, almost since the beginning of the 

 radio art, the most important problem in radio engineering. It is 

 now well known that static disturbances have definite points of 

 origin and that the impulses which are detected at a receiving station 

 ha\-e definite directions of propagation. A receiving system having 

 no directional selectivity is, therefore, affected by static impulses 

 from all directions and, in spite of many inventions, it has not yet 

 been possible to improve its signal-static ratio except by limiting the 

 frequency band transmitted. A system which, however, is so de- 

 signed as freely to receive waves arriving from a limited range of 

 directions is susceptible only to static disturbances propagated within 

 that range, and large improvements in signal static ratio have been 

 claimed for different types of directive antenna systems during the 

 past few years. 



A directional receiving system for radio telephony in which direc- 

 tional selectivity is obtained by combining the output voltages from 

 two antennas is described in this paper. The main feature of the 

 system is the arrangement for controlling the output voltages of the 

 antennas, so that they may be combined to netitralize each other or 

 to reinforce each other as desired. A double detection (super-hetero- 

 dyne) receiver is employed and the output voltages, which are com- 

 bined so as to produce the directional characteristic, are the inter- 

 mediate frequency currents due to the waves received by the an- 

 tennas and the beating oscillator currents. The control of these 

 output voltages is effected by operating upon the beating oscillator 

 currents. 



High-Power Metallography — Some Recent ' Developments iyi Photo- 

 micrography and Metallurgical Research. Franxis F. Lucas. The 



^ Proceedings of the Institute of Radio Engineers, \'o!. 13, No. 6, pp. 685-707, 

 Dec, 1925. 



''Journal of the Franklin Institute, Vol. 201, Xo. 2, pp. 177-216, Feb., 1926. 



