526 foote: pyrometer color screens 



antenna, or again rise nearly in a straight line if the ground con- 

 ditions are poor. This rise may be very rapid in the case of 

 peculiarly poor grounds. For instance, the resistance of the 

 Bureau of Standards antenna rises from 13 ohms at 800 meters 

 wave length to 28 ohms at 2000 meters. Great difficulty has 

 been found in. explaining this increase of resistance with increas- 

 ing wave length, but it is believed that the following explanation 

 is the true one: 



The antenna system must be looked upon, as a condenser, 

 the anten'n.a itself being the upper plate and the ground water 

 the lower plate. Between the ground water and the surface 

 there is usually a layer of semi-conducting material which 

 would correspond to a poor dielectric in. the case of an ordinary 

 condenser. It is well known that the dielectric losses in im- 

 perfect condensers generally increase in proportion to the wave 

 length of the current employed in. the measurement. It is found 

 that by covering the surface of the ground under and around the 

 antenna with a wire net, thus making the net the lower plate of 

 the condenser, the ground losses nearly disappear. 



PHYSICS. — The "center of gravity'' and '^ effective wave length'' 

 of transmission of pyrometer color screens. Paul D. Foot]^, 

 Bureau of Standards. 



Some ten years ago Waidner and Burgess^ called attention to 

 the shift in effective wave length of the color screens used with 

 optical pyrometers, when the temperature of the source sighted 

 upon changes. Pirani^ has contributed two papers upon this 

 subject and in the past month a paper by Hyde, Cady, and 

 Forsythe^ has appeared. 



Heretofore it has been, assumed that the effective wave length 

 is the so-called "center of gravity" of the luminosity curve. 

 Hyde, Cady, and Forsythe have given a new definition of the 

 mean effective wave length between, any two temperatures, 



'Waidner and Burgess. Bureau of Standards Scientific Paper, No. 55, p. 175 

 ' M. V. Pirani. Verh. d. Phys. Ges., 15: 826-838. 19L3; 17: 47-62. 1915. 

 3 Hyde, Cady, and Forsythe. Phys. Review, 6: 70-74. 1915. 



