138 BELL SYSTEM TECHNICAL JOURNAL 



during the period when the sunset condition intervenes in the trans- 

 mission path. This dip is particularly noticeable in the signal strength 

 curve of Fig. 10. It is not noticeable in Fig. 9, except for the fact that 

 the rise in signal strength corresponding to night conditions in London 

 is delayed until the major part of the transmission path is in darkness. 

 Strength of Received Noise. The variation in the strength of received 

 noise is shown by the noise curves of Figs. 9 and 10. 



1. The diurnal variation of that portion of the noise which is due 

 to atmospheric or "static" disturbances is somewhat obscured by 

 the presence of artificial noise, i.e., noise caused by interference from 

 other stations. The rise in the noise curve at 12 noon is known to 

 be due to artificial interference. In general, however, the large noise 

 values shown to prevail throughout the night in London between 

 about 6 p. m. and 4 a. m. are known to be due to atmospherics. This 

 diurnal variation shows up quite prominently in both figures. 



The maximum noise is reached at 2 a. m. London time. Up to this 

 time the night belt extends over London and a sector of the earth 

 considerably to the east and including Europe, Africa and Asia. The 

 noise begins to drop off shortly thereafter and reaches its minimum 

 at sunrise in London. This could be accounted for on the assumption 

 that the major source of the noise lies considerably to the east of 

 London and that transmission of the stray electric waves to London 

 is gradually diminished in efficiency as daylight overtakes the path 

 of transmission. 



2. The seasonal variation, as shown by a comparison of the noise 

 curve of Fig. 9 with that of Fig. 10, is not so great as is the case with 

 the transmission efficiency of the signal. However, the noise level is 

 noticeably higher during the second period of the tests, as shown 

 by the average curve of Fig. 10, particularly during the night when the 

 maximum noise obtains. 



This indicates that the noise is largely of continental origin lying 

 to the east or south east of London which is in agreement with rough 

 observations made by means of a loop and suggests that the employ- 

 ment of directional antennas would be of considerable advantage. It 

 is expected to include such antennas in the further measurement work. 



In connection with these noise curves it should be noted that what 

 they represent is in reality the strength of a local warbling tone-signal, 

 expressed in terms of equivalent field strength in microvolts, which 

 which is just definitely audible through the noise. The actual value 

 of the noise currents, were they measured by an integrating device such 

 as a thermocouple, for example, would be a number of times larger 

 than indicated. 



