174 BELL SYSTEM TECHNICAL JOURNAL 



mission is a pretty definite proposition, following closely the 

 Austin-Cohen formula. The night transmission appears in the 

 nature of a "bob-up" from the day condition but seems to be 

 limited in the extent of its "come-back" by the loss imposed 

 by the simple inverse-with-distance law. The fact that the 

 difference between curves A and B is entirely one of absorption 

 suggests that the very large and rapid night fluctuations, 

 which are now so well known to broadcast listeners, may be 

 explained in large part if not in whole, by variations in atmos- 

 pheric absorption. 



Occasional Long Distance Transmissions 



Many of the long distance records which have been made on short 

 waves and low power can be accounted for simply on this basis — 

 that the absorption which ordinarily obtains during daylight has 

 been temporarily wiped out. The way in which it is possible for the 

 range to "open up" tremendously under exceptionally favorable 

 conditions will be seen from this: Referring to Fig. 20, assume that 

 the normal daylight range between S. S. America and New York was 

 250 miles (400 km.) as fixed by a limit taken as 200 micro-volts per 

 meter. Then, at night, this same field strength may be delivered over 

 a distance of about 700 miles (1,100 km.) if the absorption is wiped out 

 in accordance with curve B. 



Furthermore, so favorable is the simple spreading-out law at such 

 distances, that the field strength is only halved in going another 700 

 miles to 1,400 miles (2,200 km.) and only halved again in doubling 

 this distance to 2,800 miles (4,500 km.), and so on. In other words 

 under no-absorption conditions, by increasing the receiving radio 

 frequency amplification by a current ratio of only ^2X^2 = /^, or 

 about 12 power radio units, the range of transmission may be increased 

 from the reliable daylight range of 250 miles (400 km.) to a possible 

 night range of ten times this distance. It is therefore seen that 

 many if not all of the long distance transmissions which have been 

 realized for short periods of time probably can be explained simply on 

 the basis of there having occurred an exceptional clearing up of 

 absorption at a time of unusually favorable interference conditions. 



Setting Up and Operating Combined Radio-Wire 

 Circuits 



The operating problems presented by the combination wire-radio 

 telephone system are more difficult than those involved in the oper- 

 ation of either a straight telephone toll line on the one hand or the 



