OVERSEAS RADIO EXTENSION 255 



last spriiiii, been coniniercialK' usable about 97 per cent of the 

 operating time. 



The difference in short-wave transmission east and west across the 

 North Atlantic and that across the tropical zone, shown in Pig. 7, 

 is quite in keeping with the general experience of other operating 

 agencies and is already a well recognized fact in short-wave trans- 

 mission. There is obviously a radical difference in the character of 

 the transmission paths involved which requires further survey and 

 analysis. 



Typical Magnetic Storm Effect 



It will be noted from the second diagram of Fig. 7 that the inter- 

 ruption of short-wave transmission across the North Atlantic some- 

 times continues for several days at a time. These periods have been 

 found to correspond to disturbances in the magnetic state of the earth 

 and to be accompanied by the appearance of relatively large differ- 

 ences of electric potential along the earth's surface. Measurements 

 which have been carried out on the strength of electric field received 

 across the Atlantic during such periods and simultaneous records 

 which have been made of earth potentials shed some light on what 

 happens during these periods. 



There is shown in Fig. 8 observations which were made during a 

 major effect of this kind which occurred in July, 1928. Short-wave 

 transmission conditions appeared to have been normal both before 

 and after the occurrence of this effect. The measurements were made 

 at New Southgate, England, upon station WND, one of the radio 

 transmitters at Deal, N. J., used before the present transmitting plant 

 at Lawrenceville was built. The measurements were made on 18,340 

 kc. during the normal hours of daylight operation. The upper curve 

 of the figure shows the variation in received field strength averaged 

 over the daylight hours for each of the several days shown. Below 

 the field strength curve there is plotted a record which was made 

 during this same period of the earth potentials in the vicinity of New 

 York. This is a smoothed transcript of a record taken on a continu- 

 ously operated recorder connected in a grounded wire circuit which 

 extended from New York westward to Reading, Pa., about 100 miles 

 distant. 



It will be observed that the time of minimum field strength coin- 

 cided approximately with the time of maximum earth potential (the 

 small wiggles of earth potential are to be neglected since they are due 

 to disturbances set up by man-made electrical systems). The drop 

 in the strength of the received field will be observed to be large, of 



