12G PROF. A. E. H. LOVE ON THE TRANSMISSION OF 



should not be less than about 400 sea miles (6 40'), or 750 km. (6 45'). For tne 

 purpose of testing it experimentally it is necessary to measure the amplitudes of 

 received antenna currents at various distances exceeding these limits. 



19. So far as I know, the only records of quantitative measurement of received 

 current at sufficiently distant places are contained in the memoir of AUSTIN( U ) and 

 the article by HoGAN( 16 ). AUSTIN'S experiments were performed by transmitting 

 signals between the U.S. station at Brant Rock and two cruisers in the Atlantic, 

 measurements being made of the strength of the signals from shore to ship, from ship 

 to shore, and from ship to ship, at various distances up to 1000 sea miles (1850 km.), 

 by day and by night, during several months in the years 1909-10. The wave-lengths 

 employed were 375 km., 1'5 km., and 1 km. HOGAN'S experiments were performed 

 in the year 1913 between the U.S. station at Arlington, Va., and one of the same two 

 cruisers. The wave-lengths employed were 3'8 km. for the signals sent from the shore 

 station, 2 km. for those sent from the ship. The range of distance was 3000 km. for the 

 shorter waves, and 4250 km. for the longer. The method of observation was to take 

 shunted telephone readings on the incoming signals, the shunt being adjusted to 

 reduce the signals to audibility, and the standard of audibility being that strength of 

 signal which permits a clear differentiation of dots and dashes. AUSTIN'S detectors 

 were of the " free wire electrolytic type," HOGAN generally used the " Fessenden 

 liquid barretter," but " some of the readings at extreme distances were taken upon 

 the heterodyne receiver." HoGAN defines the " audibilty factor " as the ratio 

 (R + S)/S, where II is the impedance of the telephone, and S that of the shunt which, 

 when connected across the terminals of the telephone receiver, reduces the signal 

 intensity to audibility. He states that this ratio is approximately proportional to the 

 square of the received antenna current. AUSTIN records the results of his experiments 

 of July, 1910, in tables. He also records all his results, including these, graphically, 

 by marking on sqxiared paper points whose abscissre are the distances, and ordinates 

 the received antenna currents. HOGAN records his results graphically in a similar 

 way, with the difference that his ordinates are proportional to the values of the 

 audibility factor. Both observers found the results for daylight signalling much more 

 regular than for night. 



20. AUSTIN sought to deduce from his observations the law of decrease of received 

 antenna current with increasing distance. In this he was guided partly by some 

 observations taken in 1905 in the Irish Sea by DUDDELL and TAYLOR, who, he says, 

 found that the received current over water fell off nearly in proportion to the distance. 

 This was to be expected for comparatively short distances. He was also guided partly 

 by the ideas of ZENNECK.( 6 ), referred to in 3 above. Accordingly he compared his 

 results with an expression of the type Ae- D /D, in which D denotes distance from the 

 sending station, and A and a are independent of D, but may depend upon the wave- 

 length. The factor e~ D he described as due to " absorption." The most regular series 

 of his observations taken in July, 1910, were found to fit fairly well the curves that 



