240 TELEGRAPHY ACROSS SPACE. 



Penartk, lie has thus operated. It is not always easy in his experi- 

 ments, particularly in those where earth connections were used, to be 

 certain how much of the effect was due to true induction, and how 

 much to earth induction. But in some of the cases there can be no 

 doubt whatever. An excellent resume of this work was given by him 

 at the Chicago congress in 1893. In this he describes how, in one 

 series of experiments, he laid out on a level plain two one-fourth-mile 

 squares of copper wire insulated with gutta-percha, the distance between 

 the two nearest sides of the two squares being also a quarter of a mile. 

 In this case, using rapidly interrupted or vibratory currents, and a 

 Morse key to break them up into Morse signals, and applying in the 

 other circuit a receiving telephone, conversation in the Morse code 

 could be held readily between the two operators. This arrangement 

 precluded all idea of earth induction. In effect, Mr. Preece was work- 

 ing with a strange species of transformer, of which his two squares 

 constituted respectively the "primary" and the "secondary," the 

 "core" of the transformer being in this case partly of earth and partly 

 of air. Mr. A. W. Heaviside has described an analogous case in which, 

 wishing to establish telephonic communication to the bottom of a 

 colliery in the north of England, he arranged a circuit in a triangular 

 form along galleries about 2\ miles in total length, at a depth of 60 

 fathoms. On the surface of the colliery another circuit was laid out in 

 triangular lines of equal size, over and parallel to the underground 

 line. Here, again, telephonic speech was perfectly clear by induction 

 from line to line, or rather, in this case, from area to area. Each area 

 inclosed something like 700,000 square yards, an ample base area when 

 the distance to be penetrated was but 120 yards. 



Earlier than the date of either of these experiments, the late Mr. 

 Willoughby Smith had shown how, using two coiled circuits of wire at 

 a distance of some yards apart, telephonic messages could be sent 

 across air, or even through walls and floors. 



The greatest distance to which Mr. Preece's experiments upon tele- 

 graph lines have been carried is 40 miles, namely, between the tele- 

 graphic lines that run across the Scottish border by the east and west 

 coasts respectively. Sounds produced in the Newcastle and Jedburg 

 line were distinctly heard on the parallel line at Gretna, though there 

 was no line connecting the two places. Here, however, since both 

 lines used earth returns, it is probable that most of the effect was due 

 to conduction, not to true induction. 



Instruments which operate by means of alternating currents of high 

 frequency, like Mr. Langdon-Davies's phonophore, are peculiarly liable 

 to set up disturbance in other circuits. A single phonophore circuit 

 can be heard in lines a hundred miles away. When this first came to 

 my notice it impressed me greatly; and, coupled in my mind with the 

 Ferranti incident, mentioned above, caused me to offer to one of my 

 financial friends in the city, some eight years ago, to undertake seri- 



