TELEGRAPHY ACROSS SPACE. 



By Silvanus P. Thompson, P. R. S. 



There is no such thing as wireless telegraphy. True, one can send 

 signals for a distance of a yard or two without any wires, but in all the 

 recent successful attempts to telegraph across space, whether by elec- 

 tric waves or by other means, wires are used. They do not indeed run 

 from the sending station to the receiving station, but are used as base 

 lines. For example, in the case of the longest distance yet reached in 

 telegraphing by electric waves — 13 miles over open country — the maxi- 

 mum distance obtained in the recent experiments of Professor Slaby, 

 the length of the wires used as base lines at each end was nearly 1,000 

 feet. As will be seen, in every case, wires or their equivalent are used 

 to serve either as base lines or as base areas in the transmission. 



Setting aside the mediaeval myth of a sympathetic magnetic tele- 

 graph with two mere compass needles to point to letters ranged 

 around a dial, there are three generic methods by which it has been 

 found possible to signal across space without any directly communicat- 

 ing wire or cable. These may be conveniently classified as follows: 

 I. Conduction methods. 

 II. Induction methods. 

 III. Wave methods. 



I. — CONDUCTION METHODS. 



These methods depend upon the use of water or earth as a means of 

 conducting a fraction of the electric current from the base line at the 

 sending end to the base line at the receiving end. 



From the earliest days of telegraphy it has been a familiar fact that 

 either earth or water might serve as a return circuit for an electric 

 current; and, under certain circumstances, that signals could be sent 

 even with an absolute gap in the metallic line, if there were provided, 

 by means of earth or water, a sufficiently good path to enable current 

 in adequate amount to be received beyond the gap in the line. This 

 method has sometimes been called the leakage method, since it depends 

 upon the circumstance that electric currents flowing in a conducting 



1 From Journal of the Society of Arts, Vol. XLVI, London, 1898, pp. 453-460. 



235 



