TELEGRAPHY ACROSS SPACE. 243 



small glass tube partly filled with loose metallic filings — iron or nickel 

 by preference — joined in the circuit. Such a coherer acts as a species 

 of relay, by means of which an electric wave, incapable in itself of 

 affecting a galvanometer or other instrument, is enabled to do so indi- 

 rectly by setting into operation a local current. Alter the coherer has 

 thus operated, it usually remains in the conductive state until subjected 

 to some mechanical jar or shock. Lodge proposed to apply for this 

 purpose a mechanical taper worked either by clockwork or by a tremb- 

 ling electric mechanism. On several occasions, and notably at Oxford, 

 in 1894, he showed how such coherers could be used in transmitting 

 telegraphic signals to a distance. He showed that they would work 

 through solid walls. Lodge's greatest distance at that time had not 

 exceeded some 100 or 150 yards. Communication was thus made 

 between the University Museum and the adjacent building of the Clar- 

 endon Laboratory. For more than eighteen months the Eev. F. 

 Jervis Smith, of Oxford, using a carbon-powder coherer, has maintained 

 communication between his house and the Millard Engineering Labora- 

 tory, over a mile away. 



Even before this Mr. Nikola Tesla, in a lecture delivered at St. Louis 

 in 1893, had made a further suggestion of great importance. He 

 proposed to transmit electric energy by oscillations to any distance, 

 without communicating wires, by erecting at each end of the stretch a 

 vertical conductor joined at its lower part to the earth, and at its upper 

 to a conducting body of a large surface. This constitutes a vertical 

 base line from which to disseminate the oscillating disturbances. 



About two years ago a young Italian, Mr. Marconi, came to this 

 country and succeeded in inducing the British telegraph department 

 to give him facilities for experimenting upon wave- method of trans- 

 mission. First upon Salisbury Plain, and then across the Bristol 

 Channel, he succeeded in transmitting Morse signals to a greater 

 distance than anyone had previously attained. He sent signals from 

 Lavernock Point to Bream Down — about 9 miles, as the crow flies, over 

 the open channel. To accomplish this he used as base lines two ver- 

 tical conductors earthed at their lower ends and carrying at the top 

 extended surfaces. He used a Righi transmitter. As receiver he 

 employed the special form of Lodge-Branly coherer, presently to be 

 described. This was connected in the manner Lodge had recommended 

 in a local circuit, and was tapped by a mechanical tapper operated by 

 a vibrating electric mechanism. The local circuit operated a post-office 

 relay connected to a Morse instrument signaling the dots and dashes. 

 The coherer was itself included in the vertical base line. So far all is 

 old. The special coherer used in these experiments by Marconi has 

 very fine metallic powder, chiefly of nickel aud silver, in a small glass 

 tube exhausted of air. He also applied shunting resistances to the 

 relay contacts, and interposed a fine iron wire, closely coiled, as an 

 impedance in the local circuit on each side of the coherer. 



