WIRELESS TELEGRAPHY 



have crossed the stream had the circuit been contin- 

 uous. If one imagines two iron bridges crossing a 

 narrow stream at some distance apart, the ends of these 

 bridges being connected by two wires running parallel 

 on opposite banks, it is obvious that if telegraph in- 

 struments are inserted anywhere along these wires, mes- 

 sages may be sent and received, a complete circuit being 

 formed by the bridges and the wires connecting them. 

 If the bridges were removed, however, and the ends 

 of the wires at the place of removal immersed in the 

 stream by means of metal plates at the points corre- 

 sponding to the position of the ends of the bridges, it 

 is found that messages may be sent as readily as before 

 the bridges were removed, the water or the bed of 

 the stream completing the circuit. This is the prin- 

 ciple upon which Morse constructed his wireless 

 telegraph. But the maximum distance at which he 

 was able to convey messages was something like a mile ; 

 and in order to do this he found that it was necessary to 

 have his wires extended along the banks at least three 

 miles. In other words the relative distances at which 

 messages could be sent through water was as one to three 

 in comparison with the length of the wires on shore. 



No particular advance was made in wireless teleg- 

 raphy from this time until about 1880, after the in- 

 vention of the telephone. About this time, however, 

 Prof. J. Trowbridge, of Harvard, found that if a dynamo 

 or coil had two terminals in the earth, an interrupted or 

 alternating current passing between them may be de- 

 tected by means of a telephone receiver, which is ex- 

 tremely sensitive to feeble interrupted currents. Mes- 



VOL. VIII. 4 [ 49 ] 



