442 On a Method of identifying a submerged Telegraph-cable. 



The distance C D being 12 yards, every signal that was sent 

 through the conductor was distinctly audible on the telephone 

 (T); reducing the distance gradually, the signals were still 

 readable so long as that distance was not less than 6 feet. At 

 a less distance the signals became faint ; and at 3 inches only 

 an exceedingly feeble sound could just be heard, and not suf- 

 ficient for reading. 



On pressing the key a sound was heard from the telephone ; 

 and on releasing the key a sound was also heard. 



The explanation, of course, is simple : — " For every current 

 made in the conductor, a current is induced in the iron guards 

 in the opposite direction to the primary current; and for every 

 interruption made, a current is induced in the iron guards in 

 the same direction as the primary current. The telephone 

 therefore receives a succession of opposite currents ; and being 

 such an exceedingly sensitive instrument, notwithstanding 

 the very minute fraction of these induced currents which 

 passes through the shunt formed by the telephone, still it 

 gives out audible sound." 



As stated before, the guards were 12 galvanized iron wires 

 each weighing 900 lb. per mile, and 



the resistance per mile of one 900-lb. wire = 7 ohms, 



the resistance per mile of twelve 900-lb. wires = -^ ohm, 



and the resistance of 6 feet of twelve 900-lb. wires = # 66 

 milliohm. 



In this case the resistance of 0*66 milliohm offered by the 

 6 feet of the guards was shunted by a telephone offering with 

 its connecting wires a resistance of 2*5 ohms. Therefore, of 

 the currents induced in the guards by the primary current 

 in the conductor, only ^qo" P asse( ^ through the telephone. 



The method which I suggest for the identification of a sub- 

 merged telegraph-cable is therefore as follows : — Attach a 

 telephone of low resistance to the guards of the raised por- 

 tion of the cable (which portion may be either in or out of 

 the water), as shown in the diagram, and send signals from 

 the bank through the conductor of one of the cables ; and 

 then, if the telephone is connected to the guards of the cable 

 through the conductor of which the signals are being sent, 

 those signals will be audible in the telephone. 



There are doubtless many other cases in which this method 

 may be found of practical service. 



The telephone in its short history resembles very much the 

 development of a boy whose father had brought him up to be a 

 priest, but who, contrary to all expectations, turned out to be 

 a general ; for the application in practical telegraphy for which 

 the telephone was invented has not turned out to be so sue- 



