760 



NAiURE 



[February 10, 192 1 



The Leader Cable System. 



SO far back as 1893 the idea was conceived of 

 using electric signals transmitted through a 

 submarine cable to guide ships past dangerous 

 places. But it was not until Prof. R. B. Owens, 

 of McGill University, began investigating the sub- 

 ject that what is now known as the leader cable 

 system took practical shape. Prof. Owens took 

 out certain patents in 1901— 3, and these were later 

 presented to the Admiralty, which has decided to 

 allow anyone who wishes to do so to use them 

 without paying royalty. 



When Prof. Owens first began his researches 

 the thermionic-valve amplifier was not in exist- 

 ence, and from lack of this important adjunct to 

 its efficiency the leader cable could not be put to 

 practical use, as the signals originated by it were 

 not strong enough for navigational purposes. 

 The invention of the amplifier enabled this defect 

 to be overcome, with the result that during the war 

 leader cables were employed both by ourselves and 

 by the Germans. Such cables have since been laid 

 at Portsmouth, New York, and Brest. 



In all these installations the underlying prin- 

 ciple employed is that devised by Prof. Owens, 

 but considerable improvements have been made in 

 the details of tlie apparatus and in the manner of 

 using it. These advances are mainly the result of 

 work carried out at various Admiralty experimen- 

 tal stations, particularly at Portsmouth, where a 

 cable 17 nautical miles in length has been laid 

 along the eastern approach to the harbour. The 

 working of this cable was demonstrated to the 

 foreign naval attaches recently. 



A leader cable system comprises a submarine 

 cable laid in any waters where it is desired to 

 facilitate navigation. The sea end of the cable is 

 earthed, whilst the shore end is taken into a trans- 

 mitting station and there connected to one ter- 

 minal of an alternating dynamo, the other terminal 

 of the alternator being connected to earth or the 

 sea. In the cable at the shore station a power- 

 operated signalling key is inserted by means of 

 which the current in the cable can be made or 

 broken so as to transmit through the cable any 

 pre-arranged signals or Morse letter. In order 

 that a ship may be able to locate the cable and 

 follow along it a receiving apparatus is fitted in 

 her, or a portable set may be taken on board by 

 the pilot. This apparatus consists of two coils 

 of wire, one on the port and one on the starboard 

 side, which are connected to an amplifier and tele- 

 phones on the vessel's bridge through a change- 

 over switch. In the telephones the signals given 

 out by the cable are heard as a sharply pitched 

 musical note. 



The electric current in a leader cable is an 

 alternating one, and the actual field distribution 

 arising therefrom is complicated by the fact that 

 the return current appears to be mainly concen- 

 trated between the cable and the sea surface. 

 Considering the case of a continuous current in 

 the cable and a return path through sea water in 



NO. 2676, VOL. 106] 



the vicinity of the cable, the resultant magnetic 

 field in the air above the cable will contain cir- 

 cular lines of force due to the constant current in 

 the cable and horizontal lines of force due to 

 the return current in the sea water. Assuming 

 the return current to be distributed uniformly and 

 thus to constitute a sheet of current the magnetic 

 field of which is horizontal and at right angles to 

 the cable, the resultant field will be horizontal 

 directly over the cable, vertical some distance 

 away, and again approximately horizontal, but in 

 the reverse direction, at a considerable distance 

 from the cable. 



If instead of a current of constant intensity an 

 alternating current is passed through the cable, 

 electric currents in a direction opposed to those 

 in the cable will be induced in the sea water, and 

 the intensity of these induced currents will be 

 greatest near the cable. Above the surface of the 

 water the lines of force due to these induced 

 currents will be slightly curved to the surface, but 

 the general direction of the field will be opposed 

 to that due to the current in the cable. In the 

 final resultant field the points of inversion are 

 moved towards the cable. With increase in the 

 frequency of alternation the induced currents 

 increase in intensity, and as a result the points of 



Li^t^: 





Fig. I. — Approximate dist ribution of -iiagnetic field caused by an altt mating 

 current in the cable. 



inversion (X and Y, Fig. i) move closer together 

 as the frequency increases. 



When the hull of a ship is brought into the 

 vicinity of the cable that part of the ship^and the 

 space adjacent to it — farthest removed from the 

 cable vi'ill be screened to some extent. The ship 

 being a good conductor, electric currents are 

 induced in its outer surface when an alternating 

 current flows through the cable. When the ship 

 is broadside on to the cable these sheets of current 

 flow fore and aft and give rise to a magnetic field 

 parallel to the surface of the ship. If the in- 

 tensity of the magnetic alternating field giving 

 rise to these induced currents is greater on one 

 side of the ship than on the other, then the 

 resulting magnetic field will be greater on the 

 former side. 



When a steel or iron ship lies directly over the 

 cable the intensity of the magnetic field is appre- 

 ciably increased because of the presence of the 

 ship, but the intensity is small over the deck 

 because of the screening effect of the hull. If on 

 each side of the ship a square frame is placed, 

 and if on these frames a number of turns of wire 

 are wound, thus forming a coil, some of the lines 



