February io, 1921J 



NATURE 



761 



of force will pass through these coils. When the 

 -hip lies directly over the cable the number of lines 



■ >t force passing through each coil will be equal, 

 .iiid the strength of the signals heard in each coil 

 will be equal also. But when the ship is on one 

 side of the cable the strength of the signals 

 received in the coil nearest the cable will be the 

 louder. By this variation in the strength of 

 signals the navigator is able to tell which side of 



■ he cable his ship is on. 



Experiments made at Portsmouth show that the 

 best position for the coils to be placed is with tHrir 

 centre not farther than within i8 in. from the 



stripped back for a distance of 6 ft. so as to make 

 good electrical connection with the sea. The 

 inboard ends of the cables are connected to the 

 receiving apparatus on the bridge through a twin 

 wire led from the stern. The chief difference 

 when using the electrodes instead of coils is that, 

 as with the former there is no screening effect, the 

 signals received do not indicate which side of the 

 leader cable the ship is on. To ascertain this it 

 is necessary to maintain a steady course for some 

 few minutes and to observe whether the strength 

 of the signals increases or decrea.ses. 



In the receiving apparatus aboard ship two leads 



S0°j«0'. 



JS- 



Fk;. •.— Pmrtion of PortMioaih leader cable. The broken line turning eait from the end of the cable indicate* the direction of a proposed 

 CKienaion of the cable which would enable <ihi|» proceeding up.Chaanel to fix ibeir position in thick weather by cro««ng the cable 

 twice at the points indicated- 



ship's side and the bottom edges of ilic toils 

 inclined outward at an angle of 15° to the vertical. 

 When they are so placed screening is satisfactory up 

 to 400 yards, the maximum range is approximately 

 '<oo yards, and fairly good signals arc obtained 

 ' vcn if the cable is approached at a steep angle. 



The range at which signals from a leader cable 

 ■ :in be received will be largely increased if in place 

 I if coils l)eing used two electrodes are towed astern 

 nf the ship, these being insulated cables of 

 .ipproximately 50 and 1 -^o yards in length, with the 

 itisiilritiori .-it the rmtboard ends of the cables 



■ '1.. 106] 



•in- i.iken from each coil to a change-over switch 

 installed on the bridge in such a position that it is 

 easv of access to the navigating officer. Two 

 leads are taken from this switch to an amplifier 

 of the three-v;«lve low-frequency transformer lypi', 

 using a 4-volt filament battery and a 6o-volt anode 

 potential. The telephones are conntjcled directly 

 with the amplifier, and can be fitted with a head- 

 piece c)r a single receiver. By using a more power- 

 ful amplifier it is possible to install .'i loucl-sp<'akin)4 

 telephone repeater which will enable the signals 

 to be heard bv all standing on the bridge. By 



