524 Mr. Sidney George Broivn [March 12, 



WEEKLY EVENING MEETING, 



Friday, March 12, 1909. 



His Grace The Duke of Noethumberland, K.G. P.C. D.C.L. 

 F.R.S., President, in the Chair. 



Sidney George Brown, Esq., M.I.E.E. M.R.I. 



Modern. Submarine Telegraphy. 



This lecture relates to modern submarine telegraphy, and, therefore, 

 I shall omit the historical part of the subject and start with the cable 

 itself, as we deal with it now. 



The signals to form the messages are sent over the submarine 

 cable as electric currents. The cable consists of a central copper 

 wire : this is the conductor for the current, and to prevent the elec- 

 tricity escaping from the wire it is insulated along its entire length 

 by gutta-percha. 



Gutta-percha is chosen for submarine work, because of its very 

 high insulating properties and its not being acted on, or suffering 

 chemical change under water. 



The gutta-percha covered wire is called the core : this core, before 

 it can be laid at the bottom of the sea, must be surrounded by jute 

 serving and steel wires for protection when being laid and during its 

 existence after. 



When deahng with the electrical properties of a cable, the core 

 only is considered, and for all practical purposes it may be taken that 

 the return conductor to the current is the water immediately outside 

 the gutta-percha. 



A core of any given length has a certain time rate of signalling ; 

 that is to say, when a voltage is applied at one end, the effective 

 current, that as a consequence flows in the wu-e, does not arrive at 

 the distant end instantaneously, but takes time to grow. 



The time rate of signalling is inversely proportional to the product 

 of the resistance of the wire and the electrostatic capacity of the core. 



This is termed the " K.R." or capacity resistance law, a law first 

 pointed out by Lord Kelvin. 



It follows from this law that if you double the length of any given 

 kind of cable you reduce its speed for signalling to one quarter. 



The time rate is inversely proportional to the resistance multiphed 

 by the capacity. If you make a certain sized core (size of gutta- 

 percha) with a large copper, up to a certain point you decrease the 

 resistance and increase the capacity ; but there is a critical value giving 



