THE LAYING OF SUBBIAKINE CABLES. 



179 



the subject the reader is referred to Dr. Lodge's able work on 

 "Lightning Conductors and Lightning Guards," and to his 

 Paper and the discussion thereon printed in the Journal of the 

 Society of Telegraph Engineers and Electricians, Vol. XIX.) 



The well-known Siemens plate lightning guard is illustrated 

 in Fig. 90. The cable and land-line are connected respectively 

 as shown to the two top plates of the guard. These are 

 separated from the lower plate by means of very thin insulat- 

 ing washers shown in black in the figure. The upper and 

 lower plates are scored in fine grooves, those on the bottom 

 plate being at right angles to the grooves on the upper plates. 

 By this device it will be seen that a large number of points 



Fig. 90. — Siemen's Lightning Guard 



are presented opposite each other. The bottom plate is con- 

 nected to earth. While perfect air insulation exists between the 

 line and earth plates by reason of the small clearance^ there are 

 a large number of possible points for lightning to discharge 

 across to earth without damaging the instruments in the line. 



Another form of lightning guard which is now largely adopted 

 for the protection of submarine cables and instruments is that 

 devised by Dr. Oliver Lodge. The guard Is made in two forms : 

 the double form illustrated in Fig. 91 and the single form in 

 Fig. 92. The double form is intended to protect a telegraphic 

 instrument such as a siphon recorder and has four terminals. 

 The instrument is connected to the protected terminals labelled 



n2 



