THE LAYING OF SUBMARINE CABLES. 159 



circuit key. The arrows indicate a given direction of current, and 

 the switch being placed to the left, as in the figure, the current 

 passes through the galvanometer in the same direction. By 

 putting the switch to the right the direction of current through 



Fig. 77.— Tonking's Key. 



the galvanometer is reversed. Battery currents can be reversed 

 by the ordinary testing key, but there is one objection to this 

 — namely, that the cable is put to earth in changing over, and 

 therefore special reversing keys are used for this purpose. One 



n 



Fig. 78.— Short Circuit Key. 



form which was used throughout the laying of the 1894 Atlantic 

 cable is shown in Figs. 76 and 77. In this key two insulated 

 strips, A and B, of spring brass are fixed on an ebonite pillar 

 capable of being rotated to the right or left by the lever H. 



