344 Prof. Ayrton and Mr. Mather on Developments in 



concentric being used as the " lead/' and the outer as the 

 " guard-wire." 



The arrangement is shown symbolically in the sketch 

 above. It will be readily understood that so long as the 

 insulation resistance of the outer of the concentric is high 

 compared with the internal resistance of the battery the 

 method indicated gives complete protection against lead 

 leakage and surface leakage, even if the concentric be laid 

 along the ground for a considerable distance. 



Other instances in which special applications of the guard- 

 wire principle have been found convenient have arisen in 

 eases where it has been necessary to determine whether 

 a defective piece of cable was bad throughout, or bad owing 

 to one or more isolated faults. When two tanks are available 

 and one of them can be moderately well insulated from 

 earth, the method of procedure is to coil about half the cable 

 in each tank, remove the metal sheathing (if any) from a short 

 length, say one inch, of the cable between the two tanks, and 

 connect the guard-wire with the insulated tank as well as 



Fiff. 2 



wrap it round the insulating covering near the extremity 

 of the cable as shown in fig. 2. In the case of braided and 

 other unsheathed cables, the covering of the portion between 



