COMPARISON OF RESISTANCES. 



This method may be applied directly to the determination of 

 the insulation resistance of cables which themselves form con- 

 ductors of great capacity. One end of the cable being insulated, 

 and the outer coating to earth, the other end is connected with 

 a source which produces the potential V and an electrometer. 

 Communication being then broken with the source, the time / is 

 observed at the end of which the potential of the cable be- 

 comes V. 



This method gives the product CR of the capacity of the 

 cable by its resistance to insulation. The experiment is compli- 

 cated in this case by the phenomenon of electrical absorption, and 

 the results are very 'different, according to the length of time in 

 which the end of the cable has been in contact with the source ; it 

 is therefore necessary to define the conditions. The rule of the 

 French Telegraph Administration is to use as source a battery 

 of 100 DanielPs, each with a resistance of 10 ohms. The cable 

 is put in connection with the battery for 15 seconds, and V is 

 then measured by an instantaneous discharge. It is again charged 

 for 15 seconds, and then the cable is discharged after being left 

 to itself for one minute. 



It is to be observed that the product CR is independent of 

 the shape and dimensions of the-, cable, and that it is simply 

 proportional to the product of the specific resistance of the insu- 

 lator by its specific inductive capacity. If o- and //, are these 

 two constants, the capacity of a cable, the core and armature of 

 which may be compared with a condenser formed of concentric 

 cylinders of length L and radii R x and R 2 , has the value (80 

 and 123) 



and its resistance (217) 



R = 

 consequently 



4?r 



This ratio is a general one. It is a consequence of the corre- 

 lation which exists between the flow of electricity and the flow 

 of electrostatic induction (213). 



